[PATCH v3 00/70] nstree: listns()

[PATCH v3 00/70] nstree: listns()

[Christian Brauner]

Hey,

As announced a while ago this is the next step building on the nstree
work from prior cycles. There's a bunch of fixes and semantic cleanups
in here and a ton of tests.

Currently listns() is relying on active namespace reference counts which
are introduced alongside this series.

While a namespace is on the namespace trees with a valid reference count
it is possible to reopen it through a namespace file handle. This is all
fine but has some issues that should be addressed.

On current kernels a namespace is visible to userspace in the
following cases:

(1) The namespace is in use by a task.
(2) The namespace is persisted through a VFS object (namespace file
    descriptor or bind-mount).
    Note that (2) only cares about direct persistence of the namespace
    itself not indirectly via e.g., file->f_cred file references or
    similar.
(3) The namespace is a hierarchical namespace type and is the parent of
    a single or multiple child namespaces.

Case (3) is interesting because it is possible that a parent namespace
might not fulfill any of (1) or (2), i.e., is invisible to userspace but
it may still be resurrected through the NS_GET_PARENT ioctl().

Currently namespace file handles allow much broader access to namespaces
than what is currently possible via (1)-(3). The reason is that
namespaces may remain pinned for completely internal reasons yet are
inaccessible to userspace.

For example, a user namespace my remain pinned by get_cred() calls to
stash the opener's credentials into file->f_cred. As it stands file
handles allow to resurrect such a users namespace even though this
should not be possible via (1)-(3). This is a fundamental uapi change
that we shouldn't do if we don't have to.

Consider the following insane case: Various architectures support the
CONFIG_MMU_LAZY_TLB_REFCOUNT option which uses lazy TLB destruction.
When this option is set a userspace task's struct mm_struct may be used
for kernel threads such as the idle task and will only be destroyed once
the cpu's runqueue switches back to another task. But because of ptrace()
permission checks struct mm_struct stashes the user namespace of the
task that struct mm_struct originally belonged to. The kernel thread
will take a reference on the struct mm_struct and thus pin it.

So on an idle system user namespaces can be persisted for arbitrary
amounts of time which also means that they can be resurrected using
namespace file handles. That makes no sense whatsoever. The problem is
of course excarabted on large systems with a huge number of cpus.

To handle this nicely we introduce an active reference count which
tracks (1)-(3). This is easy to do as all of these things are already
managed centrally. Only (1)-(3) will count towards the active reference
count and only namespaces which are active may be opened via namespace
file handles.

The problem is that namespaces may be resurrected. Which means that they
can become temporarily inactive and will be reactived some time later.
Currently the only example of this is the SIOGCSKNS socket ioctl. The
SIOCGSKNS ioctl allows to open a network namespace file descriptor based
on a socket file descriptor.

If a socket is tied to a network namespace that subsequently becomes
inactive but that socket is persisted by another process in another
network namespace (e.g., via SCM_RIGHTS of pidfd_getfd()) then the
SIOCGSKNS ioctl will resurrect this network namespace.

So calls to open_related_ns() and open_namespace() will end up
resurrecting the corresponding namespace tree.

Note that the active reference count does not regulate the lifetime of
the namespace itself. This is still done by the normal reference count.
The active reference count can only be elevated if the regular reference
count is elevated.

The active reference count also doesn't regulate the presence of a
namespace on the namespace trees. It only regulates its visiblity to
namespace file handles (and in later patches to listns()).

A namespace remains on the namespace trees from creation until its
actual destruction. This will allow the kernel to always reach any
namespace trivially and it will also enable subsystems like bpf to walk
the namespace lists on the system for tracing or general introspection
purposes.

Note that different namespaces have different visibility lifetimes on
current kernels. While most namespace are immediately released when the
last task using them exits, the user- and pid namespace are persisted
and thus both remain accessible via /proc/<pid>/ns/<ns_type>.

The user namespace lifetime is aliged with struct cred and is only
released through exit_creds(). However, it becomes inaccessible to
userspace once the last task using it is reaped, i.e., when
release_task() is called and all proc entries are flushed. Similarly,
the pid namespace is also visible until the last task using it has been
reaped and the associated pid numbers are freed.

The active reference counts of the user- and pid namespace are
decremented once the task is reaped.

Based on the namespace trees and the active reference count, a new
listns() system call that allows userspace to iterate through namespaces
in the system. This provides a programmatic interface to discover and
inspect namespaces, enhancing existing namespace apis.

Currently, there is no direct way for userspace to enumerate namespaces
in the system. Applications must resort to scanning /proc/<pid>/ns/
across all processes, which is:

1. Inefficient - requires iterating over all processes
2. Incomplete - misses inactive namespaces that aren't attached to any
   running process but are kept alive by file descriptors, bind mounts,
   or parent namespace references
3. Permission-heavy - requires access to /proc for many processes
4. No ordering or ownership.
5. No filtering per namespace type: Must always iterate and check all
   namespaces.

The list goes on. The listns() system call solves these problems by
providing direct kernel-level enumeration of namespaces. It is similar
to listmount() but obviously tailored to namespaces.

/*
 * @req: Pointer to struct ns_id_req specifying search parameters
 * @ns_ids: User buffer to receive namespace IDs
 * @nr_ns_ids: Size of ns_ids buffer (maximum number of IDs to return)
 * @flags: Reserved for future use (must be 0)
 */
ssize_t listns(const struct ns_id_req *req, u64 *ns_ids,
               size_t nr_ns_ids, unsigned int flags);

Returns:
- On success: Number of namespace IDs written to ns_ids
- On error: Negative error code

/*
 * @size: Structure size
 * @ns_id: Starting point for iteration; use 0 for first call, then
 *         use the last returned ID for subsequent calls to paginate
 * @ns_type: Bitmask of namespace types to include (from enum ns_type):
 *           0: Return all namespace types
 *           MNT_NS: Mount namespaces
 *           NET_NS: Network namespaces
 *           USER_NS: User namespaces
 *           etc. Can be OR'd together
 * @user_ns_id: Filter results to namespaces owned by this user namespace:
 *              0: Return all namespaces (subject to permission checks)
 *              LISTNS_CURRENT_USER: Namespaces owned by caller's user namespace
 *              Other value: Namespaces owned by the specified user namespace ID
 */
struct ns_id_req {
        __u32 size;         /* sizeof(struct ns_id_req) */
        __u32 spare;        /* Reserved, must be 0 */
        __u64 ns_id;        /* Last seen namespace ID (for pagination) */
        __u32 ns_type;      /* Filter by namespace type(s) */
        __u32 spare2;       /* Reserved, must be 0 */
        __u64 user_ns_id;   /* Filter by owning user namespace */
};

Example 1: List all namespaces

void list_all_namespaces(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,      /* Start from beginning */
		.ns_type = 0,    /* All types */
		.user_ns_id = 0, /* All user namespaces */
	};
	uint64_t ids[100];
	ssize_t ret;

	printf("All namespaces in the system:\n");
	do {
		ret = listns(&req, ids, 100, 0);
		if (ret < 0) {
			perror("listns");
			break;
		}

		for (ssize_t i = 0; i < ret; i++)
			printf("  Namespace ID: %llu\n", (unsigned long long)ids[i]);

		/* Continue from last seen ID */
		if (ret > 0)
			req.ns_id = ids[ret - 1];
	} while (ret == 100); /* Buffer was full, more may exist */
}

Example 2 : List network namespaces only

void list_network_namespaces(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,
		.ns_type = NET_NS, /* Only network namespaces */
		.user_ns_id = 0,
	};
	uint64_t ids[100];
	ssize_t ret;

	ret = listns(&req, ids, 100, 0);
	if (ret < 0) {
		perror("listns");
		return;
	}

	printf("Network namespaces: %zd found\n", ret);
	for (ssize_t i = 0; i < ret; i++)
		printf("  netns ID: %llu\n", (unsigned long long)ids[i]);
}

Example 3 : List namespaces owned by current user namespace

void list_owned_namespaces(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,
		.ns_type = 0,                      /* All types */
		.user_ns_id = LISTNS_CURRENT_USER, /* Current userns */
	};
	uint64_t ids[100];
	ssize_t ret;

	ret = listns(&req, ids, 100, 0);
	if (ret < 0) {
		perror("listns");
		return;
	}

	printf("Namespaces owned by my user namespace: %zd\n", ret);
	for (ssize_t i = 0; i < ret; i++)
		printf("  ns ID: %llu\n", (unsigned long long)ids[i]);
}

Example 4 : List multiple namespace types

void list_network_and_mount_namespaces(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,
		.ns_type = NET_NS | MNT_NS, /* Network and mount */
		.user_ns_id = 0,
	};
	uint64_t ids[100];
	ssize_t ret;

	ret = listns(&req, ids, 100, 0);
	printf("Network and mount namespaces: %zd found\n", ret);
}

Example 5 : Pagination through large namespace sets

void list_all_with_pagination(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,
		.ns_type = 0,
		.user_ns_id = 0,
	};
	uint64_t ids[50];
	size_t total = 0;
	ssize_t ret;

	printf("Enumerating all namespaces with pagination:\n");

	while (1) {
		ret = listns(&req, ids, 50, 0);
		if (ret < 0) {
			perror("listns");
			break;
		}
		if (ret == 0)
			break; /* No more namespaces */

		total += ret;
		printf("  Batch: %zd namespaces\n", ret);

		/* Last ID in this batch becomes start of next batch */
		req.ns_id = ids[ret - 1];

		if (ret < 50)
			break; /* Partial batch = end of results */
	}

	printf("Total: %zu namespaces\n", total);
}

listns() respects namespace isolation and capabilities:

(1) Global listing (user_ns_id = 0):
    - Requires CAP_SYS_ADMIN in the namespace's owning user namespace
    - OR the namespace must be in the caller's namespace context (e.g.,
      a namespace the caller is currently using)
    - User namespaces additionally allow listing if the caller has
      CAP_SYS_ADMIN in that user namespace itself
(2) Owner-filtered listing (user_ns_id != 0):
    - Requires CAP_SYS_ADMIN in the specified owner user namespace
    - OR the namespace must be in the caller's namespace context
    - This allows unprivileged processes to enumerate namespaces they own
(3) Visibility:
    - Only "active" namespaces are listed
    - A namespace is active if it has a non-zero __ns_ref_active count
    - This includes namespaces used by running processes, held by open
      file descriptors, or kept active by bind mounts
    - Inactive namespaces (kept alive only by internal kernel
      references) are not visible via listns()

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Changes in v3:
- Expanded test-suite.
- Moved active reference count tracking for task-attached namespaces to
  dedicated helpers.
- Fixed active reference count leaks when creating a new process fails.
- Allow to be rescheduled when walking a a long namespace list.
- Grab reference count when accessing a namespace when walking the list.
- Link to v2: https://patch.msgid.link/20251022-work-namespace-nstree-listns-v2-0-71a588572371@kernel.org

Changes in v2:
- Fully implement the active reference count.
- Fix various minor issues.
- Expand the testsuite to test complex resurrection scenarios due to SIOCGSKNS.
- Currently each task takes an active reference on the user namespace as
  credentials can be persisted for a very long time and completely
  arbitrary reasons but we don't want to tie the lifetime of a user
  namespace being visible to userspace to the existence of some
  credentials being stashed somewhere. We want to tie it to it being
  in-use by actual tasks or vfs objects and then go away. There might be
  more clever ways of doing this but for now this is good enough.
- TODO: Add detailed tests for multi-threaded namespace sharing.
- Link to v1: https://patch.msgid.link/20251021-work-namespace-nstree-listns-v1-0-ad44261a8a5b@kernel.org

---
Christian Brauner (70):
      libfs: allow to specify s_d_flags
      nsfs: use inode_just_drop()
      nsfs: raise DCACHE_DONTCACHE explicitly
      pidfs: raise DCACHE_DONTCACHE explicitly
      nsfs: raise SB_I_NODEV and SB_I_NOEXEC
      cgroup: add cgroup namespace to tree after owner is set
      nstree: simplify return
      ns: initialize ns_list_node for initial namespaces
      ns: add __ns_ref_read()
      ns: rename to exit_nsproxy_namespaces()
      ns: add active reference count
      ns: use anonymous struct to group list member
      nstree: introduce a unified tree
      nstree: allow lookup solely based on inode
      nstree: assign fixed ids to the initial namespaces
      ns: maintain list of owned namespaces
      nstree: add listns()
      arch: hookup listns() system call
      nsfs: update tools header
      selftests/filesystems: remove CLONE_NEWPIDNS from setup_userns() helper
      selftests/namespaces: first active reference count tests
      selftests/namespaces: second active reference count tests
      selftests/namespaces: third active reference count tests
      selftests/namespaces: fourth active reference count tests
      selftests/namespaces: fifth active reference count tests
      selftests/namespaces: sixth active reference count tests
      selftests/namespaces: seventh active reference count tests
      selftests/namespaces: eigth active reference count tests
      selftests/namespaces: ninth active reference count tests
      selftests/namespaces: tenth active reference count tests
      selftests/namespaces: eleventh active reference count tests
      selftests/namespaces: twelth active reference count tests
      selftests/namespaces: thirteenth active reference count tests
      selftests/namespaces: fourteenth active reference count tests
      selftests/namespaces: fifteenth active reference count tests
      selftests/namespaces: add listns() wrapper
      selftests/namespaces: first listns() test
      selftests/namespaces: second listns() test
      selftests/namespaces: third listns() test
      selftests/namespaces: fourth listns() test
      selftests/namespaces: fifth listns() test
      selftests/namespaces: sixth listns() test
      selftests/namespaces: seventh listns() test
      selftests/namespaces: eigth listns() test
      selftests/namespaces: ninth listns() test
      selftests/namespaces: first listns() permission test
      selftests/namespaces: second listns() permission test
      selftests/namespaces: third listns() permission test
      selftests/namespaces: fourth listns() permission test
      selftests/namespaces: fifth listns() permission test
      selftests/namespaces: sixth listns() permission test
      selftests/namespaces: seventh listns() permission test
      selftests/namespaces: first inactive namespace resurrection test
      selftests/namespaces: second inactive namespace resurrection test
      selftests/namespaces: third inactive namespace resurrection test
      selftests/namespaces: fourth inactive namespace resurrection test
      selftests/namespaces: fifth inactive namespace resurrection test
      selftests/namespaces: sixth inactive namespace resurrection test
      selftests/namespaces: seventh inactive namespace resurrection test
      selftests/namespaces: eigth inactive namespace resurrection test
      selftests/namespaces: ninth inactive namespace resurrection test
      selftests/namespaces: tenth inactive namespace resurrection test
      selftests/namespaces: eleventh inactive namespace resurrection test
      selftests/namespaces: twelth inactive namespace resurrection test
      selftests/namespace: first threaded active reference count test
      selftests/namespace: second threaded active reference count test
      selftests/namespace: third threaded active reference count test
      selftests/namespace: commit_creds() active reference tests
      selftests/namespace: add stress test
      selftests/namespace: test listns() pagination

 arch/alpha/kernel/syscalls/syscall.tbl             |    1 +
 arch/arm/tools/syscall.tbl                         |    1 +
 arch/arm64/tools/syscall_32.tbl                    |    1 +
 arch/m68k/kernel/syscalls/syscall.tbl              |    1 +
 arch/microblaze/kernel/syscalls/syscall.tbl        |    1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl          |    1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl          |    1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl          |    1 +
 arch/parisc/kernel/syscalls/syscall.tbl            |    1 +
 arch/powerpc/kernel/syscalls/syscall.tbl           |    1 +
 arch/s390/kernel/syscalls/syscall.tbl              |    1 +
 arch/sh/kernel/syscalls/syscall.tbl                |    1 +
 arch/sparc/kernel/syscalls/syscall.tbl             |    1 +
 arch/x86/entry/syscalls/syscall_32.tbl             |    1 +
 arch/x86/entry/syscalls/syscall_64.tbl             |    1 +
 arch/xtensa/kernel/syscalls/syscall.tbl            |    1 +
 fs/libfs.c                                         |    1 +
 fs/namespace.c                                     |    8 +-
 fs/nsfs.c                                          |   95 +-
 fs/pidfs.c                                         |    1 +
 include/linux/ns_common.h                          |  166 +-
 include/linux/nsfs.h                               |    3 +
 include/linux/nsproxy.h                            |    5 +-
 include/linux/nstree.h                             |   26 +-
 include/linux/pseudo_fs.h                          |    1 +
 include/linux/syscalls.h                           |    4 +
 include/linux/user_namespace.h                     |    4 +-
 include/uapi/asm-generic/unistd.h                  |    4 +-
 include/uapi/linux/nsfs.h                          |   58 +
 init/version-timestamp.c                           |    5 +
 ipc/msgutil.c                                      |    5 +
 ipc/namespace.c                                    |    1 +
 kernel/cgroup/cgroup.c                             |   11 +-
 kernel/cgroup/namespace.c                          |    3 +-
 kernel/cred.c                                      |    6 +
 kernel/exit.c                                      |    3 +-
 kernel/fork.c                                      |    3 +-
 kernel/nscommon.c                                  |  227 +-
 kernel/nsproxy.c                                   |   25 +-
 kernel/nstree.c                                    |  540 +++-
 kernel/pid.c                                       |   10 +
 kernel/pid_namespace.c                             |    1 +
 kernel/time/namespace.c                            |    6 +
 kernel/user.c                                      |    5 +
 kernel/user_namespace.c                            |    1 +
 kernel/utsname.c                                   |    1 +
 net/core/net_namespace.c                           |    3 +-
 scripts/syscall.tbl                                |    1 +
 tools/include/uapi/linux/nsfs.h                    |   70 +
 tools/testing/selftests/filesystems/utils.c        |    2 +-
 tools/testing/selftests/namespaces/.gitignore      |    7 +
 tools/testing/selftests/namespaces/Makefile        |   20 +-
 .../selftests/namespaces/cred_change_test.c        |  814 ++++++
 .../selftests/namespaces/listns_pagination_bug.c   |  138 +
 .../selftests/namespaces/listns_permissions_test.c |  759 ++++++
 tools/testing/selftests/namespaces/listns_test.c   |  679 +++++
 .../selftests/namespaces/ns_active_ref_test.c      | 2672 ++++++++++++++++++++
 .../testing/selftests/namespaces/siocgskns_test.c  | 1824 +++++++++++++
 tools/testing/selftests/namespaces/stress_test.c   |  626 +++++
 tools/testing/selftests/namespaces/wrappers.h      |   35 +
 60 files changed, 8835 insertions(+), 60 deletions(-)
---
base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
change-id: 20251020-work-namespace-nstree-listns-9fd71518515c

[PATCH v3 01/70] libfs: allow to specify s_d_flags

[Christian Brauner]

Make it possible for pseudo filesystems to specify default dentry flags.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/libfs.c                | 1 +
 include/linux/pseudo_fs.h | 1 +
 2 files changed, 2 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index ce8c496a6940..4bb4d8a313e7 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -680,6 +680,7 @@ static int pseudo_fs_fill_super(struct super_block *s, struct fs_context *fc)
 	s->s_export_op = ctx->eops;
 	s->s_xattr = ctx->xattr;
 	s->s_time_gran = 1;
+	s->s_d_flags |= ctx->s_d_flags;
 	root = new_inode(s);
 	if (!root)
 		return -ENOMEM;
diff --git a/include/linux/pseudo_fs.h b/include/linux/pseudo_fs.h
index 2503f7625d65..a651e60d9410 100644
--- a/include/linux/pseudo_fs.h
+++ b/include/linux/pseudo_fs.h
@@ -9,6 +9,7 @@ struct pseudo_fs_context {
 	const struct xattr_handler * const *xattr;
 	const struct dentry_operations *dops;
 	unsigned long magic;
+	unsigned int s_d_flags;
 };
 
 struct pseudo_fs_context *init_pseudo(struct fs_context *fc,

-- 
2.47.3

[PATCH v3 02/70] nsfs: use inode_just_drop()

[Christian Brauner]

Currently nsfs uses the default inode_generic_drop() fallback which
drops the inode when it's unlinked or when it's unhashed. Since nsfs
never hashes inodes that always amounts to dropping the inode.

But that's just annoying to have to reason through every time we look at
this code. Switch to inode_just_drop() which always drops the inode
explicitly. This also aligns the behavior with pidfs which does the
same.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/nsfs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 648dc59bef7f..4e77eba0c8fc 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -408,6 +408,7 @@ static const struct super_operations nsfs_ops = {
 	.statfs = simple_statfs,
 	.evict_inode = nsfs_evict,
 	.show_path = nsfs_show_path,
+	.drop_inode = inode_just_drop,
 };
 
 static int nsfs_init_inode(struct inode *inode, void *data)

-- 
2.47.3

[PATCH v3 03/70] nsfs: raise DCACHE_DONTCACHE explicitly

[Christian Brauner]

While nsfs dentries are never hashed and thus retain_dentry() will never
consider them for placing them on the LRU it isn't great to always have
to go and remember that. Raise DCACHE_DONTCACHE explicitly as a visual
marker that dentries aren't kept but freed immediately instead.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/nsfs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 4e77eba0c8fc..0e3fe8fda5bf 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -589,6 +589,7 @@ static int nsfs_init_fs_context(struct fs_context *fc)
 	struct pseudo_fs_context *ctx = init_pseudo(fc, NSFS_MAGIC);
 	if (!ctx)
 		return -ENOMEM;
+	ctx->s_d_flags |= DCACHE_DONTCACHE;
 	ctx->ops = &nsfs_ops;
 	ctx->eops = &nsfs_export_operations;
 	ctx->dops = &ns_dentry_operations;

-- 
2.47.3

[PATCH v3 04/70] pidfs: raise DCACHE_DONTCACHE explicitly

[Christian Brauner]

While pidfs dentries are never hashed and thus retain_dentry() will never
consider them for placing them on the LRU it isn't great to always have
to go and remember that. Raise DCACHE_DONTCACHE explicitly as a visual
marker that dentries aren't kept but freed immediately instead.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/pidfs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 0ef5b47d796a..db236427fc2c 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -1022,6 +1022,7 @@ static int pidfs_init_fs_context(struct fs_context *fc)
 
 	fc->s_iflags |= SB_I_NOEXEC;
 	fc->s_iflags |= SB_I_NODEV;
+	ctx->s_d_flags |= DCACHE_DONTCACHE;
 	ctx->ops = &pidfs_sops;
 	ctx->eops = &pidfs_export_operations;
 	ctx->dops = &pidfs_dentry_operations;

-- 
2.47.3

[PATCH v3 05/70] nsfs: raise SB_I_NODEV and SB_I_NOEXEC

[Christian Brauner]

There's zero need for nsfs to allow device nodes or execution.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/nsfs.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 0e3fe8fda5bf..363be226e357 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -589,6 +589,8 @@ static int nsfs_init_fs_context(struct fs_context *fc)
 	struct pseudo_fs_context *ctx = init_pseudo(fc, NSFS_MAGIC);
 	if (!ctx)
 		return -ENOMEM;
+	fc->s_iflags |= SB_I_NOEXEC;
+	fc->s_iflags |= SB_I_NODEV;
 	ctx->s_d_flags |= DCACHE_DONTCACHE;
 	ctx->ops = &nsfs_ops;
 	ctx->eops = &nsfs_export_operations;

-- 
2.47.3

Re: [PATCH v3 05/70] nsfs: raise SB_I_NODEV and SB_I_NOEXEC

[Jeff Layton]

On Fri, 2025-10-24 at 12:52 +0200, Christian Brauner wrote:
> There's zero need for nsfs to allow device nodes or execution.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/nsfs.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index 0e3fe8fda5bf..363be226e357 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -589,6 +589,8 @@ static int nsfs_init_fs_context(struct fs_context *fc)
>  	struct pseudo_fs_context *ctx = init_pseudo(fc, NSFS_MAGIC);
>  	if (!ctx)
>  		return -ENOMEM;
> +	fc->s_iflags |= SB_I_NOEXEC;
> +	fc->s_iflags |= SB_I_NODEV;

nit: why not do this in one?

	fc->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;

>  	ctx->s_d_flags |= DCACHE_DONTCACHE;
>  	ctx->ops = &nsfs_ops;
>  	ctx->eops = &nsfs_export_operations;

-- 
Jeff Layton <jlayton@kernel.org>

Re: [PATCH v3 05/70] nsfs: raise SB_I_NODEV and SB_I_NOEXEC

[Christian Brauner]

On Mon, Oct 27, 2025 at 09:13:21AM -0400, Jeff Layton wrote:
> On Fri, 2025-10-24 at 12:52 +0200, Christian Brauner wrote:
> > There's zero need for nsfs to allow device nodes or execution.
> > 
> > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > ---
> >  fs/nsfs.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/fs/nsfs.c b/fs/nsfs.c
> > index 0e3fe8fda5bf..363be226e357 100644
> > --- a/fs/nsfs.c
> > +++ b/fs/nsfs.c
> > @@ -589,6 +589,8 @@ static int nsfs_init_fs_context(struct fs_context *fc)
> >  	struct pseudo_fs_context *ctx = init_pseudo(fc, NSFS_MAGIC);
> >  	if (!ctx)
> >  		return -ENOMEM;
> > +	fc->s_iflags |= SB_I_NOEXEC;
> > +	fc->s_iflags |= SB_I_NODEV;
> 
> nit: why not do this in one?
> 
> 	fc->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;

done.

[PATCH v3 06/70] cgroup: add cgroup namespace to tree after owner is set

[Christian Brauner]

Otherwise we trip VFS_WARN_ON_ONC() in __ns_tree_add_raw().

Fixes: 7c6059398533 ("cgroup: support ns lookup")
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 kernel/cgroup/namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cgroup/namespace.c b/kernel/cgroup/namespace.c
index fdbe57578e68..db9617556dd7 100644
--- a/kernel/cgroup/namespace.c
+++ b/kernel/cgroup/namespace.c
@@ -30,7 +30,6 @@ static struct cgroup_namespace *alloc_cgroup_ns(void)
 	ret = ns_common_init(new_ns);
 	if (ret)
 		return ERR_PTR(ret);
-	ns_tree_add(new_ns);
 	return no_free_ptr(new_ns);
 }
 
@@ -86,6 +85,7 @@ struct cgroup_namespace *copy_cgroup_ns(u64 flags,
 	new_ns->ucounts = ucounts;
 	new_ns->root_cset = cset;
 
+	ns_tree_add(new_ns);
 	return new_ns;
 }
 

-- 
2.47.3

[PATCH v3 07/70] nstree: simplify return

[Christian Brauner]

node_to_ns() checks for NULL and the assert isn't really helpful and
will have to be dropped later anyway.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 kernel/nstree.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/kernel/nstree.c b/kernel/nstree.c
index b24a320a11a6..369fd1675c6a 100644
--- a/kernel/nstree.c
+++ b/kernel/nstree.c
@@ -194,11 +194,6 @@ struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type)
 			break;
 	} while (read_seqretry(&ns_tree->ns_tree_lock, seq));
 
-	if (!node)
-		return NULL;
-
-	VFS_WARN_ON_ONCE(node_to_ns(node)->ns_type != ns_type);
-
 	return node_to_ns(node);
 }
 

-- 
2.47.3

[PATCH v3 08/70] ns: initialize ns_list_node for initial namespaces

[Christian Brauner]

Make sure that the list is always initialized for initial namespaces.

Fixes: 885fc8ac0a4d ("nstree: make iterator generic")
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c           | 1 +
 init/version-timestamp.c | 1 +
 ipc/msgutil.c            | 1 +
 kernel/cgroup/cgroup.c   | 1 +
 kernel/pid.c             | 1 +
 kernel/time/namespace.c  | 1 +
 kernel/user.c            | 1 +
 7 files changed, 7 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index d82910f33dc4..8ef8ba3dd316 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5993,6 +5993,7 @@ struct mnt_namespace init_mnt_ns = {
 	.passive	= REFCOUNT_INIT(1),
 	.mounts		= RB_ROOT,
 	.poll		= __WAIT_QUEUE_HEAD_INITIALIZER(init_mnt_ns.poll),
+	.ns.ns_list_node = LIST_HEAD_INIT(init_mnt_ns.ns.ns_list_node),
 };
 
 static void __init init_mount_tree(void)
diff --git a/init/version-timestamp.c b/init/version-timestamp.c
index d071835121c2..61b2405d97f9 100644
--- a/init/version-timestamp.c
+++ b/init/version-timestamp.c
@@ -20,6 +20,7 @@ struct uts_namespace init_uts_ns = {
 	},
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_uts_ns),
+	.ns.ns_list_node = LIST_HEAD_INIT(init_uts_ns.ns.ns_list_node),
 #ifdef CONFIG_UTS_NS
 	.ns.ops = &utsns_operations,
 #endif
diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index 7a03f6d03de3..c9469fbce27c 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -30,6 +30,7 @@ struct ipc_namespace init_ipc_ns = {
 	.ns.__ns_ref = REFCOUNT_INIT(1),
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_ipc_ns),
+	.ns.ns_list_node = LIST_HEAD_INIT(init_ipc_ns.ns.ns_list_node),
 #ifdef CONFIG_IPC_NS
 	.ns.ops = &ipcns_operations,
 #endif
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 6ae5f48cf64e..a82918da8bae 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -256,6 +256,7 @@ struct cgroup_namespace init_cgroup_ns = {
 	.ns.inum	= ns_init_inum(&init_cgroup_ns),
 	.root_cset	= &init_css_set,
 	.ns.ns_type	= ns_common_type(&init_cgroup_ns),
+	.ns.ns_list_node = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_list_node),
 };
 
 static struct file_system_type cgroup2_fs_type;
diff --git a/kernel/pid.c b/kernel/pid.c
index 4fffec767a63..cb7574ca00f7 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -78,6 +78,7 @@ struct pid_namespace init_pid_ns = {
 	.child_reaper = &init_task,
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_pid_ns),
+	.ns.ns_list_node = LIST_HEAD_INIT(init_pid_ns.ns.ns_list_node),
 #ifdef CONFIG_PID_NS
 	.ns.ops = &pidns_operations,
 #endif
diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
index 5b6997f4dc3d..ee05cad288da 100644
--- a/kernel/time/namespace.c
+++ b/kernel/time/namespace.c
@@ -484,6 +484,7 @@ struct time_namespace init_time_ns = {
 	.ns.inum	= ns_init_inum(&init_time_ns),
 	.ns.ops		= &timens_operations,
 	.frozen_offsets	= true,
+	.ns.ns_list_node = LIST_HEAD_INIT(init_time_ns.ns.ns_list_node),
 };
 
 void __init time_ns_init(void)
diff --git a/kernel/user.c b/kernel/user.c
index 0163665914c9..b9cf3b056a71 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -70,6 +70,7 @@ struct user_namespace init_user_ns = {
 	.owner = GLOBAL_ROOT_UID,
 	.group = GLOBAL_ROOT_GID,
 	.ns.inum = ns_init_inum(&init_user_ns),
+	.ns.ns_list_node = LIST_HEAD_INIT(init_user_ns.ns.ns_list_node),
 #ifdef CONFIG_USER_NS
 	.ns.ops = &userns_operations,
 #endif

-- 
2.47.3

[PATCH v3 09/70] ns: add __ns_ref_read()

[Christian Brauner]

Implement ns_ref_read() the same way as ns_ref_{get,put}().
No point in making that any more special or different from the other
helpers.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/linux/ns_common.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index f5b68b8abb54..32114d5698dc 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -143,7 +143,12 @@ static __always_inline __must_check bool __ns_ref_get(struct ns_common *ns)
 	return refcount_inc_not_zero(&ns->__ns_ref);
 }
 
-#define ns_ref_read(__ns) refcount_read(&to_ns_common((__ns))->__ns_ref)
+static __always_inline __must_check int __ns_ref_read(const struct ns_common *ns)
+{
+	return refcount_read(&ns->__ns_ref);
+}
+
+#define ns_ref_read(__ns) __ns_ref_read(to_ns_common((__ns)))
 #define ns_ref_inc(__ns) refcount_inc(&to_ns_common((__ns))->__ns_ref)
 #define ns_ref_get(__ns) __ns_ref_get(to_ns_common((__ns)))
 #define ns_ref_put(__ns) __ns_ref_put(to_ns_common((__ns)))

-- 
2.47.3

[PATCH v3 10/70] ns: rename to exit_nsproxy_namespaces()

[Christian Brauner]

The current naming is very misleading as this really isn't exiting all
of the task's namespaces. It is only exiting the namespaces that hang of
off nsproxy. Reflect that in the name.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/linux/nsproxy.h | 2 +-
 kernel/cgroup/cgroup.c  | 6 +++---
 kernel/exit.c           | 2 +-
 kernel/fork.c           | 2 +-
 kernel/nsproxy.c        | 2 +-
 5 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
index bd118a187dec..538ba8dba184 100644
--- a/include/linux/nsproxy.h
+++ b/include/linux/nsproxy.h
@@ -93,7 +93,7 @@ static inline struct cred *nsset_cred(struct nsset *set)
  */
 
 int copy_namespaces(u64 flags, struct task_struct *tsk);
-void exit_task_namespaces(struct task_struct *tsk);
+void exit_nsproxy_namespaces(struct task_struct *tsk);
 void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new);
 int exec_task_namespaces(void);
 void free_nsproxy(struct nsproxy *ns);
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index a82918da8bae..ce4d227a9ca2 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1523,9 +1523,9 @@ static struct cgroup *current_cgns_cgroup_dfl(void)
 	} else {
 		/*
 		 * NOTE: This function may be called from bpf_cgroup_from_id()
-		 * on a task which has already passed exit_task_namespaces() and
-		 * nsproxy == NULL. Fall back to cgrp_dfl_root which will make all
-		 * cgroups visible for lookups.
+		 * on a task which has already passed exit_nsproxy_namespaces()
+		 * and nsproxy == NULL. Fall back to cgrp_dfl_root which will
+		 * make all cgroups visible for lookups.
 		 */
 		return &cgrp_dfl_root.cgrp;
 	}
diff --git a/kernel/exit.c b/kernel/exit.c
index 9f74e8f1c431..825998103520 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -962,7 +962,7 @@ void __noreturn do_exit(long code)
 	exit_fs(tsk);
 	if (group_dead)
 		disassociate_ctty(1);
-	exit_task_namespaces(tsk);
+	exit_nsproxy_namespaces(tsk);
 	exit_task_work(tsk);
 	exit_thread(tsk);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 3da0f08615a9..0926bfe4b8df 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2453,7 +2453,7 @@ __latent_entropy struct task_struct *copy_process(
 	if (p->io_context)
 		exit_io_context(p);
 bad_fork_cleanup_namespaces:
-	exit_task_namespaces(p);
+	exit_nsproxy_namespaces(p);
 bad_fork_cleanup_mm:
 	if (p->mm) {
 		mm_clear_owner(p->mm, p);
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index 19aa64ab08c8..6ce76a0278ab 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -241,7 +241,7 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
 		put_nsproxy(ns);
 }
 
-void exit_task_namespaces(struct task_struct *p)
+void exit_nsproxy_namespaces(struct task_struct *p)
 {
 	switch_task_namespaces(p, NULL);
 }

-- 
2.47.3

[PATCH v3 11/70] ns: add active reference count

[Christian Brauner]

The namespace tree is, among other things, currently used to support
file handles for namespaces. When a namespace is created it is placed on
the namespace trees and when it is destroyed it is removed from the
namespace trees.

While a namespace is on the namespace trees with a valid reference count
it is possible to reopen it through a namespace file handle. This is all
fine but has some issues that should be addressed.

On current kernels a namespace is visible to userspace in the
following cases:

(1) The namespace is in use by a task.
(2) The namespace is persisted through a VFS object (namespace file
    descriptor or bind-mount).
    Note that (2) only cares about direct persistence of the namespace
    itself not indirectly via e.g., file->f_cred file references or
    similar.
(3) The namespace is a hierarchical namespace type and is the parent of
    a single or multiple child namespaces.

Case (3) is interesting because it is possible that a parent namespace
might not fulfill any of (1) or (2), i.e., is invisible to userspace but
it may still be resurrected through the NS_GET_PARENT ioctl().

Currently namespace file handles allow much broader access to namespaces
than what is currently possible via (1)-(4). The reason is that
namespaces may remain pinned for completely internal reasons yet are
inaccessible to userspace.

For example, a user namespace my remain pinned by get_cred() calls to
stash the opener's credentials into file->f_cred. As it stands file
handles allow to resurrect such a users namespace even though this
should not be possible via (1)-(3). This is a fundamental uapi change
that we shouldn't do if we don't have to.

Consider the following insane case: Various architectures support the
CONFIG_MMU_LAZY_TLB_REFCOUNT option which uses lazy TLB destruction.
When this option is set a userspace task's struct mm_struct may be used
for kernel threads such as the idle task and will only be destroyed once
the cpu's runqueue switches back to another task. But because of ptrace()
permission checks struct mm_struct stashes the user namespace of the
task that struct mm_struct originally belonged to. The kernel thread
will take a reference on the struct mm_struct and thus pin it.

So on an idle system user namespaces can be persisted for arbitrary
amounts of time which also means that they can be resurrected using
namespace file handles. That makes no sense whatsoever. The problem is
of course excarabted on large systems with a huge number of cpus.

To handle this nicely we introduce an active reference count which
tracks (1)-(3). This is easy to do as all of these things are already
managed centrally. Only (1)-(3) will count towards the active reference
count and only namespaces which are active may be opened via namespace
file handles.

The problem is that namespaces may be resurrected. Which means that they
can become temporarily inactive and will be reactived some time later.
Currently the only example of this is the SIOGCSKNS socket ioctl. The
SIOCGSKNS ioctl allows to open a network namespace file descriptor based
on a socket file descriptor.

If a socket is tied to a network namespace that subsequently becomes
inactive but that socket is persisted by another process in another
network namespace (e.g., via SCM_RIGHTS of pidfd_getfd()) then the
SIOCGSKNS ioctl will resurrect this network namespace.

So calls to open_related_ns() and open_namespace() will end up
resurrecting the corresponding namespace tree.

Note that the active reference count does not regulate the lifetime of
the namespace itself. This is still done by the normal reference count.
The active reference count can only be elevated if the regular reference
count is elevated.

The active reference count also doesn't regulate the presence of a
namespace on the namespace trees. It only regulates its visiblity to
namespace file handles (and in later patches to listns()).

A namespace remains on the namespace trees from creation until its
actual destruction. This will allow the kernel to always reach any
namespace trivially and it will also enable subsystems like bpf to walk
the namespace lists on the system for tracing or general introspection
purposes.

Note that different namespaces have different visibility lifetimes on
current kernels. While most namespace are immediately released when the
last task using them exits, the user- and pid namespace are persisted
and thus both remain accessible via /proc/<pid>/ns/<ns_type>.

The user namespace lifetime is aliged with struct cred and is only
released through exit_creds(). However, it becomes inaccessible to
userspace once the last task using it is reaped, i.e., when
release_task() is called and all proc entries are flushed. Similarly,
the pid namespace is also visible until the last task using it has been
reaped and the associated pid numbers are freed.

The active reference counts of the user- and pid namespace are
decremented once the task is reaped.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c            |   2 +
 fs/nsfs.c                 |  48 +++++++++-
 include/linux/ns_common.h | 142 ++++++++++++++++++++++++++++-
 include/linux/nsfs.h      |   3 +
 include/linux/nsproxy.h   |   3 +
 init/version-timestamp.c  |   1 +
 ipc/msgutil.c             |   1 +
 ipc/namespace.c           |   1 +
 kernel/cgroup/cgroup.c    |   1 +
 kernel/cgroup/namespace.c |   1 +
 kernel/cred.c             |   6 ++
 kernel/exit.c             |   1 +
 kernel/fork.c             |   1 +
 kernel/nscommon.c         | 221 +++++++++++++++++++++++++++++++++++++++++++++-
 kernel/nsproxy.c          |  23 +++++
 kernel/pid.c              |   6 ++
 kernel/pid_namespace.c    |   1 +
 kernel/time/namespace.c   |   2 +
 kernel/user.c             |   1 +
 kernel/user_namespace.c   |   1 +
 kernel/utsname.c          |   1 +
 net/core/net_namespace.c  |   1 +
 22 files changed, 464 insertions(+), 4 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 8ef8ba3dd316..87116def5ee3 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4173,6 +4173,7 @@ struct mnt_namespace *copy_mnt_ns(u64 flags, struct mnt_namespace *ns,
 			p = next_mnt(skip_mnt_tree(p), old);
 	}
 	ns_tree_add_raw(new_ns);
+	ns_ref_active_get_owner(new_ns);
 	return new_ns;
 }
 
@@ -5989,6 +5990,7 @@ struct mnt_namespace init_mnt_ns = {
 	.ns.ops		= &mntns_operations,
 	.user_ns	= &init_user_ns,
 	.ns.__ns_ref	= REFCOUNT_INIT(1),
+	.ns.__ns_ref_active = ATOMIC_INIT(1),
 	.ns.ns_type	= ns_common_type(&init_mnt_ns),
 	.passive	= REFCOUNT_INIT(1),
 	.mounts		= RB_ROOT,
diff --git a/fs/nsfs.c b/fs/nsfs.c
index 363be226e357..8b9bebd11c49 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -58,6 +58,8 @@ const struct dentry_operations ns_dentry_operations = {
 static void nsfs_evict(struct inode *inode)
 {
 	struct ns_common *ns = inode->i_private;
+
+	__ns_ref_active_put(ns);
 	clear_inode(inode);
 	ns->ops->put(ns);
 }
@@ -419,6 +421,16 @@ static int nsfs_init_inode(struct inode *inode, void *data)
 	inode->i_mode |= S_IRUGO;
 	inode->i_fop = &ns_file_operations;
 	inode->i_ino = ns->inum;
+
+	/*
+	 * Bring the namespace subtree back to life if we have to. This
+	 * can happen when e.g., all processes using a network namespace
+	 * and all namespace files or namespace file bind-mounts have
+	 * died but there are still sockets pinning it. The SIOCGSKNS
+	 * ioctl on such a socket will resurrect the relevant namespace
+	 * subtree.
+	 */
+	__ns_ref_active_resurrect(ns);
 	return 0;
 }
 
@@ -493,7 +505,17 @@ static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
 		VFS_WARN_ON_ONCE(ns->ns_type != fid->ns_type);
 		VFS_WARN_ON_ONCE(ns->inum != fid->ns_inum);
 
-		if (!__ns_ref_get(ns))
+		/*
+		 * This is racy because we're not actually taking an
+		 * active reference. IOW, it could happen that the
+		 * namespace becomes inactive after this check.
+		 * We don't care because nsfs_init_inode() will just
+		 * resurrect the relevant namespace tree for us. If it
+		 * has been active here we just allow it's resurrection.
+		 * We could try to take an active reference here and
+		 * then drop it again. But really, why bother.
+		 */
+		if (!ns_get_unless_inactive(ns))
 			return NULL;
 	}
 
@@ -614,3 +636,27 @@ void __init nsfs_init(void)
 	nsfs_root_path.mnt = nsfs_mnt;
 	nsfs_root_path.dentry = nsfs_mnt->mnt_root;
 }
+
+void nsproxy_ns_active_get(struct nsproxy *ns)
+{
+	ns_ref_active_get(ns->mnt_ns);
+	ns_ref_active_get(ns->uts_ns);
+	ns_ref_active_get(ns->ipc_ns);
+	ns_ref_active_get(ns->pid_ns_for_children);
+	ns_ref_active_get(ns->cgroup_ns);
+	ns_ref_active_get(ns->net_ns);
+	ns_ref_active_get(ns->time_ns);
+	ns_ref_active_get(ns->time_ns_for_children);
+}
+
+void nsproxy_ns_active_put(struct nsproxy *ns)
+{
+	ns_ref_active_put(ns->mnt_ns);
+	ns_ref_active_put(ns->uts_ns);
+	ns_ref_active_put(ns->ipc_ns);
+	ns_ref_active_put(ns->pid_ns_for_children);
+	ns_ref_active_put(ns->cgroup_ns);
+	ns_ref_active_put(ns->net_ns);
+	ns_ref_active_put(ns->time_ns);
+	ns_ref_active_put(ns->time_ns_for_children);
+}
diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index 32114d5698dc..77ea38d8bac9 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -4,7 +4,9 @@
 
 #include <linux/refcount.h>
 #include <linux/rbtree.h>
+#include <linux/vfsdebug.h>
 #include <uapi/linux/sched.h>
+#include <uapi/linux/nsfs.h>
 
 struct proc_ns_operations;
 
@@ -37,6 +39,73 @@ extern const struct proc_ns_operations cgroupns_operations;
 extern const struct proc_ns_operations timens_operations;
 extern const struct proc_ns_operations timens_for_children_operations;
 
+/*
+ * Namespace lifetimes are managed via a two-tier reference counting model:
+ *
+ * (1) __ns_ref (refcount_t): Main reference count tracking memory lifetime.
+ *     Controls when the namespace structure itself is freed. It also
+ *     pins the namespace on the namespace trees whereas (2) only
+ *     regulates their visibility to userspace.
+ *
+ * (2) __ns_ref_active (atomic_t): Reference count tracking active users.
+ *     Controls visibility of the namespace in the namespace trees.
+ *     Any live task that uses the namespace (via nsproxy or cred) holds
+ *     an active reference. Any open file descriptor or bind-mount of
+ *     the namespace holds an active reference. Once all tasks have
+ *     called exit_task_namespaces() and all file descriptors and
+ *     bind-mounts have been released the active reference count drops
+ *     to zero and the namespace becomes inactive. IOW, the namespace
+ *     cannot be listed or opened via file handles anymore.
+ *
+ *     Note that it is valid to transition from active to inactive and
+ *     back from inactive to active e.g., when walking a hierarchical
+ *     namespace tree upwards and reopening parent namespaces via
+ *     NS_GET_PARENT that only exist because they are a parent of an
+ *     actively used namespace.
+ *
+ * Relationship and lifecycle states:
+ *
+ * - Active (__ns_ref_active > 0):
+ *   Namespace is actively used by one or more tasks. The namespace can
+ *   be reopened via /proc/<pid>/ns/<ns_type> or discovered in the
+ *   namespace tree.
+ *
+ * - Inactive (__ns_ref_active == 0, __ns_ref > 0):
+ *   No tasks are actively using the namespace and it isn't pinned by
+ *   any bind-mounts or open file descriptors anymore. But the namespace
+ *   is still kept alive by internal references. For example, the user
+ *   namespace could be pinned by an open file through file->f_cred
+ *   references when one of the now defunct tasks had opened a file and
+ *   handed the file descriptor off to another process via a UNIX
+ *   socket. Such references keep the namespace structure alive through
+ *   __ns_ref but will not take an active reference.
+ *
+ * - Destroyed (__ns_ref == 0):
+ *   No references remain. The namespace is removed from the tree and freed.
+ *
+ * State transitions:
+ *
+ * Active -> Inactive:
+ *   When the last task using the namespace exits (via
+ *   exit_task_namespaces()), it drops its active references to all
+ *   namespaces apart from the pid namespace which remains accessible
+ *   until the task has been reaped and its pid number is released.
+ *
+ * Inactive -> Active:
+ *   When walking a hierarchical namespace tree upwards and reopening
+ *   parent namespaces via NS_GET_PARENT that only exist because they
+ *   are a parent of an actively used namespace it is possible to
+ *   necrobump an inactive namespace back to the active state.
+ *
+ * Inactive -> Destroyed:
+ *   When __ns_ref drops to zero (last file descriptor closed, last bind
+ *   mount removed, parent namespace released), the namespace is removed
+ *   from the tree and the memory is freed (after RCU grace period).
+ *
+ * Initial namespaces:
+ *   Boot-time namespaces (init_net, init_pid_ns, etc.) start with
+ *   __ns_ref_active = 1 and remain active forever.
+ */
 struct ns_common {
 	u32 ns_type;
 	struct dentry *stashed;
@@ -48,6 +117,7 @@ struct ns_common {
 			u64 ns_id;
 			struct rb_node ns_tree_node;
 			struct list_head ns_list_node;
+			atomic_t __ns_ref_active; /* do not use directly */
 		};
 		struct rcu_head ns_rcu;
 	};
@@ -56,6 +126,13 @@ struct ns_common {
 int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_operations *ops, int inum);
 void __ns_common_free(struct ns_common *ns);
 
+static __always_inline bool is_initial_namespace(struct ns_common *ns)
+{
+	VFS_WARN_ON_ONCE(ns->inum == 0);
+	return unlikely(in_range(ns->inum, MNT_NS_INIT_INO,
+				 IPC_NS_INIT_INO - MNT_NS_INIT_INO + 1));
+}
+
 #define to_ns_common(__ns)                                    \
 	_Generic((__ns),                                      \
 		struct cgroup_namespace *:       &(__ns)->ns, \
@@ -133,14 +210,26 @@ void __ns_common_free(struct ns_common *ns);
 
 #define ns_common_free(__ns) __ns_common_free(to_ns_common((__ns)))
 
+static __always_inline __must_check int __ns_ref_active_read(const struct ns_common *ns)
+{
+	return atomic_read(&ns->__ns_ref_active);
+}
+
 static __always_inline __must_check bool __ns_ref_put(struct ns_common *ns)
 {
-	return refcount_dec_and_test(&ns->__ns_ref);
+	if (refcount_dec_and_test(&ns->__ns_ref)) {
+		VFS_WARN_ON_ONCE(__ns_ref_active_read(ns));
+		return true;
+	}
+	return false;
 }
 
 static __always_inline __must_check bool __ns_ref_get(struct ns_common *ns)
 {
-	return refcount_inc_not_zero(&ns->__ns_ref);
+	if (refcount_inc_not_zero(&ns->__ns_ref))
+		return true;
+	VFS_WARN_ON_ONCE(__ns_ref_active_read(ns));
+	return false;
 }
 
 static __always_inline __must_check int __ns_ref_read(const struct ns_common *ns)
@@ -155,4 +244,53 @@ static __always_inline __must_check int __ns_ref_read(const struct ns_common *ns
 #define ns_ref_put_and_lock(__ns, __lock) \
 	refcount_dec_and_lock(&to_ns_common((__ns))->__ns_ref, (__lock))
 
+#define ns_ref_active_read(__ns) \
+	((__ns) ? __ns_ref_active_read(to_ns_common(__ns)) : 0)
+
+void __ns_ref_active_get_owner(struct ns_common *ns);
+
+static __always_inline void __ns_ref_active_get(struct ns_common *ns)
+{
+	WARN_ON_ONCE(atomic_add_negative(1, &ns->__ns_ref_active));
+	VFS_WARN_ON_ONCE(is_initial_namespace(ns) && __ns_ref_active_read(ns) <= 0);
+}
+#define ns_ref_active_get(__ns) \
+	do { if (__ns) __ns_ref_active_get(to_ns_common(__ns)); } while (0)
+
+static __always_inline bool __ns_ref_active_get_not_zero(struct ns_common *ns)
+{
+	if (atomic_inc_not_zero(&ns->__ns_ref_active)) {
+		VFS_WARN_ON_ONCE(!__ns_ref_read(ns));
+		return true;
+	}
+	return false;
+}
+
+#define ns_ref_active_get_owner(__ns) \
+	do { if (__ns) __ns_ref_active_get_owner(to_ns_common(__ns)); } while (0)
+
+void __ns_ref_active_put_owner(struct ns_common *ns);
+
+static __always_inline void __ns_ref_active_put(struct ns_common *ns)
+{
+	if (atomic_dec_and_test(&ns->__ns_ref_active)) {
+		VFS_WARN_ON_ONCE(is_initial_namespace(ns));
+		VFS_WARN_ON_ONCE(!__ns_ref_read(ns));
+		__ns_ref_active_put_owner(ns);
+	}
+}
+#define ns_ref_active_put(__ns) \
+	do { if (__ns) __ns_ref_active_put(to_ns_common(__ns)); } while (0)
+
+static __always_inline bool ns_get_unless_inactive(struct ns_common *ns)
+{
+	VFS_WARN_ON_ONCE(__ns_ref_active_read(ns) && !__ns_ref_read(ns));
+	return __ns_ref_active_read(ns) && __ns_ref_get(ns);
+}
+
+void __ns_ref_active_resurrect(struct ns_common *ns);
+
+#define ns_ref_active_resurrect(__ns) \
+	do { if (__ns) __ns_ref_active_resurrect(to_ns_common(__ns)); } while (0)
+
 #endif
diff --git a/include/linux/nsfs.h b/include/linux/nsfs.h
index e5a5fa83d36b..731b67fc2fec 100644
--- a/include/linux/nsfs.h
+++ b/include/linux/nsfs.h
@@ -37,4 +37,7 @@ void nsfs_init(void);
 
 #define current_in_namespace(__ns) (__current_namespace_from_type(__ns) == __ns)
 
+void nsproxy_ns_active_get(struct nsproxy *ns);
+void nsproxy_ns_active_put(struct nsproxy *ns);
+
 #endif /* _LINUX_NSFS_H */
diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
index 538ba8dba184..ac825eddec59 100644
--- a/include/linux/nsproxy.h
+++ b/include/linux/nsproxy.h
@@ -93,7 +93,10 @@ static inline struct cred *nsset_cred(struct nsset *set)
  */
 
 int copy_namespaces(u64 flags, struct task_struct *tsk);
+void switch_cred_namespaces(const struct cred *old, const struct cred *new);
 void exit_nsproxy_namespaces(struct task_struct *tsk);
+void get_cred_namespaces(struct task_struct *tsk);
+void exit_cred_namespaces(struct task_struct *tsk);
 void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new);
 int exec_task_namespaces(void);
 void free_nsproxy(struct nsproxy *ns);
diff --git a/init/version-timestamp.c b/init/version-timestamp.c
index 61b2405d97f9..c38498f94646 100644
--- a/init/version-timestamp.c
+++ b/init/version-timestamp.c
@@ -10,6 +10,7 @@
 struct uts_namespace init_uts_ns = {
 	.ns.ns_type = ns_common_type(&init_uts_ns),
 	.ns.__ns_ref = REFCOUNT_INIT(2),
+	.ns.__ns_ref_active = ATOMIC_INIT(1),
 	.name = {
 		.sysname	= UTS_SYSNAME,
 		.nodename	= UTS_NODENAME,
diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index c9469fbce27c..d7c66b430470 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -28,6 +28,7 @@ DEFINE_SPINLOCK(mq_lock);
  */
 struct ipc_namespace init_ipc_ns = {
 	.ns.__ns_ref = REFCOUNT_INIT(1),
+	.ns.__ns_ref_active = ATOMIC_INIT(1),
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_ipc_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_ipc_ns.ns.ns_list_node),
diff --git a/ipc/namespace.c b/ipc/namespace.c
index 59b12fcb40bd..23390d4f9b1f 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -87,6 +87,7 @@ static struct ipc_namespace *create_ipc_ns(struct user_namespace *user_ns,
 	sem_init_ns(ns);
 	shm_init_ns(ns);
 	ns_tree_add(ns);
+	ns_ref_active_get_owner(ns);
 
 	return ns;
 
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index ce4d227a9ca2..45e470011c77 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -251,6 +251,7 @@ bool cgroup_enable_per_threadgroup_rwsem __read_mostly;
 /* cgroup namespace for init task */
 struct cgroup_namespace init_cgroup_ns = {
 	.ns.__ns_ref	= REFCOUNT_INIT(2),
+	.ns.__ns_ref_active = ATOMIC_INIT(1),
 	.user_ns	= &init_user_ns,
 	.ns.ops		= &cgroupns_operations,
 	.ns.inum	= ns_init_inum(&init_cgroup_ns),
diff --git a/kernel/cgroup/namespace.c b/kernel/cgroup/namespace.c
index db9617556dd7..1aeb4303a3d8 100644
--- a/kernel/cgroup/namespace.c
+++ b/kernel/cgroup/namespace.c
@@ -86,6 +86,7 @@ struct cgroup_namespace *copy_cgroup_ns(u64 flags,
 	new_ns->root_cset = cset;
 
 	ns_tree_add(new_ns);
+	ns_ref_active_get_owner(new_ns);
 	return new_ns;
 }
 
diff --git a/kernel/cred.c b/kernel/cred.c
index dbf6b687dc5c..a6e7f580df14 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -306,6 +306,7 @@ int copy_creds(struct task_struct *p, u64 clone_flags)
 		kdebug("share_creds(%p{%ld})",
 		       p->cred, atomic_long_read(&p->cred->usage));
 		inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
+		get_cred_namespaces(p);
 		return 0;
 	}
 
@@ -343,6 +344,8 @@ int copy_creds(struct task_struct *p, u64 clone_flags)
 
 	p->cred = p->real_cred = get_cred(new);
 	inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
+	get_cred_namespaces(p);
+
 	return 0;
 
 error_put:
@@ -435,10 +438,13 @@ int commit_creds(struct cred *new)
 	 */
 	if (new->user != old->user || new->user_ns != old->user_ns)
 		inc_rlimit_ucounts(new->ucounts, UCOUNT_RLIMIT_NPROC, 1);
+
 	rcu_assign_pointer(task->real_cred, new);
 	rcu_assign_pointer(task->cred, new);
 	if (new->user != old->user || new->user_ns != old->user_ns)
 		dec_rlimit_ucounts(old->ucounts, UCOUNT_RLIMIT_NPROC, 1);
+	if (new->user_ns != old->user_ns)
+		switch_cred_namespaces(old, new);
 
 	/* send notifications */
 	if (!uid_eq(new->uid,   old->uid)  ||
diff --git a/kernel/exit.c b/kernel/exit.c
index 825998103520..988e16efd66b 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -291,6 +291,7 @@ void release_task(struct task_struct *p)
 	write_unlock_irq(&tasklist_lock);
 	/* @thread_pid can't go away until free_pids() below */
 	proc_flush_pid(thread_pid);
+	exit_cred_namespaces(p);
 	add_device_randomness(&p->se.sum_exec_runtime,
 			      sizeof(p->se.sum_exec_runtime));
 	free_pids(post.pids);
diff --git a/kernel/fork.c b/kernel/fork.c
index 0926bfe4b8df..f1857672426e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2487,6 +2487,7 @@ __latent_entropy struct task_struct *copy_process(
 	delayacct_tsk_free(p);
 bad_fork_cleanup_count:
 	dec_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
+	exit_cred_namespaces(p);
 	exit_creds(p);
 bad_fork_free:
 	WRITE_ONCE(p->__state, TASK_DEAD);
diff --git a/kernel/nscommon.c b/kernel/nscommon.c
index c1fb2bad6d72..d750e554d50b 100644
--- a/kernel/nscommon.c
+++ b/kernel/nscommon.c
@@ -2,6 +2,7 @@
 
 #include <linux/ns_common.h>
 #include <linux/proc_ns.h>
+#include <linux/user_namespace.h>
 #include <linux/vfsdebug.h>
 
 #ifdef CONFIG_DEBUG_VFS
@@ -52,6 +53,8 @@ static void ns_debug(struct ns_common *ns, const struct proc_ns_operations *ops)
 
 int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_operations *ops, int inum)
 {
+	int ret;
+
 	refcount_set(&ns->__ns_ref, 1);
 	ns->stashed = NULL;
 	ns->ops = ops;
@@ -68,10 +71,226 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
 		ns->inum = inum;
 		return 0;
 	}
-	return proc_alloc_inum(&ns->inum);
+	ret = proc_alloc_inum(&ns->inum);
+	if (ret)
+		return ret;
+	/*
+	 * Tree ref starts at 0. It's incremented when namespace enters
+	 * active use (installed in nsproxy) and decremented when all
+	 * active uses are gone. Initial namespaces are always active.
+	 */
+	if (is_initial_namespace(ns))
+		atomic_set(&ns->__ns_ref_active, 1);
+	else
+		atomic_set(&ns->__ns_ref_active, 0);
+	return 0;
 }
 
 void __ns_common_free(struct ns_common *ns)
 {
 	proc_free_inum(ns->inum);
 }
+
+static struct ns_common *ns_owner(struct ns_common *ns)
+{
+	struct user_namespace *owner;
+
+	if (unlikely(!ns->ops))
+		return NULL;
+	VFS_WARN_ON_ONCE(!ns->ops->owner);
+	owner = ns->ops->owner(ns);
+	VFS_WARN_ON_ONCE(!owner && ns != to_ns_common(&init_user_ns));
+	if (!owner)
+		return NULL;
+	/* Skip init_user_ns as it's always active */
+	if (owner == &init_user_ns)
+		return NULL;
+	return to_ns_common(owner);
+}
+
+void __ns_ref_active_get_owner(struct ns_common *ns)
+{
+	ns = ns_owner(ns);
+	if (ns)
+		WARN_ON_ONCE(atomic_add_negative(1, &ns->__ns_ref_active));
+}
+
+/*
+ * The active reference count works by having each namespace that gets
+ * created take a single active reference on its owning user namespace.
+ * That single reference is only released once the child namespace's
+ * active count itself goes down. This makes it possible to efficiently
+ * resurrect a namespace tree:
+ *
+ * On namespace creation each new namespace will take a single reference
+ * count on its owning user namespace.
+ *
+ * A regular namespace tree might look as follow:
+ * Legend:
+ * + : adding active reference
+ * - : dropping active reference
+ * x : always active (initial namespace)
+ *
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        +      +
+ *                        user_ns1 (2)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   +   +
+ *                        user_ns2 (3)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * If both net_ns and pid_ns put their last active reference on
+ * themselves it will cascade to user_ns1 dropping its own active
+ * reference and dropping one active reference on user_ns2:
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        -      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   -   +
+ *                        user_ns2 (2)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * The iteration stops once we reach a namespace that still has active
+ * references.
+ */
+void __ns_ref_active_put_owner(struct ns_common *ns)
+{
+	for (;;) {
+		ns = ns_owner(ns);
+		if (!ns)
+			return;
+		if (!atomic_dec_and_test(&ns->__ns_ref_active))
+			return;
+	}
+}
+
+/*
+ * The active reference count works by having each namespace that gets
+ * created take a single active reference on its owning user namespace.
+ * That single reference is only released once the child namespace's
+ * active count itself goes down. This makes it possible to efficiently
+ * resurrect a namespace tree:
+ *
+ * On namespace creation each new namespace will take a single reference
+ * count on its owning user namespace.
+ *
+ * A regular namespace tree might look as follow:
+ * Legend:
+ * + : adding active reference
+ * - : dropping active reference
+ * x : always active (initial namespace)
+ *
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        +      +
+ *                        user_ns1 (2)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   +   +
+ *                        user_ns2 (3)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * If both net_ns and pid_ns put their last active reference on
+ * themselves it will cascade to user_ns1 dropping its own active
+ * reference and dropping one active reference on user_ns2:
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        -      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   -   +
+ *                        user_ns2 (2)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * Assume the whole tree is dead but all namespaces are still active:
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        -      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        -   -   -
+ *                        user_ns2 (0)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * Now assume the net_ns gets resurrected (.e.g., via the SIOCGSKNS ioctl()):
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        +      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        -   +   -
+ *                        user_ns2 (0)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * If net_ns had a zero reference count and we bumped it we also need to
+ * take another reference on its owning user namespace. Similarly, if
+ * pid_ns had a zero reference count it also needs to take another
+ * reference on its owning user namespace. So both net_ns and pid_ns
+ * will each have their own reference on the owning user namespace.
+ *
+ * If the owning user namespace user_ns1 had a zero reference count then
+ * it also needs to take another reference on its owning user namespace
+ * and so on.
+ */
+void __ns_ref_active_resurrect(struct ns_common *ns)
+{
+	/* If we didn't resurrect the namespace we're done. */
+	if (atomic_fetch_add(1, &ns->__ns_ref_active))
+		return;
+
+	/*
+	 * We did resurrect it. Walk the ownership hierarchy upwards
+	 * until we found an owning user namespace that is active.
+	 */
+	for (;;) {
+		ns = ns_owner(ns);
+		if (!ns)
+			return;
+
+		if (atomic_fetch_add(1, &ns->__ns_ref_active))
+			return;
+	}
+}
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index 6ce76a0278ab..94c2cfe0afa1 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -26,6 +26,7 @@
 #include <linux/syscalls.h>
 #include <linux/cgroup.h>
 #include <linux/perf_event.h>
+#include <linux/nstree.h>
 
 static struct kmem_cache *nsproxy_cachep;
 
@@ -179,12 +180,15 @@ int copy_namespaces(u64 flags, struct task_struct *tsk)
 	if ((flags & CLONE_VM) == 0)
 		timens_on_fork(new_ns, tsk);
 
+	nsproxy_ns_active_get(new_ns);
 	tsk->nsproxy = new_ns;
 	return 0;
 }
 
 void free_nsproxy(struct nsproxy *ns)
 {
+	nsproxy_ns_active_put(ns);
+
 	put_mnt_ns(ns->mnt_ns);
 	put_uts_ns(ns->uts_ns);
 	put_ipc_ns(ns->ipc_ns);
@@ -232,6 +236,9 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
 
 	might_sleep();
 
+	if (new)
+		nsproxy_ns_active_get(new);
+
 	task_lock(p);
 	ns = p->nsproxy;
 	p->nsproxy = new;
@@ -246,6 +253,22 @@ void exit_nsproxy_namespaces(struct task_struct *p)
 	switch_task_namespaces(p, NULL);
 }
 
+void switch_cred_namespaces(const struct cred *old, const struct cred *new)
+{
+	ns_ref_active_get(new->user_ns);
+	ns_ref_active_put(old->user_ns);
+}
+
+void get_cred_namespaces(struct task_struct *tsk)
+{
+	ns_ref_active_get(tsk->real_cred->user_ns);
+}
+
+void exit_cred_namespaces(struct task_struct *tsk)
+{
+	ns_ref_active_put(tsk->real_cred->user_ns);
+}
+
 int exec_task_namespaces(void)
 {
 	struct task_struct *tsk = current;
diff --git a/kernel/pid.c b/kernel/pid.c
index cb7574ca00f7..ec9051d387ee 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -72,6 +72,7 @@ static int pid_max_max = PID_MAX_LIMIT;
  */
 struct pid_namespace init_pid_ns = {
 	.ns.__ns_ref = REFCOUNT_INIT(2),
+	.ns.__ns_ref_active = ATOMIC_INIT(1),
 	.idr = IDR_INIT(init_pid_ns.idr),
 	.pid_allocated = PIDNS_ADDING,
 	.level = 0,
@@ -118,9 +119,13 @@ static void delayed_put_pid(struct rcu_head *rhp)
 void free_pid(struct pid *pid)
 {
 	int i;
+	struct pid_namespace *active_ns;
 
 	lockdep_assert_not_held(&tasklist_lock);
 
+	active_ns = pid->numbers[pid->level].ns;
+	ns_ref_active_put(active_ns);
+
 	spin_lock(&pidmap_lock);
 	for (i = 0; i <= pid->level; i++) {
 		struct upid *upid = pid->numbers + i;
@@ -284,6 +289,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 	}
 	spin_unlock(&pidmap_lock);
 	idr_preload_end();
+	ns_ref_active_get(ns);
 
 	return pid;
 
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 650be58d8d18..2e678338c6d1 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -124,6 +124,7 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 #endif
 
 	ns_tree_add(ns);
+	ns_ref_active_get_owner(ns);
 	return ns;
 
 out_free_inum:
diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
index ee05cad288da..2e7c110bd13f 100644
--- a/kernel/time/namespace.c
+++ b/kernel/time/namespace.c
@@ -106,6 +106,7 @@ static struct time_namespace *clone_time_ns(struct user_namespace *user_ns,
 	ns->offsets = old_ns->offsets;
 	ns->frozen_offsets = false;
 	ns_tree_add(ns);
+	ns_ref_active_get_owner(ns);
 	return ns;
 
 fail_free_page:
@@ -480,6 +481,7 @@ const struct proc_ns_operations timens_for_children_operations = {
 struct time_namespace init_time_ns = {
 	.ns.ns_type	= ns_common_type(&init_time_ns),
 	.ns.__ns_ref	= REFCOUNT_INIT(3),
+	.ns.__ns_ref_active = ATOMIC_INIT(1),
 	.user_ns	= &init_user_ns,
 	.ns.inum	= ns_init_inum(&init_time_ns),
 	.ns.ops		= &timens_operations,
diff --git a/kernel/user.c b/kernel/user.c
index b9cf3b056a71..bf60532856db 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -67,6 +67,7 @@ struct user_namespace init_user_ns = {
 	},
 	.ns.ns_type = ns_common_type(&init_user_ns),
 	.ns.__ns_ref = REFCOUNT_INIT(3),
+	.ns.__ns_ref_active = ATOMIC_INIT(1),
 	.owner = GLOBAL_ROOT_UID,
 	.group = GLOBAL_ROOT_GID,
 	.ns.inum = ns_init_inum(&init_user_ns),
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 03cb63883d04..9da3ee9e2eb1 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -160,6 +160,7 @@ int create_user_ns(struct cred *new)
 
 	set_cred_user_ns(new, ns);
 	ns_tree_add(ns);
+	ns_ref_active_get_owner(ns);
 	return 0;
 fail_keyring:
 #ifdef CONFIG_PERSISTENT_KEYRINGS
diff --git a/kernel/utsname.c b/kernel/utsname.c
index ebbfc578a9d3..18f55a05ad5b 100644
--- a/kernel/utsname.c
+++ b/kernel/utsname.c
@@ -60,6 +60,7 @@ static struct uts_namespace *clone_uts_ns(struct user_namespace *user_ns,
 	ns->user_ns = get_user_ns(user_ns);
 	up_read(&uts_sem);
 	ns_tree_add(ns);
+	ns_ref_active_get_owner(ns);
 	return ns;
 
 fail_free:
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index b0e0f22d7b21..f30fb78f020c 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -450,6 +450,7 @@ static __net_init int setup_net(struct net *net)
 	list_add_tail_rcu(&net->list, &net_namespace_list);
 	up_write(&net_rwsem);
 	ns_tree_add_raw(net);
+	ns_ref_active_get_owner(net);
 out:
 	return error;
 

-- 
2.47.3

Re: [PATCH v3 11/70] ns: add active reference count

[Thomas Gleixner]

On Fri, Oct 24 2025 at 12:52, Christian Brauner wrote:
> diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
> index ee05cad288da..2e7c110bd13f 100644
> --- a/kernel/time/namespace.c
> +++ b/kernel/time/namespace.c
> @@ -106,6 +106,7 @@ static struct time_namespace *clone_time_ns(struct user_namespace *user_ns,
>  	ns->offsets = old_ns->offsets;
>  	ns->frozen_offsets = false;
>  	ns_tree_add(ns);
> +	ns_ref_active_get_owner(ns);

It seems all places where ns_ref_active_get_owner() is added it is
preceeded by a variant of ns_tree_add(). So why don't you stilck that
refcount thing into ns_tree_add()? I'm probably missing something here.

Thanks,

        tglx

Re: [PATCH v3 11/70] ns: add active reference count

[Christian Brauner]

On Mon, Oct 27, 2025 at 05:36:27PM +0100, Thomas Gleixner wrote:
> On Fri, Oct 24 2025 at 12:52, Christian Brauner wrote:
> > diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
> > index ee05cad288da..2e7c110bd13f 100644
> > --- a/kernel/time/namespace.c
> > +++ b/kernel/time/namespace.c
> > @@ -106,6 +106,7 @@ static struct time_namespace *clone_time_ns(struct user_namespace *user_ns,
> >  	ns->offsets = old_ns->offsets;
> >  	ns->frozen_offsets = false;
> >  	ns_tree_add(ns);
> > +	ns_ref_active_get_owner(ns);
> 
> It seems all places where ns_ref_active_get_owner() is added it is
> preceeded by a variant of ns_tree_add(). So why don't you stilck that
> refcount thing into ns_tree_add()? I'm probably missing something here.

Yes, as usual a very astute observation. I've taken your advice and
folded the parent count into __ns_tree_add_raw(). Thanks!

Re: [PATCH v3 11/70] ns: add active reference count

[Simon Horman]

On Fri, Oct 24, 2025 at 12:52:40PM +0200, Christian Brauner wrote:

...

> diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c

...

> +void get_cred_namespaces(struct task_struct *tsk)
> +{
> +	ns_ref_active_get(tsk->real_cred->user_ns);

Hi Christian,

real_cred is protected by RCU, but this code doesn't seem to take
that into account. Or, at least Sparse doesn't think so:

.../nsproxy.c:264:9: error: no generic selection for 'struct user_namespace *const [noderef] __rcu user_ns'
.../nsproxy.c:264:9: warning: dereference of noderef expression

> +}
> +
> +void exit_cred_namespaces(struct task_struct *tsk)
> +{
> +	ns_ref_active_put(tsk->real_cred->user_ns);

Likewise here.

> +}
> +
>  int exec_task_namespaces(void)
>  {
>  	struct task_struct *tsk = current;

...

Re: [PATCH v3 11/70] ns: add active reference count

[Christian Brauner]

On Tue, Oct 28, 2025 at 10:30:06AM +0000, Simon Horman wrote:
> On Fri, Oct 24, 2025 at 12:52:40PM +0200, Christian Brauner wrote:
> 
> ...
> 
> > diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
> 
> ...
> 
> > +void get_cred_namespaces(struct task_struct *tsk)
> > +{
> > +	ns_ref_active_get(tsk->real_cred->user_ns);
> 
> Hi Christian,
> 
> real_cred is protected by RCU, but this code doesn't seem to take
> that into account. Or, at least Sparse doesn't think so:
> 
> .../nsproxy.c:264:9: error: no generic selection for 'struct user_namespace *const [noderef] __rcu user_ns'
> .../nsproxy.c:264:9: warning: dereference of noderef expression
> 
> > +}
> > +
> > +void exit_cred_namespaces(struct task_struct *tsk)
> > +{
> > +	ns_ref_active_put(tsk->real_cred->user_ns);
> 
> Likewise here.

get_cred_namespaces() is called during copy_creds() which is called
during process creation aka from copy_process(). So copy_creds() always
takes the creds of current (the parent process in this case) which can't
change in any way.

Simplifying a bit:
Either we created a thread via CLONE_THREAD in which case we can't
specify CLONE_NEWUSER (little know fact, I guess) and so we just bump
the reference count on the existing user namespace from the parent's
creds, or we're creating a new set of credentials that no one has ever
seen before possibly even a new user namespace if CLONE_NEWUSER has been
specified.

In both case the credentials are completely stable. The call to
exit_cred_namespaces() has similar reasoning when called from the
cleanup/failure path of copy_process().

The other callsite is release_task() which is called - simplifying -
after the task has been reaped. That thing is deader than dead and
nothing can mess with its creds anymore.

In other words, the get/put patterns for namespace management generally
happens at edges where the relevant structures are stable and can't be
changed by anyone other than the calling thread. And at no point are we
putting references on creds themselves.

Let me know if I missed something obvious.

Re: [PATCH v3 11/70] ns: add active reference count

[Askar Safin]

Christian Brauner <brauner@kernel.org>:
> Currently namespace file handles allow much broader access to namespaces
> than what is currently possible via (1)-(4). The reason is that

There is no any (4) here.


> On current kernels a namespace is visible to userspace in the
> following cases:
[...]
> (3) The namespace is a hierarchical namespace type and is the parent of
>     a single or multiple child namespaces.
[...]
> To handle this nicely we introduce an active reference count which
> tracks (1)-(3). This is easy to do as all of these things are already
[...]
> + * Inactive -> Active:
> + *   When walking a hierarchical namespace tree upwards and reopening
> + *   parent namespaces via NS_GET_PARENT that only exist because they
> + *   are a parent of an actively used namespace it is possible to
> + *   necrobump an inactive namespace back to the active state.

These quoted parts contradict to each other. You say "we introduce an
active reference count which tracks (1)-(3)", and (3) says "The namespace
is a hierarchical namespace type and is the parent of a single or multiple
child namespaces". I. e. active reference will count such parents. But then
in code you say:

> + * Inactive -> Active:
> + *   When walking a hierarchical namespace tree upwards and reopening
> + *   parent namespaces via NS_GET_PARENT that only exist because they
> + *   are a parent of an actively used namespace it is possible to
> + *   necrobump an inactive namespace back to the active state.

I. e. now you say that such parents are inactive and can become active.




-- 
Askar Safin

[PATCH v3 12/70] ns: use anonymous struct to group list member

[Christian Brauner]

Make it easier to spot that they belong together conceptually.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/linux/ns_common.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index 77ea38d8bac9..f340a279acc2 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -115,8 +115,10 @@ struct ns_common {
 	union {
 		struct {
 			u64 ns_id;
-			struct rb_node ns_tree_node;
-			struct list_head ns_list_node;
+			struct /* per type rbtree and list */ {
+				struct rb_node ns_tree_node;
+				struct list_head ns_list_node;
+			};
 			atomic_t __ns_ref_active; /* do not use directly */
 		};
 		struct rcu_head ns_rcu;

-- 
2.47.3

[PATCH v3 13/70] nstree: introduce a unified tree

[Christian Brauner]

This will allow userspace to lookup and stat a namespace simply by its
identifier without having to know what type of namespace it is.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/linux/ns_common.h |  4 ++
 kernel/nscommon.c         |  1 +
 kernel/nstree.c           | 94 ++++++++++++++++++++++++++++++++++++-----------
 3 files changed, 77 insertions(+), 22 deletions(-)

diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index f340a279acc2..b7d2123a9d19 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -115,6 +115,10 @@ struct ns_common {
 	union {
 		struct {
 			u64 ns_id;
+			struct /* global namespace rbtree and list */ {
+				struct rb_node ns_unified_tree_node;
+				struct list_head ns_unified_list_node;
+			};
 			struct /* per type rbtree and list */ {
 				struct rb_node ns_tree_node;
 				struct list_head ns_list_node;
diff --git a/kernel/nscommon.c b/kernel/nscommon.c
index d750e554d50b..bdd32e14a587 100644
--- a/kernel/nscommon.c
+++ b/kernel/nscommon.c
@@ -61,6 +61,7 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
 	ns->ns_id = 0;
 	ns->ns_type = ns_type;
 	RB_CLEAR_NODE(&ns->ns_tree_node);
+	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
 	INIT_LIST_HEAD(&ns->ns_list_node);
 
 #ifdef CONFIG_DEBUG_VFS
diff --git a/kernel/nstree.c b/kernel/nstree.c
index 369fd1675c6a..d21df06b6747 100644
--- a/kernel/nstree.c
+++ b/kernel/nstree.c
@@ -4,31 +4,30 @@
 #include <linux/proc_ns.h>
 #include <linux/vfsdebug.h>
 
+__cacheline_aligned_in_smp DEFINE_SEQLOCK(ns_tree_lock);
+static struct rb_root ns_unified_tree = RB_ROOT; /* protected by ns_tree_lock */
+
 /**
  * struct ns_tree - Namespace tree
  * @ns_tree: Rbtree of namespaces of a particular type
  * @ns_list: Sequentially walkable list of all namespaces of this type
- * @ns_tree_lock: Seqlock to protect the tree and list
  * @type: type of namespaces in this tree
  */
 struct ns_tree {
-       struct rb_root ns_tree;
-       struct list_head ns_list;
-       seqlock_t ns_tree_lock;
-       int type;
+	struct rb_root ns_tree;
+	struct list_head ns_list;
+	int type;
 };
 
 struct ns_tree mnt_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(mnt_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(mnt_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWNS,
 };
 
 struct ns_tree net_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(net_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(net_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWNET,
 };
 EXPORT_SYMBOL_GPL(net_ns_tree);
@@ -36,42 +35,36 @@ EXPORT_SYMBOL_GPL(net_ns_tree);
 struct ns_tree uts_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(uts_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(uts_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWUTS,
 };
 
 struct ns_tree user_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(user_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(user_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWUSER,
 };
 
 struct ns_tree ipc_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(ipc_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(ipc_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWIPC,
 };
 
 struct ns_tree pid_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(pid_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(pid_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWPID,
 };
 
 struct ns_tree cgroup_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(cgroup_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(cgroup_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWCGROUP,
 };
 
 struct ns_tree time_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(time_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(time_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWTIME,
 };
 
@@ -84,6 +77,13 @@ static inline struct ns_common *node_to_ns(const struct rb_node *node)
 	return rb_entry(node, struct ns_common, ns_tree_node);
 }
 
+static inline struct ns_common *node_to_ns_unified(const struct rb_node *node)
+{
+	if (!node)
+		return NULL;
+	return rb_entry(node, struct ns_common, ns_unified_tree_node);
+}
+
 static inline int ns_cmp(struct rb_node *a, const struct rb_node *b)
 {
 	struct ns_common *ns_a = node_to_ns(a);
@@ -98,13 +98,27 @@ static inline int ns_cmp(struct rb_node *a, const struct rb_node *b)
 	return 0;
 }
 
+static inline int ns_cmp_unified(struct rb_node *a, const struct rb_node *b)
+{
+	struct ns_common *ns_a = node_to_ns_unified(a);
+	struct ns_common *ns_b = node_to_ns_unified(b);
+	u64 ns_id_a = ns_a->ns_id;
+	u64 ns_id_b = ns_b->ns_id;
+
+	if (ns_id_a < ns_id_b)
+		return -1;
+	if (ns_id_a > ns_id_b)
+		return 1;
+	return 0;
+}
+
 void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 {
 	struct rb_node *node, *prev;
 
 	VFS_WARN_ON_ONCE(!ns->ns_id);
 
-	write_seqlock(&ns_tree->ns_tree_lock);
+	write_seqlock(&ns_tree_lock);
 
 	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
 
@@ -119,7 +133,8 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 	else
 		list_add_rcu(&ns->ns_list_node, &node_to_ns(prev)->ns_list_node);
 
-	write_sequnlock(&ns_tree->ns_tree_lock);
+	rb_find_add_rcu(&ns->ns_unified_tree_node, &ns_unified_tree, ns_cmp_unified);
+	write_sequnlock(&ns_tree_lock);
 
 	VFS_WARN_ON_ONCE(node);
 }
@@ -130,11 +145,12 @@ void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree)
 	VFS_WARN_ON_ONCE(list_empty(&ns->ns_list_node));
 	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
 
-	write_seqlock(&ns_tree->ns_tree_lock);
+	write_seqlock(&ns_tree_lock);
 	rb_erase(&ns->ns_tree_node, &ns_tree->ns_tree);
+	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
 	list_bidir_del_rcu(&ns->ns_list_node);
 	RB_CLEAR_NODE(&ns->ns_tree_node);
-	write_sequnlock(&ns_tree->ns_tree_lock);
+	write_sequnlock(&ns_tree_lock);
 }
 EXPORT_SYMBOL_GPL(__ns_tree_remove);
 
@@ -150,6 +166,17 @@ static int ns_find(const void *key, const struct rb_node *node)
 	return 0;
 }
 
+static int ns_find_unified(const void *key, const struct rb_node *node)
+{
+	const u64 ns_id = *(u64 *)key;
+	const struct ns_common *ns = node_to_ns_unified(node);
+
+	if (ns_id < ns->ns_id)
+		return -1;
+	if (ns_id > ns->ns_id)
+		return 1;
+	return 0;
+}
 
 static struct ns_tree *ns_tree_from_type(int ns_type)
 {
@@ -175,28 +202,51 @@ static struct ns_tree *ns_tree_from_type(int ns_type)
 	return NULL;
 }
 
-struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type)
+static struct ns_common *__ns_unified_tree_lookup_rcu(u64 ns_id)
 {
-	struct ns_tree *ns_tree;
 	struct rb_node *node;
 	unsigned int seq;
 
-	RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "suspicious ns_tree_lookup_rcu() usage");
+	do {
+		seq = read_seqbegin(&ns_tree_lock);
+		node = rb_find_rcu(&ns_id, &ns_unified_tree, ns_find_unified);
+		if (node)
+			break;
+	} while (read_seqretry(&ns_tree_lock, seq));
+
+	return node_to_ns_unified(node);
+}
+
+static struct ns_common *__ns_tree_lookup_rcu(u64 ns_id, int ns_type)
+{
+	struct ns_tree *ns_tree;
+	struct rb_node *node;
+	unsigned int seq;
 
 	ns_tree = ns_tree_from_type(ns_type);
 	if (!ns_tree)
 		return NULL;
 
 	do {
-		seq = read_seqbegin(&ns_tree->ns_tree_lock);
+		seq = read_seqbegin(&ns_tree_lock);
 		node = rb_find_rcu(&ns_id, &ns_tree->ns_tree, ns_find);
 		if (node)
 			break;
-	} while (read_seqretry(&ns_tree->ns_tree_lock, seq));
+	} while (read_seqretry(&ns_tree_lock, seq));
 
 	return node_to_ns(node);
 }
 
+struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type)
+{
+	RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "suspicious ns_tree_lookup_rcu() usage");
+
+	if (ns_type)
+		return __ns_tree_lookup_rcu(ns_id, ns_type);
+
+	return __ns_unified_tree_lookup_rcu(ns_id);
+}
+
 /**
  * ns_tree_adjoined_rcu - find the next/previous namespace in the same
  * tree

-- 
2.47.3

[PATCH v3 14/70] nstree: allow lookup solely based on inode

[Christian Brauner]

The namespace file handle struct nsfs_file_handle is uapi and userspace
is expressly allowed to generate file handles without going through
name_to_handle_at().

Allow userspace to generate a file handle where both the inode number
and the namespace type are zero and just pass in the unique namespace
id. The kernel uses the unified namespace tree to find the namespace and
open the file handle.

When the kernel creates a file handle via name_to_handle_at() it will
always fill in the type and the inode number allowing userspace to
retrieve core information.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/nsfs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 8b9bebd11c49..19dc28742a42 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -502,8 +502,8 @@ static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
 			return NULL;
 
 		VFS_WARN_ON_ONCE(ns->ns_id != fid->ns_id);
-		VFS_WARN_ON_ONCE(ns->ns_type != fid->ns_type);
-		VFS_WARN_ON_ONCE(ns->inum != fid->ns_inum);
+		if (fid->ns_inum && (fid->ns_inum != ns->inum))
+			return NULL;
 
 		/*
 		 * This is racy because we're not actually taking an

-- 
2.47.3

Re: [PATCH v3 14/70] nstree: allow lookup solely based on inode

[Jeff Layton]

On Fri, 2025-10-24 at 12:52 +0200, Christian Brauner wrote:
> The namespace file handle struct nsfs_file_handle is uapi and userspace
> is expressly allowed to generate file handles without going through
> name_to_handle_at().
> 
> Allow userspace to generate a file handle where both the inode number
> and the namespace type are zero and just pass in the unique namespace
> id. The kernel uses the unified namespace tree to find the namespace and
> open the file handle.
> 
> When the kernel creates a file handle via name_to_handle_at() it will
> always fill in the type and the inode number allowing userspace to
> retrieve core information.
> 

Where is the nsfs filehandle format documented? I didn't see any
reference to it in the manpages.

> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/nsfs.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index 8b9bebd11c49..19dc28742a42 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -502,8 +502,8 @@ static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
>  			return NULL;
>  
>  		VFS_WARN_ON_ONCE(ns->ns_id != fid->ns_id);
> -		VFS_WARN_ON_ONCE(ns->ns_type != fid->ns_type);
> -		VFS_WARN_ON_ONCE(ns->inum != fid->ns_inum);
> +		if (fid->ns_inum && (fid->ns_inum != ns->inum))
> +			return NULL;
>  
>  		/*
>  		 * This is racy because we're not actually taking an

-- 
Jeff Layton <jlayton@kernel.org>

[PATCH v3 15/70] nstree: assign fixed ids to the initial namespaces

[Christian Brauner]

The initial set of namespace comes with fixed inode numbers making it
easy for userspace to identify them solely based on that information.
This has long preceeded anything here.

Similarly, let's assign fixed namespace ids for the initial namespaces.

Kill the cookie and use a sequentially increasing number. This has the
nice side-effect that the owning user namespace will always have a
namespace id that is smaller than any of it's descendant namespaces.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c            |  2 +-
 include/linux/nstree.h    | 26 ++++++++++++++++++++++----
 include/uapi/linux/nsfs.h | 14 ++++++++++++++
 kernel/nstree.c           | 13 ++++++++-----
 net/core/net_namespace.c  |  2 +-
 5 files changed, 46 insertions(+), 11 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 87116def5ee3..5d8a80e1e944 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4094,7 +4094,7 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns, bool a
 		return ERR_PTR(ret);
 	}
 	if (!anon)
-		ns_tree_gen_id(&new_ns->ns);
+		ns_tree_gen_id(new_ns);
 	refcount_set(&new_ns->passive, 1);
 	new_ns->mounts = RB_ROOT;
 	init_waitqueue_head(&new_ns->poll);
diff --git a/include/linux/nstree.h b/include/linux/nstree.h
index 8b8636690473..96ee71622517 100644
--- a/include/linux/nstree.h
+++ b/include/linux/nstree.h
@@ -8,6 +8,7 @@
 #include <linux/seqlock.h>
 #include <linux/rculist.h>
 #include <linux/cookie.h>
+#include <uapi/linux/nsfs.h>
 
 extern struct ns_tree cgroup_ns_tree;
 extern struct ns_tree ipc_ns_tree;
@@ -29,7 +30,22 @@ extern struct ns_tree uts_ns_tree;
 		struct user_namespace *:   &(user_ns_tree),	\
 		struct uts_namespace *:    &(uts_ns_tree))
 
-u64 ns_tree_gen_id(struct ns_common *ns);
+#define ns_init_id(__ns)				      \
+	_Generic((__ns),                                      \
+		struct cgroup_namespace *: CGROUP_NS_INIT_ID, \
+		struct ipc_namespace *:    IPC_NS_INIT_ID,    \
+		struct mnt_namespace *:    MNT_NS_INIT_ID,    \
+		struct net *:              NET_NS_INIT_ID,    \
+		struct pid_namespace *:    PID_NS_INIT_ID,    \
+		struct time_namespace *:   TIME_NS_INIT_ID,   \
+		struct user_namespace *:   USER_NS_INIT_ID,   \
+		struct uts_namespace *:    UTS_NS_INIT_ID)
+
+#define ns_tree_gen_id(__ns)                 \
+	__ns_tree_gen_id(to_ns_common(__ns), \
+			 (((__ns) == ns_init_ns(__ns)) ? ns_init_id(__ns) : 0))
+
+u64 __ns_tree_gen_id(struct ns_common *ns, u64 id);
 void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree);
 void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree);
 struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type);
@@ -37,9 +53,9 @@ struct ns_common *__ns_tree_adjoined_rcu(struct ns_common *ns,
 					 struct ns_tree *ns_tree,
 					 bool previous);
 
-static inline void __ns_tree_add(struct ns_common *ns, struct ns_tree *ns_tree)
+static inline void __ns_tree_add(struct ns_common *ns, struct ns_tree *ns_tree, u64 id)
 {
-	ns_tree_gen_id(ns);
+	__ns_tree_gen_id(ns, id);
 	__ns_tree_add_raw(ns, ns_tree);
 }
 
@@ -59,7 +75,9 @@ static inline void __ns_tree_add(struct ns_common *ns, struct ns_tree *ns_tree)
  * This function assigns a new id to the namespace and adds it to the
  * appropriate namespace tree and list.
  */
-#define ns_tree_add(__ns) __ns_tree_add(to_ns_common(__ns), to_ns_tree(__ns))
+#define ns_tree_add(__ns)                                   \
+	__ns_tree_add(to_ns_common(__ns), to_ns_tree(__ns), \
+		      (((__ns) == ns_init_ns(__ns)) ? ns_init_id(__ns) : 0))
 
 /**
  * ns_tree_remove - Remove a namespace from a namespace tree
diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
index e098759ec917..f8bc2aad74d6 100644
--- a/include/uapi/linux/nsfs.h
+++ b/include/uapi/linux/nsfs.h
@@ -67,4 +67,18 @@ struct nsfs_file_handle {
 #define NSFS_FILE_HANDLE_SIZE_VER0 16 /* sizeof first published struct */
 #define NSFS_FILE_HANDLE_SIZE_LATEST sizeof(struct nsfs_file_handle) /* sizeof latest published struct */
 
+enum init_ns_id {
+	IPC_NS_INIT_ID		= 1ULL,
+	UTS_NS_INIT_ID		= 2ULL,
+	USER_NS_INIT_ID		= 3ULL,
+	PID_NS_INIT_ID		= 4ULL,
+	CGROUP_NS_INIT_ID	= 5ULL,
+	TIME_NS_INIT_ID		= 6ULL,
+	NET_NS_INIT_ID		= 7ULL,
+	MNT_NS_INIT_ID		= 8ULL,
+#ifdef __KERNEL__
+	NS_LAST_INIT_ID		= MNT_NS_INIT_ID,
+#endif
+};
+
 #endif /* __LINUX_NSFS_H */
diff --git a/kernel/nstree.c b/kernel/nstree.c
index d21df06b6747..de5ceda44637 100644
--- a/kernel/nstree.c
+++ b/kernel/nstree.c
@@ -68,8 +68,6 @@ struct ns_tree time_ns_tree = {
 	.type = CLONE_NEWTIME,
 };
 
-DEFINE_COOKIE(namespace_cookie);
-
 static inline struct ns_common *node_to_ns(const struct rb_node *node)
 {
 	if (!node)
@@ -278,15 +276,20 @@ struct ns_common *__ns_tree_adjoined_rcu(struct ns_common *ns,
 /**
  * ns_tree_gen_id - generate a new namespace id
  * @ns: namespace to generate id for
+ * @id: if non-zero, this is the initial namespace and this is a fixed id
  *
  * Generates a new namespace id and assigns it to the namespace. All
  * namespaces types share the same id space and thus can be compared
  * directly. IOW, when two ids of two namespace are equal, they are
  * identical.
  */
-u64 ns_tree_gen_id(struct ns_common *ns)
+u64 __ns_tree_gen_id(struct ns_common *ns, u64 id)
 {
-	guard(preempt)();
-	ns->ns_id = gen_cookie_next(&namespace_cookie);
+	static atomic64_t namespace_cookie = ATOMIC64_INIT(NS_LAST_INIT_ID + 1);
+
+	if (id)
+		ns->ns_id = id;
+	else
+		ns->ns_id = atomic64_inc_return(&namespace_cookie);
 	return ns->ns_id;
 }
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index f30fb78f020c..a76b9b9709d6 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -439,7 +439,7 @@ static __net_init int setup_net(struct net *net)
 	LIST_HEAD(net_exit_list);
 	int error = 0;
 
-	net->net_cookie = ns_tree_gen_id(&net->ns);
+	net->net_cookie = ns_tree_gen_id(net);
 
 	list_for_each_entry(ops, &pernet_list, list) {
 		error = ops_init(ops, net);

-- 
2.47.3

[PATCH v3 16/70] ns: maintain list of owned namespaces

[Christian Brauner]

The namespace tree doesn't express the ownership concept of namespace
appropriately. Maintain a list of directly owned namespaces per user
namespace. This will allow userspace and the kernel to use the listns()
system call to walk the namespace tree by owning user namespace.
Read-only list operations are completely lockless.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c            |  2 ++
 include/linux/ns_common.h |  4 ++++
 init/version-timestamp.c  |  2 ++
 ipc/msgutil.c             |  2 ++
 kernel/cgroup/cgroup.c    |  2 ++
 kernel/nscommon.c         |  2 ++
 kernel/nstree.c           | 19 +++++++++++++++++++
 kernel/pid.c              |  2 ++
 kernel/time/namespace.c   |  2 ++
 kernel/user.c             |  2 ++
 10 files changed, 39 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 5d8a80e1e944..d460ca79f0e7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5996,6 +5996,8 @@ struct mnt_namespace init_mnt_ns = {
 	.mounts		= RB_ROOT,
 	.poll		= __WAIT_QUEUE_HEAD_INITIALIZER(init_mnt_ns.poll),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_mnt_ns.ns.ns_list_node),
+	.ns.ns_owner_entry = LIST_HEAD_INIT(init_mnt_ns.ns.ns_owner_entry),
+	.ns.ns_owner = LIST_HEAD_INIT(init_mnt_ns.ns.ns_owner),
 };
 
 static void __init init_mount_tree(void)
diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index b7d2123a9d19..88dce67e06e4 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -123,6 +123,10 @@ struct ns_common {
 				struct rb_node ns_tree_node;
 				struct list_head ns_list_node;
 			};
+			struct /* namespace ownership list */ {
+				struct list_head ns_owner; /* list of namespaces owned by this namespace */
+				struct list_head ns_owner_entry; /* node in the owner namespace's ns_owned list */
+			};
 			atomic_t __ns_ref_active; /* do not use directly */
 		};
 		struct rcu_head ns_rcu;
diff --git a/init/version-timestamp.c b/init/version-timestamp.c
index c38498f94646..e5c278dabecf 100644
--- a/init/version-timestamp.c
+++ b/init/version-timestamp.c
@@ -22,6 +22,8 @@ struct uts_namespace init_uts_ns = {
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_uts_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_uts_ns.ns.ns_list_node),
+	.ns.ns_owner_entry = LIST_HEAD_INIT(init_uts_ns.ns.ns_owner_entry),
+	.ns.ns_owner = LIST_HEAD_INIT(init_uts_ns.ns.ns_owner),
 #ifdef CONFIG_UTS_NS
 	.ns.ops = &utsns_operations,
 #endif
diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index d7c66b430470..ce1de73725c0 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -32,6 +32,8 @@ struct ipc_namespace init_ipc_ns = {
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_ipc_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_ipc_ns.ns.ns_list_node),
+	.ns.ns_owner_entry = LIST_HEAD_INIT(init_ipc_ns.ns.ns_owner_entry),
+	.ns.ns_owner = LIST_HEAD_INIT(init_ipc_ns.ns.ns_owner),
 #ifdef CONFIG_IPC_NS
 	.ns.ops = &ipcns_operations,
 #endif
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 45e470011c77..9fa082e2eb1a 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -258,6 +258,8 @@ struct cgroup_namespace init_cgroup_ns = {
 	.root_cset	= &init_css_set,
 	.ns.ns_type	= ns_common_type(&init_cgroup_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_list_node),
+	.ns.ns_owner_entry = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_owner_entry),
+	.ns.ns_owner = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_owner),
 };
 
 static struct file_system_type cgroup2_fs_type;
diff --git a/kernel/nscommon.c b/kernel/nscommon.c
index bdd32e14a587..ba46de0637c3 100644
--- a/kernel/nscommon.c
+++ b/kernel/nscommon.c
@@ -63,6 +63,8 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
 	RB_CLEAR_NODE(&ns->ns_tree_node);
 	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
 	INIT_LIST_HEAD(&ns->ns_list_node);
+	INIT_LIST_HEAD(&ns->ns_owner);
+	INIT_LIST_HEAD(&ns->ns_owner_entry);
 
 #ifdef CONFIG_DEBUG_VFS
 	ns_debug(ns, ops);
diff --git a/kernel/nstree.c b/kernel/nstree.c
index de5ceda44637..829682bb04a1 100644
--- a/kernel/nstree.c
+++ b/kernel/nstree.c
@@ -3,6 +3,7 @@
 #include <linux/nstree.h>
 #include <linux/proc_ns.h>
 #include <linux/vfsdebug.h>
+#include <linux/user_namespace.h>
 
 __cacheline_aligned_in_smp DEFINE_SEQLOCK(ns_tree_lock);
 static struct rb_root ns_unified_tree = RB_ROOT; /* protected by ns_tree_lock */
@@ -113,8 +114,10 @@ static inline int ns_cmp_unified(struct rb_node *a, const struct rb_node *b)
 void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 {
 	struct rb_node *node, *prev;
+	const struct proc_ns_operations *ops = ns->ops;
 
 	VFS_WARN_ON_ONCE(!ns->ns_id);
+	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
 
 	write_seqlock(&ns_tree_lock);
 
@@ -132,6 +135,21 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 		list_add_rcu(&ns->ns_list_node, &node_to_ns(prev)->ns_list_node);
 
 	rb_find_add_rcu(&ns->ns_unified_tree_node, &ns_unified_tree, ns_cmp_unified);
+
+	if (ops) {
+		struct user_namespace *user_ns;
+
+		VFS_WARN_ON_ONCE(!ops->owner);
+		user_ns = ops->owner(ns);
+		if (user_ns) {
+			struct ns_common *owner = &user_ns->ns;
+			VFS_WARN_ON_ONCE(owner->ns_type != CLONE_NEWUSER);
+			list_add_tail_rcu(&ns->ns_owner_entry, &owner->ns_owner);
+		} else {
+			/* Only the initial user namespace doesn't have an owner. */
+			VFS_WARN_ON_ONCE(ns != to_ns_common(&init_user_ns));
+		}
+	}
 	write_sequnlock(&ns_tree_lock);
 
 	VFS_WARN_ON_ONCE(node);
@@ -148,6 +166,7 @@ void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree)
 	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
 	list_bidir_del_rcu(&ns->ns_list_node);
 	RB_CLEAR_NODE(&ns->ns_tree_node);
+	list_bidir_del_rcu(&ns->ns_owner_entry);
 	write_sequnlock(&ns_tree_lock);
 }
 EXPORT_SYMBOL_GPL(__ns_tree_remove);
diff --git a/kernel/pid.c b/kernel/pid.c
index ec9051d387ee..8134c40b2584 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -80,6 +80,8 @@ struct pid_namespace init_pid_ns = {
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_pid_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_pid_ns.ns.ns_list_node),
+	.ns.ns_owner_entry = LIST_HEAD_INIT(init_pid_ns.ns.ns_owner_entry),
+	.ns.ns_owner = LIST_HEAD_INIT(init_pid_ns.ns.ns_owner),
 #ifdef CONFIG_PID_NS
 	.ns.ops = &pidns_operations,
 #endif
diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
index 2e7c110bd13f..15cb74267c75 100644
--- a/kernel/time/namespace.c
+++ b/kernel/time/namespace.c
@@ -485,6 +485,8 @@ struct time_namespace init_time_ns = {
 	.user_ns	= &init_user_ns,
 	.ns.inum	= ns_init_inum(&init_time_ns),
 	.ns.ops		= &timens_operations,
+	.ns.ns_owner_entry = LIST_HEAD_INIT(init_time_ns.ns.ns_owner_entry),
+	.ns.ns_owner = LIST_HEAD_INIT(init_time_ns.ns.ns_owner),
 	.frozen_offsets	= true,
 	.ns.ns_list_node = LIST_HEAD_INIT(init_time_ns.ns.ns_list_node),
 };
diff --git a/kernel/user.c b/kernel/user.c
index bf60532856db..e392768ccd44 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -72,6 +72,8 @@ struct user_namespace init_user_ns = {
 	.group = GLOBAL_ROOT_GID,
 	.ns.inum = ns_init_inum(&init_user_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_user_ns.ns.ns_list_node),
+	.ns.ns_owner_entry = LIST_HEAD_INIT(init_user_ns.ns.ns_owner_entry),
+	.ns.ns_owner = LIST_HEAD_INIT(init_user_ns.ns.ns_owner),
 #ifdef CONFIG_USER_NS
 	.ns.ops = &userns_operations,
 #endif

-- 
2.47.3

[PATCH v3 17/70] nstree: add listns()

[Christian Brauner]

Add a new listns() system call that allows userspace to iterate through
namespaces in the system. This provides a programmatic interface to
discover and inspect namespaces, enhancing existing namespace apis.

Currently, there is no direct way for userspace to enumerate namespaces
in the system. Applications must resort to scanning /proc/<pid>/ns/
across all processes, which is:

1. Inefficient - requires iterating over all processes
2. Incomplete - misses inactive namespaces that aren't attached to any
   running process but are kept alive by file descriptors, bind mounts,
   or parent namespace references
3. Permission-heavy - requires access to /proc for many processes
4. No ordering or ownership.
5. No filtering per namespace type: Must always iterate and check all
   namespaces.

The list goes on. The listns() system call solves these problems by
providing direct kernel-level enumeration of namespaces. It is similar
to listmount() but obviously tailored to namespaces.

/*
 * @req: Pointer to struct ns_id_req specifying search parameters
 * @ns_ids: User buffer to receive namespace IDs
 * @nr_ns_ids: Size of ns_ids buffer (maximum number of IDs to return)
 * @flags: Reserved for future use (must be 0)
 */
ssize_t listns(const struct ns_id_req *req, u64 *ns_ids,
               size_t nr_ns_ids, unsigned int flags);

Returns:
- On success: Number of namespace IDs written to ns_ids
- On error: Negative error code

/*
 * @size: Structure size
 * @ns_id: Starting point for iteration; use 0 for first call, then
 *         use the last returned ID for subsequent calls to paginate
 * @ns_type: Bitmask of namespace types to include (from enum ns_type):
 *           0: Return all namespace types
 *           MNT_NS: Mount namespaces
 *           NET_NS: Network namespaces
 *           USER_NS: User namespaces
 *           etc. Can be OR'd together
 * @user_ns_id: Filter results to namespaces owned by this user namespace:
 *              0: Return all namespaces (subject to permission checks)
 *              LISTNS_CURRENT_USER: Namespaces owned by caller's user namespace
 *              Other value: Namespaces owned by the specified user namespace ID
 */
struct ns_id_req {
        __u32 size;         /* sizeof(struct ns_id_req) */
        __u32 spare;        /* Reserved, must be 0 */
        __u64 ns_id;        /* Last seen namespace ID (for pagination) */
        __u32 ns_type;      /* Filter by namespace type(s) */
        __u32 spare2;       /* Reserved, must be 0 */
        __u64 user_ns_id;   /* Filter by owning user namespace */
};

Example 1: List all namespaces

void list_all_namespaces(void)
{
    struct ns_id_req req = {
        .size = sizeof(req),
        .ns_id = 0,          /* Start from beginning */
        .ns_type = 0,        /* All types */
        .user_ns_id = 0,     /* All user namespaces */
    };
    uint64_t ids[100];
    ssize_t ret;

    printf("All namespaces in the system:\n");
    do {
        ret = listns(&req, ids, 100, 0);
        if (ret < 0) {
            perror("listns");
            break;
        }

        for (ssize_t i = 0; i < ret; i++)
            printf("  Namespace ID: %llu\n", (unsigned long long)ids[i]);

        /* Continue from last seen ID */
        if (ret > 0)
            req.ns_id = ids[ret - 1];
    } while (ret == 100);  /* Buffer was full, more may exist */
}

Example 2: List network namespaces only

void list_network_namespaces(void)
{
    struct ns_id_req req = {
        .size = sizeof(req),
        .ns_id = 0,
        .ns_type = NET_NS,   /* Only network namespaces */
        .user_ns_id = 0,
    };
    uint64_t ids[100];
    ssize_t ret;

    ret = listns(&req, ids, 100, 0);
    if (ret < 0) {
        perror("listns");
        return;
    }

    printf("Network namespaces: %zd found\n", ret);
    for (ssize_t i = 0; i < ret; i++)
        printf("  netns ID: %llu\n", (unsigned long long)ids[i]);
}

Example 3: List namespaces owned by current user namespace

void list_owned_namespaces(void)
{
    struct ns_id_req req = {
        .size = sizeof(req),
        .ns_id = 0,
        .ns_type = 0,                      /* All types */
        .user_ns_id = LISTNS_CURRENT_USER, /* Current userns */
    };
    uint64_t ids[100];
    ssize_t ret;

    ret = listns(&req, ids, 100, 0);
    if (ret < 0) {
        perror("listns");
        return;
    }

    printf("Namespaces owned by my user namespace: %zd\n", ret);
    for (ssize_t i = 0; i < ret; i++)
        printf("  ns ID: %llu\n", (unsigned long long)ids[i]);
}

Example 4: List multiple namespace types

void list_network_and_mount_namespaces(void)
{
    struct ns_id_req req = {
        .size = sizeof(req),
        .ns_id = 0,
        .ns_type = NET_NS | MNT_NS,  /* Network and mount */
        .user_ns_id = 0,
    };
    uint64_t ids[100];
    ssize_t ret;

    ret = listns(&req, ids, 100, 0);
    printf("Network and mount namespaces: %zd found\n", ret);
}

Example 5: Pagination through large namespace sets

void list_all_with_pagination(void)
{
    struct ns_id_req req = {
        .size = sizeof(req),
        .ns_id = 0,
        .ns_type = 0,
        .user_ns_id = 0,
    };
    uint64_t ids[50];
    size_t total = 0;
    ssize_t ret;

    printf("Enumerating all namespaces with pagination:\n");

    while (1) {
        ret = listns(&req, ids, 50, 0);
        if (ret < 0) {
            perror("listns");
            break;
        }
        if (ret == 0)
            break;  /* No more namespaces */

        total += ret;
        printf("  Batch: %zd namespaces\n", ret);

        /* Last ID in this batch becomes start of next batch */
        req.ns_id = ids[ret - 1];

        if (ret < 50)
            break;  /* Partial batch = end of results */
    }

    printf("Total: %zu namespaces\n", total);
}

Permission Model

listns() respects namespace isolation and capabilities:

(1) Global listing (user_ns_id = 0):
    - Requires CAP_SYS_ADMIN in the namespace's owning user namespace
    - OR the namespace must be in the caller's namespace context (e.g.,
      a namespace the caller is currently using)
    - User namespaces additionally allow listing if the caller has
      CAP_SYS_ADMIN in that user namespace itself
(2) Owner-filtered listing (user_ns_id != 0):
    - Requires CAP_SYS_ADMIN in the specified owner user namespace
    - OR the namespace must be in the caller's namespace context
    - This allows unprivileged processes to enumerate namespaces they own
(3) Visibility:
    - Only "active" namespaces are listed
    - A namespace is active if it has a non-zero __ns_ref_active count
    - This includes namespaces used by running processes, held by open
      file descriptors, or kept active by bind mounts
    - Inactive namespaces (kept alive only by internal kernel
      references) are not visible via listns()

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c                 |   1 +
 fs/nsfs.c                      |  39 ++++
 include/linux/ns_common.h      |   5 +-
 include/linux/syscalls.h       |   4 +
 include/linux/user_namespace.h |   4 +-
 include/uapi/linux/nsfs.h      |  44 +++++
 init/version-timestamp.c       |   1 +
 ipc/msgutil.c                  |   1 +
 kernel/cgroup/cgroup.c         |   1 +
 kernel/nscommon.c              |   3 +
 kernel/nstree.c                | 417 ++++++++++++++++++++++++++++++++++++++++-
 kernel/pid.c                   |   1 +
 kernel/time/namespace.c        |   1 +
 kernel/user.c                  |   1 +
 14 files changed, 516 insertions(+), 7 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index d460ca79f0e7..980296b0ec86 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5996,6 +5996,7 @@ struct mnt_namespace init_mnt_ns = {
 	.mounts		= RB_ROOT,
 	.poll		= __WAIT_QUEUE_HEAD_INITIALIZER(init_mnt_ns.poll),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_mnt_ns.ns.ns_list_node),
+	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_mnt_ns.ns.ns_unified_list_node),
 	.ns.ns_owner_entry = LIST_HEAD_INIT(init_mnt_ns.ns.ns_owner_entry),
 	.ns.ns_owner = LIST_HEAD_INIT(init_mnt_ns.ns.ns_owner),
 };
diff --git a/fs/nsfs.c b/fs/nsfs.c
index 19dc28742a42..5c21fdc79796 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -471,6 +471,45 @@ static int nsfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 	return FILEID_NSFS;
 }
 
+bool is_current_namespace(struct ns_common *ns)
+{
+	switch (ns->ns_type) {
+#ifdef CONFIG_CGROUPS
+	case CLONE_NEWCGROUP:
+		return current_in_namespace(to_cg_ns(ns));
+#endif
+#ifdef CONFIG_IPC_NS
+	case CLONE_NEWIPC:
+		return current_in_namespace(to_ipc_ns(ns));
+#endif
+	case CLONE_NEWNS:
+		return current_in_namespace(to_mnt_ns(ns));
+#ifdef CONFIG_NET_NS
+	case CLONE_NEWNET:
+		return current_in_namespace(to_net_ns(ns));
+#endif
+#ifdef CONFIG_PID_NS
+	case CLONE_NEWPID:
+		return current_in_namespace(to_pid_ns(ns));
+#endif
+#ifdef CONFIG_TIME_NS
+	case CLONE_NEWTIME:
+		return current_in_namespace(to_time_ns(ns));
+#endif
+#ifdef CONFIG_USER_NS
+	case CLONE_NEWUSER:
+		return current_in_namespace(to_user_ns(ns));
+#endif
+#ifdef CONFIG_UTS_NS
+	case CLONE_NEWUTS:
+		return current_in_namespace(to_uts_ns(ns));
+#endif
+	default:
+		VFS_WARN_ON_ONCE(true);
+		return false;
+	}
+}
+
 static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
 					int fh_len, int fh_type)
 {
diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index 88dce67e06e4..95b3e2aa177d 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -123,8 +123,10 @@ struct ns_common {
 				struct rb_node ns_tree_node;
 				struct list_head ns_list_node;
 			};
-			struct /* namespace ownership list */ {
+			struct /* namespace ownership rbtree and list */ {
+				struct rb_root ns_owner_tree; /* rbtree of namespaces owned by this namespace */
 				struct list_head ns_owner; /* list of namespaces owned by this namespace */
+				struct rb_node ns_owner_tree_node; /* node in the owner namespace's rbtree */
 				struct list_head ns_owner_entry; /* node in the owner namespace's ns_owned list */
 			};
 			atomic_t __ns_ref_active; /* do not use directly */
@@ -133,6 +135,7 @@ struct ns_common {
 	};
 };
 
+bool is_current_namespace(struct ns_common *ns);
 int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_operations *ops, int inum);
 void __ns_common_free(struct ns_common *ns);
 
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 66c06fcdfe19..cf84d98964b2 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -77,6 +77,7 @@ struct cachestat_range;
 struct cachestat;
 struct statmount;
 struct mnt_id_req;
+struct ns_id_req;
 struct xattr_args;
 struct file_attr;
 
@@ -437,6 +438,9 @@ asmlinkage long sys_statmount(const struct mnt_id_req __user *req,
 asmlinkage long sys_listmount(const struct mnt_id_req __user *req,
 			      u64 __user *mnt_ids, size_t nr_mnt_ids,
 			      unsigned int flags);
+asmlinkage long sys_listns(const struct ns_id_req __user *req,
+			   u64 __user *ns_ids, size_t nr_ns_ids,
+			   unsigned int flags);
 asmlinkage long sys_truncate(const char __user *path, long length);
 asmlinkage long sys_ftruncate(unsigned int fd, off_t length);
 #if BITS_PER_LONG == 32
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 9a9aebbf96b9..9c3be157397e 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -166,13 +166,13 @@ static inline void set_userns_rlimit_max(struct user_namespace *ns,
 	ns->rlimit_max[type] = max <= LONG_MAX ? max : LONG_MAX;
 }
 
-#ifdef CONFIG_USER_NS
-
 static inline struct user_namespace *to_user_ns(struct ns_common *ns)
 {
 	return container_of(ns, struct user_namespace, ns);
 }
 
+#ifdef CONFIG_USER_NS
+
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
 {
 	if (ns)
diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
index f8bc2aad74d6..a25e38d1c874 100644
--- a/include/uapi/linux/nsfs.h
+++ b/include/uapi/linux/nsfs.h
@@ -81,4 +81,48 @@ enum init_ns_id {
 #endif
 };
 
+enum ns_type {
+	TIME_NS    = (1ULL << 7),  /* CLONE_NEWTIME */
+	MNT_NS     = (1ULL << 17), /* CLONE_NEWNS */
+	CGROUP_NS  = (1ULL << 25), /* CLONE_NEWCGROUP */
+	UTS_NS     = (1ULL << 26), /* CLONE_NEWUTS */
+	IPC_NS     = (1ULL << 27), /* CLONE_NEWIPC */
+	USER_NS    = (1ULL << 28), /* CLONE_NEWUSER */
+	PID_NS     = (1ULL << 29), /* CLONE_NEWPID */
+	NET_NS     = (1ULL << 30), /* CLONE_NEWNET */
+};
+
+/**
+ * struct ns_id_req - namespace ID request structure
+ * @size: size of this structure
+ * @spare: reserved for future use
+ * @filter: filter mask
+ * @ns_id: last namespace id
+ * @user_ns_id: owning user namespace ID
+ *
+ * Structure for passing namespace ID and miscellaneous parameters to
+ * statns(2) and listns(2).
+ *
+ * For statns(2) @param represents the request mask.
+ * For listns(2) @param represents the last listed mount id (or zero).
+ */
+struct ns_id_req {
+	__u32 size;
+	__u32 spare;
+	__u64 ns_id;
+	struct /* listns */ {
+		__u32 ns_type;
+		__u32 spare2;
+		__u64 user_ns_id;
+	};
+};
+
+/*
+ * Special @user_ns_id value that can be passed to listns()
+ */
+#define LISTNS_CURRENT_USER 0xffffffffffffffff /* Caller's userns */
+
+/* List of all ns_id_req versions. */
+#define NS_ID_REQ_SIZE_VER0 32 /* sizeof first published struct */
+
 #endif /* __LINUX_NSFS_H */
diff --git a/init/version-timestamp.c b/init/version-timestamp.c
index e5c278dabecf..cd6f435d5fde 100644
--- a/init/version-timestamp.c
+++ b/init/version-timestamp.c
@@ -22,6 +22,7 @@ struct uts_namespace init_uts_ns = {
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_uts_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_uts_ns.ns.ns_list_node),
+	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_uts_ns.ns.ns_unified_list_node),
 	.ns.ns_owner_entry = LIST_HEAD_INIT(init_uts_ns.ns.ns_owner_entry),
 	.ns.ns_owner = LIST_HEAD_INIT(init_uts_ns.ns.ns_owner),
 #ifdef CONFIG_UTS_NS
diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index ce1de73725c0..3708f325228d 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -32,6 +32,7 @@ struct ipc_namespace init_ipc_ns = {
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_ipc_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_ipc_ns.ns.ns_list_node),
+	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_ipc_ns.ns.ns_unified_list_node),
 	.ns.ns_owner_entry = LIST_HEAD_INIT(init_ipc_ns.ns.ns_owner_entry),
 	.ns.ns_owner = LIST_HEAD_INIT(init_ipc_ns.ns.ns_owner),
 #ifdef CONFIG_IPC_NS
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 9fa082e2eb1a..a0eee0785080 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -258,6 +258,7 @@ struct cgroup_namespace init_cgroup_ns = {
 	.root_cset	= &init_css_set,
 	.ns.ns_type	= ns_common_type(&init_cgroup_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_list_node),
+	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_unified_list_node),
 	.ns.ns_owner_entry = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_owner_entry),
 	.ns.ns_owner = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_owner),
 };
diff --git a/kernel/nscommon.c b/kernel/nscommon.c
index ba46de0637c3..def79b549c52 100644
--- a/kernel/nscommon.c
+++ b/kernel/nscommon.c
@@ -62,7 +62,10 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
 	ns->ns_type = ns_type;
 	RB_CLEAR_NODE(&ns->ns_tree_node);
 	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
+	RB_CLEAR_NODE(&ns->ns_owner_tree_node);
 	INIT_LIST_HEAD(&ns->ns_list_node);
+	INIT_LIST_HEAD(&ns->ns_unified_list_node);
+	ns->ns_owner_tree = RB_ROOT;
 	INIT_LIST_HEAD(&ns->ns_owner);
 	INIT_LIST_HEAD(&ns->ns_owner_entry);
 
diff --git a/kernel/nstree.c b/kernel/nstree.c
index 829682bb04a1..5fd50d73f0ae 100644
--- a/kernel/nstree.c
+++ b/kernel/nstree.c
@@ -2,11 +2,15 @@
 
 #include <linux/nstree.h>
 #include <linux/proc_ns.h>
+#include <linux/rculist.h>
+#include <linux/syscalls.h>
 #include <linux/vfsdebug.h>
 #include <linux/user_namespace.h>
+#include <linux/rcupdate_wait.h>
 
 __cacheline_aligned_in_smp DEFINE_SEQLOCK(ns_tree_lock);
 static struct rb_root ns_unified_tree = RB_ROOT; /* protected by ns_tree_lock */
+static LIST_HEAD(ns_unified_list); /* protected by ns_tree_lock */
 
 /**
  * struct ns_tree - Namespace tree
@@ -83,6 +87,13 @@ static inline struct ns_common *node_to_ns_unified(const struct rb_node *node)
 	return rb_entry(node, struct ns_common, ns_unified_tree_node);
 }
 
+static inline struct ns_common *node_to_ns_owner(const struct rb_node *node)
+{
+	if (!node)
+		return NULL;
+	return rb_entry(node, struct ns_common, ns_owner_tree_node);
+}
+
 static inline int ns_cmp(struct rb_node *a, const struct rb_node *b)
 {
 	struct ns_common *ns_a = node_to_ns(a);
@@ -111,6 +122,20 @@ static inline int ns_cmp_unified(struct rb_node *a, const struct rb_node *b)
 	return 0;
 }
 
+static inline int ns_cmp_owner(struct rb_node *a, const struct rb_node *b)
+{
+	struct ns_common *ns_a = node_to_ns_owner(a);
+	struct ns_common *ns_b = node_to_ns_owner(b);
+	u64 ns_id_a = ns_a->ns_id;
+	u64 ns_id_b = ns_b->ns_id;
+
+	if (ns_id_a < ns_id_b)
+		return -1;
+	if (ns_id_a > ns_id_b)
+		return 1;
+	return 0;
+}
+
 void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 {
 	struct rb_node *node, *prev;
@@ -134,7 +159,13 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 	else
 		list_add_rcu(&ns->ns_list_node, &node_to_ns(prev)->ns_list_node);
 
+	/* Add to unified tree and list */
 	rb_find_add_rcu(&ns->ns_unified_tree_node, &ns_unified_tree, ns_cmp_unified);
+	prev = rb_prev(&ns->ns_unified_tree_node);
+	if (!prev)
+		list_add_rcu(&ns->ns_unified_list_node, &ns_unified_list);
+	else
+		list_add_rcu(&ns->ns_unified_list_node, &node_to_ns_unified(prev)->ns_unified_list_node);
 
 	if (ops) {
 		struct user_namespace *user_ns;
@@ -144,7 +175,16 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 		if (user_ns) {
 			struct ns_common *owner = &user_ns->ns;
 			VFS_WARN_ON_ONCE(owner->ns_type != CLONE_NEWUSER);
-			list_add_tail_rcu(&ns->ns_owner_entry, &owner->ns_owner);
+
+			/* Insert into owner's rbtree */
+			rb_find_add_rcu(&ns->ns_owner_tree_node, &owner->ns_owner_tree, ns_cmp_owner);
+
+			/* Insert into owner's list in sorted order */
+			prev = rb_prev(&ns->ns_owner_tree_node);
+			if (!prev)
+				list_add_rcu(&ns->ns_owner_entry, &owner->ns_owner);
+			else
+				list_add_rcu(&ns->ns_owner_entry, &node_to_ns_owner(prev)->ns_owner_entry);
 		} else {
 			/* Only the initial user namespace doesn't have an owner. */
 			VFS_WARN_ON_ONCE(ns != to_ns_common(&init_user_ns));
@@ -157,16 +197,36 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 
 void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree)
 {
+	const struct proc_ns_operations *ops = ns->ops;
+	struct user_namespace *user_ns;
+
 	VFS_WARN_ON_ONCE(RB_EMPTY_NODE(&ns->ns_tree_node));
 	VFS_WARN_ON_ONCE(list_empty(&ns->ns_list_node));
 	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
 
 	write_seqlock(&ns_tree_lock);
 	rb_erase(&ns->ns_tree_node, &ns_tree->ns_tree);
-	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
-	list_bidir_del_rcu(&ns->ns_list_node);
 	RB_CLEAR_NODE(&ns->ns_tree_node);
-	list_bidir_del_rcu(&ns->ns_owner_entry);
+
+	list_bidir_del_rcu(&ns->ns_list_node);
+
+	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
+	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
+
+	list_bidir_del_rcu(&ns->ns_unified_list_node);
+
+	/* Remove from owner's rbtree if this namespace has an owner */
+	if (ops) {
+		user_ns = ops->owner(ns);
+		if (user_ns) {
+			struct ns_common *owner = &user_ns->ns;
+			rb_erase(&ns->ns_owner_tree_node, &owner->ns_owner_tree);
+			RB_CLEAR_NODE(&ns->ns_owner_tree_node);
+		}
+
+		list_bidir_del_rcu(&ns->ns_owner_entry);
+	}
+
 	write_sequnlock(&ns_tree_lock);
 }
 EXPORT_SYMBOL_GPL(__ns_tree_remove);
@@ -312,3 +372,352 @@ u64 __ns_tree_gen_id(struct ns_common *ns, u64 id)
 		ns->ns_id = atomic64_inc_return(&namespace_cookie);
 	return ns->ns_id;
 }
+
+struct klistns {
+	u64 *kns_ids;
+	u32 nr_ns_ids;
+	u64 last_ns_id;
+	u64 user_ns_id;
+	u32 ns_type;
+	struct user_namespace *user_ns;
+	struct ns_common *first_ns;
+};
+
+static void __free_klistns_free(const struct klistns *kls)
+{
+	if (kls->user_ns_id != LISTNS_CURRENT_USER)
+		put_user_ns(kls->user_ns);
+	if (kls->first_ns)
+		kls->first_ns->ops->put(kls->first_ns);
+	kvfree(kls->kns_ids);
+}
+
+#define NS_ALL (PID_NS | USER_NS | MNT_NS | UTS_NS | IPC_NS | NET_NS | CGROUP_NS | TIME_NS)
+
+static int copy_ns_id_req(const struct ns_id_req __user *req,
+			  struct ns_id_req *kreq)
+{
+	int ret;
+	size_t usize;
+
+	BUILD_BUG_ON(sizeof(struct ns_id_req) != NS_ID_REQ_SIZE_VER0);
+
+	ret = get_user(usize, &req->size);
+	if (ret)
+		return -EFAULT;
+	if (unlikely(usize > PAGE_SIZE))
+		return -E2BIG;
+	if (unlikely(usize < NS_ID_REQ_SIZE_VER0))
+		return -EINVAL;
+	memset(kreq, 0, sizeof(*kreq));
+	ret = copy_struct_from_user(kreq, sizeof(*kreq), req, usize);
+	if (ret)
+		return ret;
+	if (kreq->spare != 0)
+		return -EINVAL;
+	if (kreq->ns_type & ~NS_ALL)
+		return -EOPNOTSUPP;
+	return 0;
+}
+
+static inline int prepare_klistns(struct klistns *kls, struct ns_id_req *kreq,
+				  size_t nr_ns_ids)
+{
+	kls->last_ns_id = kreq->ns_id;
+	kls->user_ns_id = kreq->user_ns_id;
+	kls->nr_ns_ids = nr_ns_ids;
+	kls->ns_type = kreq->ns_type;
+
+	kls->kns_ids = kvmalloc_array(nr_ns_ids, sizeof(*kls->kns_ids),
+				      GFP_KERNEL_ACCOUNT);
+	if (!kls->kns_ids)
+		return -ENOMEM;
+
+	return 0;
+}
+
+/*
+ * Lookup a namespace owned by owner with id >= ns_id.
+ * Returns the namespace with the smallest id that is >= ns_id.
+ */
+static struct ns_common *lookup_ns_owner_at(u64 ns_id, struct ns_common *owner)
+{
+	struct ns_common *ret = NULL;
+	struct rb_node *node;
+
+	VFS_WARN_ON_ONCE(owner->ns_type != CLONE_NEWUSER);
+
+	read_seqlock_excl(&ns_tree_lock);
+	node = owner->ns_owner_tree.rb_node;
+
+	while (node) {
+		struct ns_common *ns = node_to_ns_owner(node);
+
+		if (ns_id <= ns->ns_id) {
+			ret = ns;
+			if (ns_id == ns->ns_id)
+				break;
+			node = node->rb_left;
+		} else {
+			node = node->rb_right;
+		}
+	}
+
+	if (ret && !ns_get_unless_inactive(ret))
+		ret = NULL;
+	read_sequnlock_excl(&ns_tree_lock);
+	return ret;
+}
+
+static struct ns_common *lookup_ns_id(u64 mnt_ns_id, int ns_type)
+{
+	struct ns_common *ns;
+
+	guard(rcu)();
+	ns = ns_tree_lookup_rcu(mnt_ns_id, ns_type);
+	if (!ns)
+		return NULL;
+
+	if (!ns_get_unless_inactive(ns))
+		return NULL;
+
+	return ns;
+}
+
+static ssize_t do_listns_userns(struct klistns *kls)
+{
+	u64 *ns_ids = kls->kns_ids;
+	size_t nr_ns_ids = kls->nr_ns_ids;
+	struct ns_common *ns = NULL, *first_ns = NULL;
+	const struct list_head *head;
+	bool userns_capable;
+	ssize_t ret;
+
+	VFS_WARN_ON_ONCE(!kls->user_ns_id);
+
+	if (kls->user_ns_id == LISTNS_CURRENT_USER)
+		ns = to_ns_common(current_user_ns());
+	else if (kls->user_ns_id)
+		ns = lookup_ns_id(kls->user_ns_id, CLONE_NEWUSER);
+	if (!ns)
+		return -EINVAL;
+	kls->user_ns = to_user_ns(ns);
+
+	/*
+	 * Use the rbtree to find the first namespace we care about and
+	 * then use it's list entry to iterate from there.
+	 */
+	if (kls->last_ns_id) {
+		kls->first_ns = lookup_ns_owner_at(kls->last_ns_id + 1, ns);
+		if (!kls->first_ns)
+			return -ENOENT;
+		first_ns = kls->first_ns;
+	}
+
+	ret = 0;
+	head = &to_ns_common(kls->user_ns)->ns_owner;
+	userns_capable = ns_capable_noaudit(kls->user_ns, CAP_SYS_ADMIN);
+	guard(rcu)();
+	if (!first_ns)
+		first_ns = list_entry_rcu(head->next, typeof(*ns), ns_owner_entry);
+	for (ns = first_ns; &ns->ns_owner_entry != head && nr_ns_ids;
+	     ns = list_entry_rcu(ns->ns_owner_entry.next, typeof(*ns), ns_owner_entry)) {
+		if (kls->ns_type && !(kls->ns_type & ns->ns_type))
+			continue;
+		if (!ns_get_unless_inactive(ns))
+			continue;
+		if (userns_capable || is_current_namespace(ns) ||
+		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
+			*ns_ids = ns->ns_id;
+			ns_ids++;
+			nr_ns_ids--;
+			ret++;
+		}
+		if (need_resched())
+			cond_resched_rcu();
+		/* doesn't sleep */
+		ns->ops->put(ns);
+	}
+
+	return ret;
+}
+
+/*
+ * Lookup a namespace with id >= ns_id in either the unified tree or a type-specific tree.
+ * Returns the namespace with the smallest id that is >= ns_id.
+ */
+static struct ns_common *lookup_ns_id_at(u64 ns_id, int ns_type)
+{
+	struct ns_common *ret = NULL;
+	struct ns_tree *ns_tree = NULL;
+	struct rb_node *node;
+
+	if (ns_type) {
+		ns_tree = ns_tree_from_type(ns_type);
+		if (!ns_tree)
+			return NULL;
+	}
+
+	read_seqlock_excl(&ns_tree_lock);
+	if (ns_tree)
+		node = ns_tree->ns_tree.rb_node;
+	else
+		node = ns_unified_tree.rb_node;
+
+	while (node) {
+		struct ns_common *ns;
+
+		if (ns_type)
+			ns = node_to_ns(node);
+		else
+			ns = node_to_ns_unified(node);
+
+		if (ns_id <= ns->ns_id) {
+			if (ns_type)
+				ret = node_to_ns(node);
+			else
+				ret = node_to_ns_unified(node);
+			if (ns_id == ns->ns_id)
+				break;
+			node = node->rb_left;
+		} else {
+			node = node->rb_right;
+		}
+	}
+
+	if (ret && !ns_get_unless_inactive(ret))
+		ret = NULL;
+	read_sequnlock_excl(&ns_tree_lock);
+	return ret;
+}
+
+static inline struct ns_common *first_ns_common(const struct list_head *head,
+						struct ns_tree *ns_tree)
+{
+	if (ns_tree)
+		return list_entry_rcu(head->next, struct ns_common, ns_list_node);
+	return list_entry_rcu(head->next, struct ns_common, ns_unified_list_node);
+}
+
+static inline struct ns_common *next_ns_common(struct ns_common *ns,
+					       struct ns_tree *ns_tree)
+{
+	if (ns_tree)
+		return list_entry_rcu(ns->ns_list_node.next, struct ns_common, ns_list_node);
+	return list_entry_rcu(ns->ns_unified_list_node.next, struct ns_common, ns_unified_list_node);
+}
+
+static inline bool ns_common_is_head(struct ns_common *ns,
+				     const struct list_head *head,
+				     struct ns_tree *ns_tree)
+{
+	if (ns_tree)
+		return &ns->ns_list_node == head;
+	return &ns->ns_unified_list_node == head;
+}
+
+static ssize_t do_listns(struct klistns *kls)
+{
+	u64 *ns_ids = kls->kns_ids;
+	size_t nr_ns_ids = kls->nr_ns_ids;
+	struct ns_common *ns, *first_ns = NULL;
+	struct ns_tree *ns_tree = NULL;
+	const struct list_head *head;
+	struct user_namespace *user_ns;
+	u32 ns_type;
+	ssize_t ret;
+
+	if (hweight32(kls->ns_type) == 1)
+		ns_type = kls->ns_type;
+	else
+		ns_type = 0;
+
+	if (ns_type) {
+		ns_tree = ns_tree_from_type(ns_type);
+		if (!ns_tree)
+			return -EINVAL;
+	}
+
+	if (kls->last_ns_id) {
+		kls->first_ns = lookup_ns_id_at(kls->last_ns_id + 1, ns_type);
+		if (!kls->first_ns)
+			return -ENOENT;
+		first_ns = kls->first_ns;
+	}
+
+	ret = 0;
+	if (ns_tree)
+		head = &ns_tree->ns_list;
+	else
+		head = &ns_unified_list;
+
+	guard(rcu)();
+	if (!first_ns)
+		first_ns = first_ns_common(head, ns_tree);
+
+	for (ns = first_ns; !ns_common_is_head(ns, head, ns_tree) && nr_ns_ids;
+	     ns = next_ns_common(ns, ns_tree)) {
+		if (kls->ns_type && !(kls->ns_type & ns->ns_type))
+			continue;
+		if (!ns_get_unless_inactive(ns))
+			continue;
+		/* Check permissions */
+		if (!ns->ops)
+			user_ns = NULL;
+		else
+			user_ns = ns->ops->owner(ns);
+		if (!user_ns)
+			user_ns = &init_user_ns;
+		if (ns_capable_noaudit(user_ns, CAP_SYS_ADMIN) ||
+		    is_current_namespace(ns) ||
+		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
+			*ns_ids++ = ns->ns_id;
+			nr_ns_ids--;
+			ret++;
+		}
+		if (need_resched())
+			cond_resched_rcu();
+		/* doesn't sleep */
+		ns->ops->put(ns);
+	}
+
+	return ret;
+}
+
+SYSCALL_DEFINE4(listns, const struct ns_id_req __user *, req,
+		u64 __user *, ns_ids, size_t, nr_ns_ids, unsigned int, flags)
+{
+	struct klistns klns __free(klistns_free) = {};
+	const size_t maxcount = 1000000;
+	struct ns_id_req kreq;
+	ssize_t ret;
+
+	if (flags)
+		return -EINVAL;
+
+	if (unlikely(nr_ns_ids > maxcount))
+		return -EOVERFLOW;
+
+	if (!access_ok(ns_ids, nr_ns_ids * sizeof(*ns_ids)))
+		return -EFAULT;
+
+	ret = copy_ns_id_req(req, &kreq);
+	if (ret)
+		return ret;
+
+	ret = prepare_klistns(&klns, &kreq, nr_ns_ids);
+	if (ret)
+		return ret;
+
+	if (kreq.user_ns_id)
+		ret = do_listns_userns(&klns);
+	else
+		ret = do_listns(&klns);
+	if (ret <= 0)
+		return ret;
+
+	if (copy_to_user(ns_ids, klns.kns_ids, ret * sizeof(*ns_ids)))
+		return -EFAULT;
+
+	return ret;
+}
diff --git a/kernel/pid.c b/kernel/pid.c
index 8134c40b2584..22a0440a62fa 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -80,6 +80,7 @@ struct pid_namespace init_pid_ns = {
 	.user_ns = &init_user_ns,
 	.ns.inum = ns_init_inum(&init_pid_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_pid_ns.ns.ns_list_node),
+	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_pid_ns.ns.ns_unified_list_node),
 	.ns.ns_owner_entry = LIST_HEAD_INIT(init_pid_ns.ns.ns_owner_entry),
 	.ns.ns_owner = LIST_HEAD_INIT(init_pid_ns.ns.ns_owner),
 #ifdef CONFIG_PID_NS
diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
index 15cb74267c75..acbeec049263 100644
--- a/kernel/time/namespace.c
+++ b/kernel/time/namespace.c
@@ -489,6 +489,7 @@ struct time_namespace init_time_ns = {
 	.ns.ns_owner = LIST_HEAD_INIT(init_time_ns.ns.ns_owner),
 	.frozen_offsets	= true,
 	.ns.ns_list_node = LIST_HEAD_INIT(init_time_ns.ns.ns_list_node),
+	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_time_ns.ns.ns_unified_list_node),
 };
 
 void __init time_ns_init(void)
diff --git a/kernel/user.c b/kernel/user.c
index e392768ccd44..68fe16617d38 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -72,6 +72,7 @@ struct user_namespace init_user_ns = {
 	.group = GLOBAL_ROOT_GID,
 	.ns.inum = ns_init_inum(&init_user_ns),
 	.ns.ns_list_node = LIST_HEAD_INIT(init_user_ns.ns.ns_list_node),
+	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_user_ns.ns.ns_unified_list_node),
 	.ns.ns_owner_entry = LIST_HEAD_INIT(init_user_ns.ns.ns_owner_entry),
 	.ns.ns_owner = LIST_HEAD_INIT(init_user_ns.ns.ns_owner),
 #ifdef CONFIG_USER_NS

-- 
2.47.3

Re: [PATCH v3 17/70] nstree: add listns()

[Arnd Bergmann]

On Fri, Oct 24, 2025, at 12:52, Christian Brauner wrote:
> Add a new listns() system call that allows userspace to iterate through
> namespaces in the system. This provides a programmatic interface to
> discover and inspect namespaces, enhancing existing namespace apis.

I double-checked that the ABI is well-formed and works the same
way on all supported architectures, though I did not check the functional
aspects.

Acked-by: Arnd Bergmann <arnd@arndb.de>

One small thing I noticed:

> +SYSCALL_DEFINE4(listns, const struct ns_id_req __user *, req,
> +		u64 __user *, ns_ids, size_t, nr_ns_ids, unsigned int, flags)
> +{
> +	struct klistns klns __free(klistns_free) = {};
> +	const size_t maxcount = 1000000;
> +	struct ns_id_req kreq;
> +	ssize_t ret;
> +
> +	if (flags)
> +		return -EINVAL;
> +
> +	if (unlikely(nr_ns_ids > maxcount))
> +		return -EOVERFLOW;
> +
> +	if (!access_ok(ns_ids, nr_ns_ids * sizeof(*ns_ids)))
> +		return -EFAULT;

I'm a bit worried about hardcoding the maxcount value here, which
seems to limit both the size of the allocation and prevent overflowing
the multiplication of the access_ok() argument, though that isn't
completely clear from the implementation.

Allowing 8MB of vmalloc space to be filled can be bad on 32-bit
systems that may only have 100MB in total. The access_ok() check
looks like it tries to provide an early-fail error return but
should not actually be needed since there is a single copy_to_user()
in the end, and that is more likely to fail for unmapped memory than
an access_ok() failure.

Would it make sense to just drop the kvmalloc() completely and
instead put_user() the output values individually? That way you
can avoid both a hardwired limit and a potential DoS from vmalloc
exhaustion.

     Arnd

Re: [PATCH v3 17/70] nstree: add listns()

[Christian Brauner]

On Fri, Oct 24, 2025 at 04:06:57PM +0200, Arnd Bergmann wrote:
> On Fri, Oct 24, 2025, at 12:52, Christian Brauner wrote:
> > Add a new listns() system call that allows userspace to iterate through
> > namespaces in the system. This provides a programmatic interface to
> > discover and inspect namespaces, enhancing existing namespace apis.
> 
> I double-checked that the ABI is well-formed and works the same
> way on all supported architectures, though I did not check the functional
> aspects.
> 
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> 
> One small thing I noticed:
> 
> > +SYSCALL_DEFINE4(listns, const struct ns_id_req __user *, req,
> > +		u64 __user *, ns_ids, size_t, nr_ns_ids, unsigned int, flags)
> > +{
> > +	struct klistns klns __free(klistns_free) = {};
> > +	const size_t maxcount = 1000000;
> > +	struct ns_id_req kreq;
> > +	ssize_t ret;
> > +
> > +	if (flags)
> > +		return -EINVAL;
> > +
> > +	if (unlikely(nr_ns_ids > maxcount))
> > +		return -EOVERFLOW;
> > +
> > +	if (!access_ok(ns_ids, nr_ns_ids * sizeof(*ns_ids)))
> > +		return -EFAULT;
> 
> I'm a bit worried about hardcoding the maxcount value here, which
> seems to limit both the size of the allocation and prevent overflowing
> the multiplication of the access_ok() argument, though that isn't
> completely clear from the implementation.
> 
> Allowing 8MB of vmalloc space to be filled can be bad on 32-bit
> systems that may only have 100MB in total. The access_ok() check
> looks like it tries to provide an early-fail error return but
> should not actually be needed since there is a single copy_to_user()
> in the end, and that is more likely to fail for unmapped memory than
> an access_ok() failure.
> 
> Would it make sense to just drop the kvmalloc() completely and
> instead put_user() the output values individually? That way you
> can avoid both a hardwired limit and a potential DoS from vmalloc
> exhaustion.

Initially this wasn't possible because we walked all of this completely
with only rcu protection. But now that we always have to take a passive
reference its possible to do what you suggest. This would mean
ping-ponging the rcu_read_lock()/rcu_read_unlock() but that's probably
fine. How do you feel about the following?: 

diff --git a/kernel/nstree.c b/kernel/nstree.c
index 1455573e774e..e4c8508e97c7 100644
--- a/kernel/nstree.c
+++ b/kernel/nstree.c
@@ -382,7 +382,7 @@ u64 __ns_tree_gen_id(struct ns_common *ns, u64 id)
 }
 
 struct klistns {
-	u64 *kns_ids;
+	u64 __user *uns_ids;
 	u32 nr_ns_ids;
 	u64 last_ns_id;
 	u64 user_ns_id;
@@ -395,9 +395,8 @@ static void __free_klistns_free(const struct klistns *kls)
 {
 	if (kls->user_ns_id != LISTNS_CURRENT_USER)
 		put_user_ns(kls->user_ns);
-	if (kls->first_ns)
+	if (kls->first_ns && kls->first_ns->ops)
 		kls->first_ns->ops->put(kls->first_ns);
-	kvfree(kls->kns_ids);
 }
 
 #define NS_ALL (PID_NS | USER_NS | MNT_NS | UTS_NS | IPC_NS | NET_NS | CGROUP_NS | TIME_NS)
@@ -429,18 +428,13 @@ static int copy_ns_id_req(const struct ns_id_req __user *req,
 }
 
 static inline int prepare_klistns(struct klistns *kls, struct ns_id_req *kreq,
-				  size_t nr_ns_ids)
+				  u64 __user *ns_ids, size_t nr_ns_ids)
 {
 	kls->last_ns_id = kreq->ns_id;
 	kls->user_ns_id = kreq->user_ns_id;
-	kls->nr_ns_ids = nr_ns_ids;
-	kls->ns_type = kreq->ns_type;
-
-	kls->kns_ids = kvmalloc_array(nr_ns_ids, sizeof(*kls->kns_ids),
-				      GFP_KERNEL_ACCOUNT);
-	if (!kls->kns_ids)
-		return -ENOMEM;
-
+	kls->nr_ns_ids	= nr_ns_ids;
+	kls->ns_type	= kreq->ns_type;
+	kls->uns_ids	= ns_ids;
 	return 0;
 }
 
@@ -459,8 +453,9 @@ static struct ns_common *lookup_ns_owner_at(u64 ns_id, struct ns_common *owner)
 	node = owner->ns_owner_tree.rb_node;
 
 	while (node) {
-		struct ns_common *ns = node_to_ns_owner(node);
+		struct ns_common *ns;
 
+		ns = node_to_ns_owner(node);
 		if (ns_id <= ns->ns_id) {
 			ret = ns;
 			if (ns_id == ns->ns_id)
@@ -494,7 +489,7 @@ static struct ns_common *lookup_ns_id(u64 mnt_ns_id, int ns_type)
 
 static ssize_t do_listns_userns(struct klistns *kls)
 {
-	u64 *ns_ids = kls->kns_ids;
+	u64 __user *ns_ids = kls->uns_ids;
 	size_t nr_ns_ids = kls->nr_ns_ids;
 	struct ns_common *ns = NULL, *first_ns = NULL;
 	const struct list_head *head;
@@ -525,7 +520,9 @@ static ssize_t do_listns_userns(struct klistns *kls)
 	ret = 0;
 	head = &to_ns_common(kls->user_ns)->ns_owner;
 	userns_capable = ns_capable_noaudit(kls->user_ns, CAP_SYS_ADMIN);
-	guard(rcu)();
+
+	rcu_read_lock();
+
 	if (!first_ns)
 		first_ns = list_entry_rcu(head->next, typeof(*ns), ns_owner_entry);
 	for (ns = first_ns; &ns->ns_owner_entry != head && nr_ns_ids;
@@ -534,19 +531,28 @@ static ssize_t do_listns_userns(struct klistns *kls)
 			continue;
 		if (!ns_get_unless_inactive(ns))
 			continue;
+
+		rcu_read_unlock();
+
 		if (userns_capable || is_current_namespace(ns) ||
 		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
-			*ns_ids = ns->ns_id;
-			ns_ids++;
+			if (put_user(ns->ns_id, ns_ids + ret))
+				return -EINVAL;
 			nr_ns_ids--;
 			ret++;
 		}
+
 		if (need_resched())
-			cond_resched_rcu();
+			cond_resched();
+
+		rcu_read_lock();
+
 		/* doesn't sleep */
-		ns->ops->put(ns);
+		if (ns->ops)
+			ns->ops->put(ns);
 	}
 
+	rcu_read_unlock();
 	return ret;
 }
 
@@ -626,7 +632,7 @@ static inline bool ns_common_is_head(struct ns_common *ns,
 
 static ssize_t do_listns(struct klistns *kls)
 {
-	u64 *ns_ids = kls->kns_ids;
+	u64 __user *ns_ids = kls->uns_ids;
 	size_t nr_ns_ids = kls->nr_ns_ids;
 	struct ns_common *ns, *first_ns = NULL;
 	struct ns_tree *ns_tree = NULL;
@@ -659,7 +665,8 @@ static ssize_t do_listns(struct klistns *kls)
 	else
 		head = &ns_unified_list;
 
-	guard(rcu)();
+	rcu_read_lock();
+
 	if (!first_ns)
 		first_ns = first_ns_common(head, ns_tree);
 
@@ -669,6 +676,9 @@ static ssize_t do_listns(struct klistns *kls)
 			continue;
 		if (!ns_get_unless_inactive(ns))
 			continue;
+
+		rcu_read_unlock();
+
 		/* Check permissions */
 		if (!ns->ops)
 			user_ns = NULL;
@@ -679,16 +689,22 @@ static ssize_t do_listns(struct klistns *kls)
 		if (ns_capable_noaudit(user_ns, CAP_SYS_ADMIN) ||
 		    is_current_namespace(ns) ||
 		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
-			*ns_ids++ = ns->ns_id;
+			if (put_user(ns->ns_id, ns_ids + ret))
+				return -EINVAL;
 			nr_ns_ids--;
 			ret++;
 		}
 		if (need_resched())
-			cond_resched_rcu();
+			cond_resched();
+
+		rcu_read_lock();
+
 		/* doesn't sleep */
-		ns->ops->put(ns);
+		if (ns->ops)
+			ns->ops->put(ns);
 	}
 
+	rcu_read_unlock();
 	return ret;
 }
 
@@ -713,19 +729,12 @@ SYSCALL_DEFINE4(listns, const struct ns_id_req __user *, req,
 	if (ret)
 		return ret;
 
-	ret = prepare_klistns(&klns, &kreq, nr_ns_ids);
+	ret = prepare_klistns(&klns, &kreq, ns_ids, nr_ns_ids);
 	if (ret)
 		return ret;
 
 	if (kreq.user_ns_id)
-		ret = do_listns_userns(&klns);
-	else
-		ret = do_listns(&klns);
-	if (ret <= 0)
-		return ret;
+		return do_listns_userns(&klns);
 
-	if (copy_to_user(ns_ids, klns.kns_ids, ret * sizeof(*ns_ids)))
-		return -EFAULT;
-
-	return ret;
+	return do_listns(&klns);
 }

Re: [PATCH v3 17/70] nstree: add listns()

[Jeff Layton]

On Fri, 2025-10-24 at 12:52 +0200, Christian Brauner wrote:
> Add a new listns() system call that allows userspace to iterate through
> namespaces in the system. This provides a programmatic interface to
> discover and inspect namespaces, enhancing existing namespace apis.
> 
> Currently, there is no direct way for userspace to enumerate namespaces
> in the system. Applications must resort to scanning /proc/<pid>/ns/
> across all processes, which is:
> 
> 1. Inefficient - requires iterating over all processes
> 2. Incomplete - misses inactive namespaces that aren't attached to any
>    running process but are kept alive by file descriptors, bind mounts,
>    or parent namespace references
> 3. Permission-heavy - requires access to /proc for many processes
> 4. No ordering or ownership.
> 5. No filtering per namespace type: Must always iterate and check all
>    namespaces.
> 
> The list goes on. The listns() system call solves these problems by
> providing direct kernel-level enumeration of namespaces. It is similar
> to listmount() but obviously tailored to namespaces.
> 
> /*
>  * @req: Pointer to struct ns_id_req specifying search parameters
>  * @ns_ids: User buffer to receive namespace IDs
>  * @nr_ns_ids: Size of ns_ids buffer (maximum number of IDs to return)
>  * @flags: Reserved for future use (must be 0)
>  */
> ssize_t listns(const struct ns_id_req *req, u64 *ns_ids,
>                size_t nr_ns_ids, unsigned int flags);
> 
> Returns:
> - On success: Number of namespace IDs written to ns_ids
> - On error: Negative error code
> 
> /*
>  * @size: Structure size
>  * @ns_id: Starting point for iteration; use 0 for first call, then
>  *         use the last returned ID for subsequent calls to paginate
>  * @ns_type: Bitmask of namespace types to include (from enum ns_type):
>  *           0: Return all namespace types
>  *           MNT_NS: Mount namespaces
>  *           NET_NS: Network namespaces
>  *           USER_NS: User namespaces
>  *           etc. Can be OR'd together
>  * @user_ns_id: Filter results to namespaces owned by this user namespace:
>  *              0: Return all namespaces (subject to permission checks)
>  *              LISTNS_CURRENT_USER: Namespaces owned by caller's user namespace
>  *              Other value: Namespaces owned by the specified user namespace ID
>  */
> struct ns_id_req {
>         __u32 size;         /* sizeof(struct ns_id_req) */
>         __u32 spare;        /* Reserved, must be 0 */
>         __u64 ns_id;        /* Last seen namespace ID (for pagination) */
>         __u32 ns_type;      /* Filter by namespace type(s) */
>         __u32 spare2;       /* Reserved, must be 0 */
>         __u64 user_ns_id;   /* Filter by owning user namespace */
> };
> 
> Example 1: List all namespaces
> 
> void list_all_namespaces(void)
> {
>     struct ns_id_req req = {
>         .size = sizeof(req),
>         .ns_id = 0,          /* Start from beginning */
>         .ns_type = 0,        /* All types */
>         .user_ns_id = 0,     /* All user namespaces */
>     };
>     uint64_t ids[100];
>     ssize_t ret;
> 
>     printf("All namespaces in the system:\n");
>     do {
>         ret = listns(&req, ids, 100, 0);
>         if (ret < 0) {
>             perror("listns");
>             break;
>         }
> 
>         for (ssize_t i = 0; i < ret; i++)
>             printf("  Namespace ID: %llu\n", (unsigned long long)ids[i]);
> 
>         /* Continue from last seen ID */
>         if (ret > 0)
>             req.ns_id = ids[ret - 1];
>     } while (ret == 100);  /* Buffer was full, more may exist */
> }
> 
> Example 2: List network namespaces only
> 
> void list_network_namespaces(void)
> {
>     struct ns_id_req req = {
>         .size = sizeof(req),
>         .ns_id = 0,
>         .ns_type = NET_NS,   /* Only network namespaces */
>         .user_ns_id = 0,
>     };
>     uint64_t ids[100];
>     ssize_t ret;
> 
>     ret = listns(&req, ids, 100, 0);
>     if (ret < 0) {
>         perror("listns");
>         return;
>     }
> 
>     printf("Network namespaces: %zd found\n", ret);
>     for (ssize_t i = 0; i < ret; i++)
>         printf("  netns ID: %llu\n", (unsigned long long)ids[i]);
> }
> 
> Example 3: List namespaces owned by current user namespace
> 
> void list_owned_namespaces(void)
> {
>     struct ns_id_req req = {
>         .size = sizeof(req),
>         .ns_id = 0,
>         .ns_type = 0,                      /* All types */
>         .user_ns_id = LISTNS_CURRENT_USER, /* Current userns */
>     };
>     uint64_t ids[100];
>     ssize_t ret;
> 
>     ret = listns(&req, ids, 100, 0);
>     if (ret < 0) {
>         perror("listns");
>         return;
>     }
> 
>     printf("Namespaces owned by my user namespace: %zd\n", ret);
>     for (ssize_t i = 0; i < ret; i++)
>         printf("  ns ID: %llu\n", (unsigned long long)ids[i]);
> }
> 
> Example 4: List multiple namespace types
> 
> void list_network_and_mount_namespaces(void)
> {
>     struct ns_id_req req = {
>         .size = sizeof(req),
>         .ns_id = 0,
>         .ns_type = NET_NS | MNT_NS,  /* Network and mount */
>         .user_ns_id = 0,
>     };
>     uint64_t ids[100];
>     ssize_t ret;
> 
>     ret = listns(&req, ids, 100, 0);
>     printf("Network and mount namespaces: %zd found\n", ret);
> }
> 
> Example 5: Pagination through large namespace sets
> 
> void list_all_with_pagination(void)
> {
>     struct ns_id_req req = {
>         .size = sizeof(req),
>         .ns_id = 0,
>         .ns_type = 0,
>         .user_ns_id = 0,
>     };
>     uint64_t ids[50];
>     size_t total = 0;
>     ssize_t ret;
> 
>     printf("Enumerating all namespaces with pagination:\n");
> 
>     while (1) {
>         ret = listns(&req, ids, 50, 0);
>         if (ret < 0) {
>             perror("listns");
>             break;
>         }
>         if (ret == 0)
>             break;  /* No more namespaces */
> 
>         total += ret;
>         printf("  Batch: %zd namespaces\n", ret);
> 
>         /* Last ID in this batch becomes start of next batch */
>         req.ns_id = ids[ret - 1];
> 
>         if (ret < 50)
>             break;  /* Partial batch = end of results */
>     }
> 
>     printf("Total: %zu namespaces\n", total);
> }
> 
> Permission Model
> 
> listns() respects namespace isolation and capabilities:
> 
> (1) Global listing (user_ns_id = 0):
>     - Requires CAP_SYS_ADMIN in the namespace's owning user namespace
>     - OR the namespace must be in the caller's namespace context (e.g.,
>       a namespace the caller is currently using)
>     - User namespaces additionally allow listing if the caller has
>       CAP_SYS_ADMIN in that user namespace itself
> (2) Owner-filtered listing (user_ns_id != 0):
>     - Requires CAP_SYS_ADMIN in the specified owner user namespace
>     - OR the namespace must be in the caller's namespace context
>     - This allows unprivileged processes to enumerate namespaces they own
> (3) Visibility:
>     - Only "active" namespaces are listed
>     - A namespace is active if it has a non-zero __ns_ref_active count
>     - This includes namespaces used by running processes, held by open
>       file descriptors, or kept active by bind mounts
>     - Inactive namespaces (kept alive only by internal kernel
>       references) are not visible via listns()
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/namespace.c                 |   1 +
>  fs/nsfs.c                      |  39 ++++
>  include/linux/ns_common.h      |   5 +-
>  include/linux/syscalls.h       |   4 +
>  include/linux/user_namespace.h |   4 +-
>  include/uapi/linux/nsfs.h      |  44 +++++
>  init/version-timestamp.c       |   1 +
>  ipc/msgutil.c                  |   1 +
>  kernel/cgroup/cgroup.c         |   1 +
>  kernel/nscommon.c              |   3 +
>  kernel/nstree.c                | 417 ++++++++++++++++++++++++++++++++++++++++-
>  kernel/pid.c                   |   1 +
>  kernel/time/namespace.c        |   1 +
>  kernel/user.c                  |   1 +
>  14 files changed, 516 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index d460ca79f0e7..980296b0ec86 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -5996,6 +5996,7 @@ struct mnt_namespace init_mnt_ns = {
>  	.mounts		= RB_ROOT,
>  	.poll		= __WAIT_QUEUE_HEAD_INITIALIZER(init_mnt_ns.poll),
>  	.ns.ns_list_node = LIST_HEAD_INIT(init_mnt_ns.ns.ns_list_node),
> +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_mnt_ns.ns.ns_unified_list_node),
>  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_mnt_ns.ns.ns_owner_entry),
>  	.ns.ns_owner = LIST_HEAD_INIT(init_mnt_ns.ns.ns_owner),
>  };
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index 19dc28742a42..5c21fdc79796 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -471,6 +471,45 @@ static int nsfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
>  	return FILEID_NSFS;
>  }
>  
> +bool is_current_namespace(struct ns_common *ns)
> +{
> +	switch (ns->ns_type) {
> +#ifdef CONFIG_CGROUPS
> +	case CLONE_NEWCGROUP:
> +		return current_in_namespace(to_cg_ns(ns));
> +#endif
> +#ifdef CONFIG_IPC_NS
> +	case CLONE_NEWIPC:
> +		return current_in_namespace(to_ipc_ns(ns));
> +#endif
> +	case CLONE_NEWNS:
> +		return current_in_namespace(to_mnt_ns(ns));
> +#ifdef CONFIG_NET_NS
> +	case CLONE_NEWNET:
> +		return current_in_namespace(to_net_ns(ns));
> +#endif
> +#ifdef CONFIG_PID_NS
> +	case CLONE_NEWPID:
> +		return current_in_namespace(to_pid_ns(ns));
> +#endif
> +#ifdef CONFIG_TIME_NS
> +	case CLONE_NEWTIME:
> +		return current_in_namespace(to_time_ns(ns));
> +#endif
> +#ifdef CONFIG_USER_NS
> +	case CLONE_NEWUSER:
> +		return current_in_namespace(to_user_ns(ns));
> +#endif
> +#ifdef CONFIG_UTS_NS
> +	case CLONE_NEWUTS:
> +		return current_in_namespace(to_uts_ns(ns));
> +#endif
> +	default:
> +		VFS_WARN_ON_ONCE(true);
> +		return false;
> +	}
> +}
> +
>  static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
>  					int fh_len, int fh_type)
>  {
> diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
> index 88dce67e06e4..95b3e2aa177d 100644
> --- a/include/linux/ns_common.h
> +++ b/include/linux/ns_common.h
> @@ -123,8 +123,10 @@ struct ns_common {
>  				struct rb_node ns_tree_node;
>  				struct list_head ns_list_node;
>  			};
> -			struct /* namespace ownership list */ {
> +			struct /* namespace ownership rbtree and list */ {
> +				struct rb_root ns_owner_tree; /* rbtree of namespaces owned by this namespace */
>  				struct list_head ns_owner; /* list of namespaces owned by this namespace */
> +				struct rb_node ns_owner_tree_node; /* node in the owner namespace's rbtree */


The changelog doesn't mention why these fields are being added.

I'd prefer to see the parts of this patch that deal with the above
fields added in a separate patch. I'm guessing that you added them here
because this patch adds the need to check ns ownership for listns(),
but I think it warrants a separate patch and changelog explaining
what's going on.

>  				struct list_head ns_owner_entry; /* node in the owner namespace's ns_owned list */
>  			};
>  			atomic_t __ns_ref_active; /* do not use directly */
> @@ -133,6 +135,7 @@ struct ns_common {
>  	};
>  };
>  
> +bool is_current_namespace(struct ns_common *ns);
>  int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_operations *ops, int inum);
>  void __ns_common_free(struct ns_common *ns);
>  
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 66c06fcdfe19..cf84d98964b2 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -77,6 +77,7 @@ struct cachestat_range;
>  struct cachestat;
>  struct statmount;
>  struct mnt_id_req;
> +struct ns_id_req;
>  struct xattr_args;
>  struct file_attr;
>  
> @@ -437,6 +438,9 @@ asmlinkage long sys_statmount(const struct mnt_id_req __user *req,
>  asmlinkage long sys_listmount(const struct mnt_id_req __user *req,
>  			      u64 __user *mnt_ids, size_t nr_mnt_ids,
>  			      unsigned int flags);
> +asmlinkage long sys_listns(const struct ns_id_req __user *req,
> +			   u64 __user *ns_ids, size_t nr_ns_ids,
> +			   unsigned int flags);
>  asmlinkage long sys_truncate(const char __user *path, long length);
>  asmlinkage long sys_ftruncate(unsigned int fd, off_t length);
>  #if BITS_PER_LONG == 32
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index 9a9aebbf96b9..9c3be157397e 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -166,13 +166,13 @@ static inline void set_userns_rlimit_max(struct user_namespace *ns,
>  	ns->rlimit_max[type] = max <= LONG_MAX ? max : LONG_MAX;
>  }
>  
> -#ifdef CONFIG_USER_NS
> -
>  static inline struct user_namespace *to_user_ns(struct ns_common *ns)
>  {
>  	return container_of(ns, struct user_namespace, ns);
>  }
>  
> +#ifdef CONFIG_USER_NS
> +
>  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
>  {
>  	if (ns)
> diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
> index f8bc2aad74d6..a25e38d1c874 100644
> --- a/include/uapi/linux/nsfs.h
> +++ b/include/uapi/linux/nsfs.h
> @@ -81,4 +81,48 @@ enum init_ns_id {
>  #endif
>  };
>  
> +enum ns_type {
> +	TIME_NS    = (1ULL << 7),  /* CLONE_NEWTIME */
> +	MNT_NS     = (1ULL << 17), /* CLONE_NEWNS */
> +	CGROUP_NS  = (1ULL << 25), /* CLONE_NEWCGROUP */
> +	UTS_NS     = (1ULL << 26), /* CLONE_NEWUTS */
> +	IPC_NS     = (1ULL << 27), /* CLONE_NEWIPC */
> +	USER_NS    = (1ULL << 28), /* CLONE_NEWUSER */
> +	PID_NS     = (1ULL << 29), /* CLONE_NEWPID */
> +	NET_NS     = (1ULL << 30), /* CLONE_NEWNET */
> +};
> +
> +/**
> + * struct ns_id_req - namespace ID request structure
> + * @size: size of this structure
> + * @spare: reserved for future use
> + * @filter: filter mask
> + * @ns_id: last namespace id
> + * @user_ns_id: owning user namespace ID
> + *
> + * Structure for passing namespace ID and miscellaneous parameters to
> + * statns(2) and listns(2).
> + *
> + * For statns(2) @param represents the request mask.
> + * For listns(2) @param represents the last listed mount id (or zero).
> + */
> +struct ns_id_req {
> +	__u32 size;
> +	__u32 spare;
> +	__u64 ns_id;
> +	struct /* listns */ {
> +		__u32 ns_type;
> +		__u32 spare2;
> +		__u64 user_ns_id;
> +	};
> +};
> +
> +/*
> + * Special @user_ns_id value that can be passed to listns()
> + */
> +#define LISTNS_CURRENT_USER 0xffffffffffffffff /* Caller's userns */
> +
> +/* List of all ns_id_req versions. */
> +#define NS_ID_REQ_SIZE_VER0 32 /* sizeof first published struct */
> +
>  #endif /* __LINUX_NSFS_H */
> diff --git a/init/version-timestamp.c b/init/version-timestamp.c
> index e5c278dabecf..cd6f435d5fde 100644
> --- a/init/version-timestamp.c
> +++ b/init/version-timestamp.c
> @@ -22,6 +22,7 @@ struct uts_namespace init_uts_ns = {
>  	.user_ns = &init_user_ns,
>  	.ns.inum = ns_init_inum(&init_uts_ns),
>  	.ns.ns_list_node = LIST_HEAD_INIT(init_uts_ns.ns.ns_list_node),
> +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_uts_ns.ns.ns_unified_list_node),
>  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_uts_ns.ns.ns_owner_entry),
>  	.ns.ns_owner = LIST_HEAD_INIT(init_uts_ns.ns.ns_owner),
>  #ifdef CONFIG_UTS_NS
> diff --git a/ipc/msgutil.c b/ipc/msgutil.c
> index ce1de73725c0..3708f325228d 100644
> --- a/ipc/msgutil.c
> +++ b/ipc/msgutil.c
> @@ -32,6 +32,7 @@ struct ipc_namespace init_ipc_ns = {
>  	.user_ns = &init_user_ns,
>  	.ns.inum = ns_init_inum(&init_ipc_ns),
>  	.ns.ns_list_node = LIST_HEAD_INIT(init_ipc_ns.ns.ns_list_node),
> +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_ipc_ns.ns.ns_unified_list_node),
>  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_ipc_ns.ns.ns_owner_entry),
>  	.ns.ns_owner = LIST_HEAD_INIT(init_ipc_ns.ns.ns_owner),
>  #ifdef CONFIG_IPC_NS
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 9fa082e2eb1a..a0eee0785080 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -258,6 +258,7 @@ struct cgroup_namespace init_cgroup_ns = {
>  	.root_cset	= &init_css_set,
>  	.ns.ns_type	= ns_common_type(&init_cgroup_ns),
>  	.ns.ns_list_node = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_list_node),
> +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_unified_list_node),
>  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_owner_entry),
>  	.ns.ns_owner = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_owner),
>  };
> diff --git a/kernel/nscommon.c b/kernel/nscommon.c
> index ba46de0637c3..def79b549c52 100644
> --- a/kernel/nscommon.c
> +++ b/kernel/nscommon.c
> @@ -62,7 +62,10 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
>  	ns->ns_type = ns_type;
>  	RB_CLEAR_NODE(&ns->ns_tree_node);
>  	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
> +	RB_CLEAR_NODE(&ns->ns_owner_tree_node);
>  	INIT_LIST_HEAD(&ns->ns_list_node);
> +	INIT_LIST_HEAD(&ns->ns_unified_list_node);
> +	ns->ns_owner_tree = RB_ROOT;
>  	INIT_LIST_HEAD(&ns->ns_owner);
>  	INIT_LIST_HEAD(&ns->ns_owner_entry);
>  
> diff --git a/kernel/nstree.c b/kernel/nstree.c
> index 829682bb04a1..5fd50d73f0ae 100644
> --- a/kernel/nstree.c
> +++ b/kernel/nstree.c
> @@ -2,11 +2,15 @@
>  
>  #include <linux/nstree.h>
>  #include <linux/proc_ns.h>
> +#include <linux/rculist.h>
> +#include <linux/syscalls.h>
>  #include <linux/vfsdebug.h>
>  #include <linux/user_namespace.h>
> +#include <linux/rcupdate_wait.h>
>  
>  __cacheline_aligned_in_smp DEFINE_SEQLOCK(ns_tree_lock);
>  static struct rb_root ns_unified_tree = RB_ROOT; /* protected by ns_tree_lock */
> +static LIST_HEAD(ns_unified_list); /* protected by ns_tree_lock */
>  
>  /**
>   * struct ns_tree - Namespace tree
> @@ -83,6 +87,13 @@ static inline struct ns_common *node_to_ns_unified(const struct rb_node *node)
>  	return rb_entry(node, struct ns_common, ns_unified_tree_node);
>  }
>  
> +static inline struct ns_common *node_to_ns_owner(const struct rb_node *node)
> +{
> +	if (!node)
> +		return NULL;
> +	return rb_entry(node, struct ns_common, ns_owner_tree_node);
> +}
> +
>  static inline int ns_cmp(struct rb_node *a, const struct rb_node *b)
>  {
>  	struct ns_common *ns_a = node_to_ns(a);
> @@ -111,6 +122,20 @@ static inline int ns_cmp_unified(struct rb_node *a, const struct rb_node *b)
>  	return 0;
>  }
>  
> +static inline int ns_cmp_owner(struct rb_node *a, const struct rb_node *b)
> +{
> +	struct ns_common *ns_a = node_to_ns_owner(a);
> +	struct ns_common *ns_b = node_to_ns_owner(b);
> +	u64 ns_id_a = ns_a->ns_id;
> +	u64 ns_id_b = ns_b->ns_id;
> +
> +	if (ns_id_a < ns_id_b)
> +		return -1;
> +	if (ns_id_a > ns_id_b)
> +		return 1;
> +	return 0;
> +}
> +
>  void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
>  {
>  	struct rb_node *node, *prev;
> @@ -134,7 +159,13 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
>  	else
>  		list_add_rcu(&ns->ns_list_node, &node_to_ns(prev)->ns_list_node);
>  
> +	/* Add to unified tree and list */
>  	rb_find_add_rcu(&ns->ns_unified_tree_node, &ns_unified_tree, ns_cmp_unified);
> +	prev = rb_prev(&ns->ns_unified_tree_node);
> +	if (!prev)
> +		list_add_rcu(&ns->ns_unified_list_node, &ns_unified_list);
> +	else
> +		list_add_rcu(&ns->ns_unified_list_node, &node_to_ns_unified(prev)->ns_unified_list_node);
>  
>  	if (ops) {
>  		struct user_namespace *user_ns;
> @@ -144,7 +175,16 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
>  		if (user_ns) {
>  			struct ns_common *owner = &user_ns->ns;
>  			VFS_WARN_ON_ONCE(owner->ns_type != CLONE_NEWUSER);
> -			list_add_tail_rcu(&ns->ns_owner_entry, &owner->ns_owner);
> +
> +			/* Insert into owner's rbtree */
> +			rb_find_add_rcu(&ns->ns_owner_tree_node, &owner->ns_owner_tree, ns_cmp_owner);
> +
> +			/* Insert into owner's list in sorted order */
> +			prev = rb_prev(&ns->ns_owner_tree_node);
> +			if (!prev)
> +				list_add_rcu(&ns->ns_owner_entry, &owner->ns_owner);
> +			else
> +				list_add_rcu(&ns->ns_owner_entry, &node_to_ns_owner(prev)->ns_owner_entry);
>  		} else {
>  			/* Only the initial user namespace doesn't have an owner. */
>  			VFS_WARN_ON_ONCE(ns != to_ns_common(&init_user_ns));
> @@ -157,16 +197,36 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
>  
>  void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree)
>  {
> +	const struct proc_ns_operations *ops = ns->ops;
> +	struct user_namespace *user_ns;
> +
>  	VFS_WARN_ON_ONCE(RB_EMPTY_NODE(&ns->ns_tree_node));
>  	VFS_WARN_ON_ONCE(list_empty(&ns->ns_list_node));
>  	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
>  
>  	write_seqlock(&ns_tree_lock);
>  	rb_erase(&ns->ns_tree_node, &ns_tree->ns_tree);
> -	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
> -	list_bidir_del_rcu(&ns->ns_list_node);
>  	RB_CLEAR_NODE(&ns->ns_tree_node);
> -	list_bidir_del_rcu(&ns->ns_owner_entry);
> +
> +	list_bidir_del_rcu(&ns->ns_list_node);
> +
> +	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
> +	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
> +
> +	list_bidir_del_rcu(&ns->ns_unified_list_node);
> +
> +	/* Remove from owner's rbtree if this namespace has an owner */
> +	if (ops) {
> +		user_ns = ops->owner(ns);
> +		if (user_ns) {
> +			struct ns_common *owner = &user_ns->ns;
> +			rb_erase(&ns->ns_owner_tree_node, &owner->ns_owner_tree);
> +			RB_CLEAR_NODE(&ns->ns_owner_tree_node);
> +		}
> +
> +		list_bidir_del_rcu(&ns->ns_owner_entry);
> +	}
> +
>  	write_sequnlock(&ns_tree_lock);
>  }

The above changes in this file seem like they ought to be part of a
different patch. This is adding 

>  EXPORT_SYMBOL_GPL(__ns_tree_remove);
> @@ -312,3 +372,352 @@ u64 __ns_tree_gen_id(struct ns_common *ns, u64 id)
>  		ns->ns_id = atomic64_inc_return(&namespace_cookie);
>  	return ns->ns_id;
>  }
> +
> +struct klistns {
> +	u64 *kns_ids;
> +	u32 nr_ns_ids;
> +	u64 last_ns_id;
> +	u64 user_ns_id;
> +	u32 ns_type;
> +	struct user_namespace *user_ns;
> +	struct ns_common *first_ns;
> +};
> +
> +static void __free_klistns_free(const struct klistns *kls)
> +{
> +	if (kls->user_ns_id != LISTNS_CURRENT_USER)
> +		put_user_ns(kls->user_ns);
> +	if (kls->first_ns)
> +		kls->first_ns->ops->put(kls->first_ns);
> +	kvfree(kls->kns_ids);
> +}
> +
> +#define NS_ALL (PID_NS | USER_NS | MNT_NS | UTS_NS | IPC_NS | NET_NS | CGROUP_NS | TIME_NS)
> +
> +static int copy_ns_id_req(const struct ns_id_req __user *req,
> +			  struct ns_id_req *kreq)
> +{
> +	int ret;
> +	size_t usize;
> +
> +	BUILD_BUG_ON(sizeof(struct ns_id_req) != NS_ID_REQ_SIZE_VER0);
> +
> +	ret = get_user(usize, &req->size);
> +	if (ret)
> +		return -EFAULT;
> +	if (unlikely(usize > PAGE_SIZE))
> +		return -E2BIG;
> +	if (unlikely(usize < NS_ID_REQ_SIZE_VER0))
> +		return -EINVAL;
> +	memset(kreq, 0, sizeof(*kreq));
> +	ret = copy_struct_from_user(kreq, sizeof(*kreq), req, usize);
> +	if (ret)
> +		return ret;
> +	if (kreq->spare != 0)
> +		return -EINVAL;
> +	if (kreq->ns_type & ~NS_ALL)
> +		return -EOPNOTSUPP;
> +	return 0;
> +}
> +
> +static inline int prepare_klistns(struct klistns *kls, struct ns_id_req *kreq,
> +				  size_t nr_ns_ids)
> +{
> +	kls->last_ns_id = kreq->ns_id;
> +	kls->user_ns_id = kreq->user_ns_id;
> +	kls->nr_ns_ids = nr_ns_ids;
> +	kls->ns_type = kreq->ns_type;
> +
> +	kls->kns_ids = kvmalloc_array(nr_ns_ids, sizeof(*kls->kns_ids),
> +				      GFP_KERNEL_ACCOUNT);
> +	if (!kls->kns_ids)
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +
> +/*
> + * Lookup a namespace owned by owner with id >= ns_id.
> + * Returns the namespace with the smallest id that is >= ns_id.
> + */
> +static struct ns_common *lookup_ns_owner_at(u64 ns_id, struct ns_common *owner)
> +{
> +	struct ns_common *ret = NULL;
> +	struct rb_node *node;
> +
> +	VFS_WARN_ON_ONCE(owner->ns_type != CLONE_NEWUSER);
> +
> +	read_seqlock_excl(&ns_tree_lock);
> +	node = owner->ns_owner_tree.rb_node;
> +
> +	while (node) {
> +		struct ns_common *ns = node_to_ns_owner(node);
> +
> +		if (ns_id <= ns->ns_id) {
> +			ret = ns;
> +			if (ns_id == ns->ns_id)
> +				break;
> +			node = node->rb_left;
> +		} else {
> +			node = node->rb_right;
> +		}
> +	}
> +
> +	if (ret && !ns_get_unless_inactive(ret))
> +		ret = NULL;
> +	read_sequnlock_excl(&ns_tree_lock);
> +	return ret;
> +}
> +
> +static struct ns_common *lookup_ns_id(u64 mnt_ns_id, int ns_type)
> +{
> +	struct ns_common *ns;
> +
> +	guard(rcu)();
> +	ns = ns_tree_lookup_rcu(mnt_ns_id, ns_type);
> +	if (!ns)
> +		return NULL;
> +
> +	if (!ns_get_unless_inactive(ns))
> +		return NULL;
> +
> +	return ns;
> +}
> +
> +static ssize_t do_listns_userns(struct klistns *kls)
> +{
> +	u64 *ns_ids = kls->kns_ids;
> +	size_t nr_ns_ids = kls->nr_ns_ids;
> +	struct ns_common *ns = NULL, *first_ns = NULL;
> +	const struct list_head *head;
> +	bool userns_capable;
> +	ssize_t ret;
> +
> +	VFS_WARN_ON_ONCE(!kls->user_ns_id);
> +
> +	if (kls->user_ns_id == LISTNS_CURRENT_USER)
> +		ns = to_ns_common(current_user_ns());
> +	else if (kls->user_ns_id)
> +		ns = lookup_ns_id(kls->user_ns_id, CLONE_NEWUSER);
> +	if (!ns)
> +		return -EINVAL;
> +	kls->user_ns = to_user_ns(ns);
> +
> +	/*
> +	 * Use the rbtree to find the first namespace we care about and
> +	 * then use it's list entry to iterate from there.
> +	 */
> +	if (kls->last_ns_id) {
> +		kls->first_ns = lookup_ns_owner_at(kls->last_ns_id + 1, ns);
> +		if (!kls->first_ns)
> +			return -ENOENT;
> +		first_ns = kls->first_ns;
> +	}
> +
> +	ret = 0;
> +	head = &to_ns_common(kls->user_ns)->ns_owner;
> +	userns_capable = ns_capable_noaudit(kls->user_ns, CAP_SYS_ADMIN);
> +	guard(rcu)();
> +	if (!first_ns)
> +		first_ns = list_entry_rcu(head->next, typeof(*ns), ns_owner_entry);
> +	for (ns = first_ns; &ns->ns_owner_entry != head && nr_ns_ids;
> +	     ns = list_entry_rcu(ns->ns_owner_entry.next, typeof(*ns), ns_owner_entry)) {
> +		if (kls->ns_type && !(kls->ns_type & ns->ns_type))
> +			continue;
> +		if (!ns_get_unless_inactive(ns))
> +			continue;
> +		if (userns_capable || is_current_namespace(ns) ||
> +		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
> +			*ns_ids = ns->ns_id;
> +			ns_ids++;
> +			nr_ns_ids--;
> +			ret++;
> +		}
> +		if (need_resched())
> +			cond_resched_rcu();
> +		/* doesn't sleep */
> +		ns->ops->put(ns);
> +	}
> +
> +	return ret;
> +}
> +
> +/*
> + * Lookup a namespace with id >= ns_id in either the unified tree or a type-specific tree.
> + * Returns the namespace with the smallest id that is >= ns_id.
> + */
> +static struct ns_common *lookup_ns_id_at(u64 ns_id, int ns_type)
> +{
> +	struct ns_common *ret = NULL;
> +	struct ns_tree *ns_tree = NULL;
> +	struct rb_node *node;
> +
> +	if (ns_type) {
> +		ns_tree = ns_tree_from_type(ns_type);
> +		if (!ns_tree)
> +			return NULL;
> +	}
> +
> +	read_seqlock_excl(&ns_tree_lock);
> +	if (ns_tree)
> +		node = ns_tree->ns_tree.rb_node;
> +	else
> +		node = ns_unified_tree.rb_node;
> +
> +	while (node) {
> +		struct ns_common *ns;
> +
> +		if (ns_type)
> +			ns = node_to_ns(node);
> +		else
> +			ns = node_to_ns_unified(node);
> +
> +		if (ns_id <= ns->ns_id) {
> +			if (ns_type)
> +				ret = node_to_ns(node);
> +			else
> +				ret = node_to_ns_unified(node);
> +			if (ns_id == ns->ns_id)
> +				break;
> +			node = node->rb_left;
> +		} else {
> +			node = node->rb_right;
> +		}
> +	}
> +
> +	if (ret && !ns_get_unless_inactive(ret))
> +		ret = NULL;
> +	read_sequnlock_excl(&ns_tree_lock);
> +	return ret;
> +}
> +
> +static inline struct ns_common *first_ns_common(const struct list_head *head,
> +						struct ns_tree *ns_tree)
> +{
> +	if (ns_tree)
> +		return list_entry_rcu(head->next, struct ns_common, ns_list_node);
> +	return list_entry_rcu(head->next, struct ns_common, ns_unified_list_node);
> +}
> +
> +static inline struct ns_common *next_ns_common(struct ns_common *ns,
> +					       struct ns_tree *ns_tree)
> +{
> +	if (ns_tree)
> +		return list_entry_rcu(ns->ns_list_node.next, struct ns_common, ns_list_node);
> +	return list_entry_rcu(ns->ns_unified_list_node.next, struct ns_common, ns_unified_list_node);
> +}
> +
> +static inline bool ns_common_is_head(struct ns_common *ns,
> +				     const struct list_head *head,
> +				     struct ns_tree *ns_tree)
> +{
> +	if (ns_tree)
> +		return &ns->ns_list_node == head;
> +	return &ns->ns_unified_list_node == head;
> +}
> +
> +static ssize_t do_listns(struct klistns *kls)
> +{
> +	u64 *ns_ids = kls->kns_ids;
> +	size_t nr_ns_ids = kls->nr_ns_ids;
> +	struct ns_common *ns, *first_ns = NULL;
> +	struct ns_tree *ns_tree = NULL;
> +	const struct list_head *head;
> +	struct user_namespace *user_ns;
> +	u32 ns_type;
> +	ssize_t ret;
> +
> +	if (hweight32(kls->ns_type) == 1)
> +		ns_type = kls->ns_type;
> +	else
> +		ns_type = 0;
> +
> +	if (ns_type) {
> +		ns_tree = ns_tree_from_type(ns_type);
> +		if (!ns_tree)
> +			return -EINVAL;
> +	}
> +
> +	if (kls->last_ns_id) {
> +		kls->first_ns = lookup_ns_id_at(kls->last_ns_id + 1, ns_type);
> +		if (!kls->first_ns)
> +			return -ENOENT;
> +		first_ns = kls->first_ns;
> +	}
> +
> +	ret = 0;
> +	if (ns_tree)
> +		head = &ns_tree->ns_list;
> +	else
> +		head = &ns_unified_list;
> +
> +	guard(rcu)();
> +	if (!first_ns)
> +		first_ns = first_ns_common(head, ns_tree);
> +
> +	for (ns = first_ns; !ns_common_is_head(ns, head, ns_tree) && nr_ns_ids;
> +	     ns = next_ns_common(ns, ns_tree)) {
> +		if (kls->ns_type && !(kls->ns_type & ns->ns_type))
> +			continue;
> +		if (!ns_get_unless_inactive(ns))
> +			continue;
> +		/* Check permissions */
> +		if (!ns->ops)
> +			user_ns = NULL;
> +		else
> +			user_ns = ns->ops->owner(ns);
> +		if (!user_ns)
> +			user_ns = &init_user_ns;
> +		if (ns_capable_noaudit(user_ns, CAP_SYS_ADMIN) ||
> +		    is_current_namespace(ns) ||
> +		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
> +			*ns_ids++ = ns->ns_id;
> +			nr_ns_ids--;
> +			ret++;
> +		}
> +		if (need_resched())
> +			cond_resched_rcu();
> +		/* doesn't sleep */
> +		ns->ops->put(ns);
> +	}
> +
> +	return ret;
> +}
> +
> +SYSCALL_DEFINE4(listns, const struct ns_id_req __user *, req,
> +		u64 __user *, ns_ids, size_t, nr_ns_ids, unsigned int, flags)
> +{
> +	struct klistns klns __free(klistns_free) = {};
> +	const size_t maxcount = 1000000;
> +	struct ns_id_req kreq;
> +	ssize_t ret;
> +
> +	if (flags)
> +		return -EINVAL;
> +
> +	if (unlikely(nr_ns_ids > maxcount))
> +		return -EOVERFLOW;
> +
> +	if (!access_ok(ns_ids, nr_ns_ids * sizeof(*ns_ids)))
> +		return -EFAULT;
> +
> +	ret = copy_ns_id_req(req, &kreq);
> +	if (ret)
> +		return ret;
> +
> +	ret = prepare_klistns(&klns, &kreq, nr_ns_ids);
> +	if (ret)
> +		return ret;
> +
> +	if (kreq.user_ns_id)
> +		ret = do_listns_userns(&klns);
> +	else
> +		ret = do_listns(&klns);
> +	if (ret <= 0)
> +		return ret;
> +
> +	if (copy_to_user(ns_ids, klns.kns_ids, ret * sizeof(*ns_ids)))
> +		return -EFAULT;
> +
> +	return ret;
> +}
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 8134c40b2584..22a0440a62fa 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -80,6 +80,7 @@ struct pid_namespace init_pid_ns = {
>  	.user_ns = &init_user_ns,
>  	.ns.inum = ns_init_inum(&init_pid_ns),
>  	.ns.ns_list_node = LIST_HEAD_INIT(init_pid_ns.ns.ns_list_node),
> +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_pid_ns.ns.ns_unified_list_node),
>  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_pid_ns.ns.ns_owner_entry),
>  	.ns.ns_owner = LIST_HEAD_INIT(init_pid_ns.ns.ns_owner),
>  #ifdef CONFIG_PID_NS
> diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
> index 15cb74267c75..acbeec049263 100644
> --- a/kernel/time/namespace.c
> +++ b/kernel/time/namespace.c
> @@ -489,6 +489,7 @@ struct time_namespace init_time_ns = {
>  	.ns.ns_owner = LIST_HEAD_INIT(init_time_ns.ns.ns_owner),
>  	.frozen_offsets	= true,
>  	.ns.ns_list_node = LIST_HEAD_INIT(init_time_ns.ns.ns_list_node),
> +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_time_ns.ns.ns_unified_list_node),
>  };
>  
>  void __init time_ns_init(void)
> diff --git a/kernel/user.c b/kernel/user.c
> index e392768ccd44..68fe16617d38 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -72,6 +72,7 @@ struct user_namespace init_user_ns = {
>  	.group = GLOBAL_ROOT_GID,
>  	.ns.inum = ns_init_inum(&init_user_ns),
>  	.ns.ns_list_node = LIST_HEAD_INIT(init_user_ns.ns.ns_list_node),
> +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_user_ns.ns.ns_unified_list_node),
>  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_user_ns.ns.ns_owner_entry),
>  	.ns.ns_owner = LIST_HEAD_INIT(init_user_ns.ns.ns_owner),
>  #ifdef CONFIG_USER_NS

-- 
Jeff Layton <jlayton@kernel.org>

Re: [PATCH v3 17/70] nstree: add listns()

[Christian Brauner]

On Mon, Oct 27, 2025 at 10:04:15AM -0400, Jeff Layton wrote:
> On Fri, 2025-10-24 at 12:52 +0200, Christian Brauner wrote:
> > Add a new listns() system call that allows userspace to iterate through
> > namespaces in the system. This provides a programmatic interface to
> > discover and inspect namespaces, enhancing existing namespace apis.
> > 
> > Currently, there is no direct way for userspace to enumerate namespaces
> > in the system. Applications must resort to scanning /proc/<pid>/ns/
> > across all processes, which is:
> > 
> > 1. Inefficient - requires iterating over all processes
> > 2. Incomplete - misses inactive namespaces that aren't attached to any
> >    running process but are kept alive by file descriptors, bind mounts,
> >    or parent namespace references
> > 3. Permission-heavy - requires access to /proc for many processes
> > 4. No ordering or ownership.
> > 5. No filtering per namespace type: Must always iterate and check all
> >    namespaces.
> > 
> > The list goes on. The listns() system call solves these problems by
> > providing direct kernel-level enumeration of namespaces. It is similar
> > to listmount() but obviously tailored to namespaces.
> > 
> > /*
> >  * @req: Pointer to struct ns_id_req specifying search parameters
> >  * @ns_ids: User buffer to receive namespace IDs
> >  * @nr_ns_ids: Size of ns_ids buffer (maximum number of IDs to return)
> >  * @flags: Reserved for future use (must be 0)
> >  */
> > ssize_t listns(const struct ns_id_req *req, u64 *ns_ids,
> >                size_t nr_ns_ids, unsigned int flags);
> > 
> > Returns:
> > - On success: Number of namespace IDs written to ns_ids
> > - On error: Negative error code
> > 
> > /*
> >  * @size: Structure size
> >  * @ns_id: Starting point for iteration; use 0 for first call, then
> >  *         use the last returned ID for subsequent calls to paginate
> >  * @ns_type: Bitmask of namespace types to include (from enum ns_type):
> >  *           0: Return all namespace types
> >  *           MNT_NS: Mount namespaces
> >  *           NET_NS: Network namespaces
> >  *           USER_NS: User namespaces
> >  *           etc. Can be OR'd together
> >  * @user_ns_id: Filter results to namespaces owned by this user namespace:
> >  *              0: Return all namespaces (subject to permission checks)
> >  *              LISTNS_CURRENT_USER: Namespaces owned by caller's user namespace
> >  *              Other value: Namespaces owned by the specified user namespace ID
> >  */
> > struct ns_id_req {
> >         __u32 size;         /* sizeof(struct ns_id_req) */
> >         __u32 spare;        /* Reserved, must be 0 */
> >         __u64 ns_id;        /* Last seen namespace ID (for pagination) */
> >         __u32 ns_type;      /* Filter by namespace type(s) */
> >         __u32 spare2;       /* Reserved, must be 0 */
> >         __u64 user_ns_id;   /* Filter by owning user namespace */
> > };
> > 
> > Example 1: List all namespaces
> > 
> > void list_all_namespaces(void)
> > {
> >     struct ns_id_req req = {
> >         .size = sizeof(req),
> >         .ns_id = 0,          /* Start from beginning */
> >         .ns_type = 0,        /* All types */
> >         .user_ns_id = 0,     /* All user namespaces */
> >     };
> >     uint64_t ids[100];
> >     ssize_t ret;
> > 
> >     printf("All namespaces in the system:\n");
> >     do {
> >         ret = listns(&req, ids, 100, 0);
> >         if (ret < 0) {
> >             perror("listns");
> >             break;
> >         }
> > 
> >         for (ssize_t i = 0; i < ret; i++)
> >             printf("  Namespace ID: %llu\n", (unsigned long long)ids[i]);
> > 
> >         /* Continue from last seen ID */
> >         if (ret > 0)
> >             req.ns_id = ids[ret - 1];
> >     } while (ret == 100);  /* Buffer was full, more may exist */
> > }
> > 
> > Example 2: List network namespaces only
> > 
> > void list_network_namespaces(void)
> > {
> >     struct ns_id_req req = {
> >         .size = sizeof(req),
> >         .ns_id = 0,
> >         .ns_type = NET_NS,   /* Only network namespaces */
> >         .user_ns_id = 0,
> >     };
> >     uint64_t ids[100];
> >     ssize_t ret;
> > 
> >     ret = listns(&req, ids, 100, 0);
> >     if (ret < 0) {
> >         perror("listns");
> >         return;
> >     }
> > 
> >     printf("Network namespaces: %zd found\n", ret);
> >     for (ssize_t i = 0; i < ret; i++)
> >         printf("  netns ID: %llu\n", (unsigned long long)ids[i]);
> > }
> > 
> > Example 3: List namespaces owned by current user namespace
> > 
> > void list_owned_namespaces(void)
> > {
> >     struct ns_id_req req = {
> >         .size = sizeof(req),
> >         .ns_id = 0,
> >         .ns_type = 0,                      /* All types */
> >         .user_ns_id = LISTNS_CURRENT_USER, /* Current userns */
> >     };
> >     uint64_t ids[100];
> >     ssize_t ret;
> > 
> >     ret = listns(&req, ids, 100, 0);
> >     if (ret < 0) {
> >         perror("listns");
> >         return;
> >     }
> > 
> >     printf("Namespaces owned by my user namespace: %zd\n", ret);
> >     for (ssize_t i = 0; i < ret; i++)
> >         printf("  ns ID: %llu\n", (unsigned long long)ids[i]);
> > }
> > 
> > Example 4: List multiple namespace types
> > 
> > void list_network_and_mount_namespaces(void)
> > {
> >     struct ns_id_req req = {
> >         .size = sizeof(req),
> >         .ns_id = 0,
> >         .ns_type = NET_NS | MNT_NS,  /* Network and mount */
> >         .user_ns_id = 0,
> >     };
> >     uint64_t ids[100];
> >     ssize_t ret;
> > 
> >     ret = listns(&req, ids, 100, 0);
> >     printf("Network and mount namespaces: %zd found\n", ret);
> > }
> > 
> > Example 5: Pagination through large namespace sets
> > 
> > void list_all_with_pagination(void)
> > {
> >     struct ns_id_req req = {
> >         .size = sizeof(req),
> >         .ns_id = 0,
> >         .ns_type = 0,
> >         .user_ns_id = 0,
> >     };
> >     uint64_t ids[50];
> >     size_t total = 0;
> >     ssize_t ret;
> > 
> >     printf("Enumerating all namespaces with pagination:\n");
> > 
> >     while (1) {
> >         ret = listns(&req, ids, 50, 0);
> >         if (ret < 0) {
> >             perror("listns");
> >             break;
> >         }
> >         if (ret == 0)
> >             break;  /* No more namespaces */
> > 
> >         total += ret;
> >         printf("  Batch: %zd namespaces\n", ret);
> > 
> >         /* Last ID in this batch becomes start of next batch */
> >         req.ns_id = ids[ret - 1];
> > 
> >         if (ret < 50)
> >             break;  /* Partial batch = end of results */
> >     }
> > 
> >     printf("Total: %zu namespaces\n", total);
> > }
> > 
> > Permission Model
> > 
> > listns() respects namespace isolation and capabilities:
> > 
> > (1) Global listing (user_ns_id = 0):
> >     - Requires CAP_SYS_ADMIN in the namespace's owning user namespace
> >     - OR the namespace must be in the caller's namespace context (e.g.,
> >       a namespace the caller is currently using)
> >     - User namespaces additionally allow listing if the caller has
> >       CAP_SYS_ADMIN in that user namespace itself
> > (2) Owner-filtered listing (user_ns_id != 0):
> >     - Requires CAP_SYS_ADMIN in the specified owner user namespace
> >     - OR the namespace must be in the caller's namespace context
> >     - This allows unprivileged processes to enumerate namespaces they own
> > (3) Visibility:
> >     - Only "active" namespaces are listed
> >     - A namespace is active if it has a non-zero __ns_ref_active count
> >     - This includes namespaces used by running processes, held by open
> >       file descriptors, or kept active by bind mounts
> >     - Inactive namespaces (kept alive only by internal kernel
> >       references) are not visible via listns()
> > 
> > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > ---
> >  fs/namespace.c                 |   1 +
> >  fs/nsfs.c                      |  39 ++++
> >  include/linux/ns_common.h      |   5 +-
> >  include/linux/syscalls.h       |   4 +
> >  include/linux/user_namespace.h |   4 +-
> >  include/uapi/linux/nsfs.h      |  44 +++++
> >  init/version-timestamp.c       |   1 +
> >  ipc/msgutil.c                  |   1 +
> >  kernel/cgroup/cgroup.c         |   1 +
> >  kernel/nscommon.c              |   3 +
> >  kernel/nstree.c                | 417 ++++++++++++++++++++++++++++++++++++++++-
> >  kernel/pid.c                   |   1 +
> >  kernel/time/namespace.c        |   1 +
> >  kernel/user.c                  |   1 +
> >  14 files changed, 516 insertions(+), 7 deletions(-)
> > 
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index d460ca79f0e7..980296b0ec86 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -5996,6 +5996,7 @@ struct mnt_namespace init_mnt_ns = {
> >  	.mounts		= RB_ROOT,
> >  	.poll		= __WAIT_QUEUE_HEAD_INITIALIZER(init_mnt_ns.poll),
> >  	.ns.ns_list_node = LIST_HEAD_INIT(init_mnt_ns.ns.ns_list_node),
> > +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_mnt_ns.ns.ns_unified_list_node),
> >  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_mnt_ns.ns.ns_owner_entry),
> >  	.ns.ns_owner = LIST_HEAD_INIT(init_mnt_ns.ns.ns_owner),
> >  };
> > diff --git a/fs/nsfs.c b/fs/nsfs.c
> > index 19dc28742a42..5c21fdc79796 100644
> > --- a/fs/nsfs.c
> > +++ b/fs/nsfs.c
> > @@ -471,6 +471,45 @@ static int nsfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
> >  	return FILEID_NSFS;
> >  }
> >  
> > +bool is_current_namespace(struct ns_common *ns)
> > +{
> > +	switch (ns->ns_type) {
> > +#ifdef CONFIG_CGROUPS
> > +	case CLONE_NEWCGROUP:
> > +		return current_in_namespace(to_cg_ns(ns));
> > +#endif
> > +#ifdef CONFIG_IPC_NS
> > +	case CLONE_NEWIPC:
> > +		return current_in_namespace(to_ipc_ns(ns));
> > +#endif
> > +	case CLONE_NEWNS:
> > +		return current_in_namespace(to_mnt_ns(ns));
> > +#ifdef CONFIG_NET_NS
> > +	case CLONE_NEWNET:
> > +		return current_in_namespace(to_net_ns(ns));
> > +#endif
> > +#ifdef CONFIG_PID_NS
> > +	case CLONE_NEWPID:
> > +		return current_in_namespace(to_pid_ns(ns));
> > +#endif
> > +#ifdef CONFIG_TIME_NS
> > +	case CLONE_NEWTIME:
> > +		return current_in_namespace(to_time_ns(ns));
> > +#endif
> > +#ifdef CONFIG_USER_NS
> > +	case CLONE_NEWUSER:
> > +		return current_in_namespace(to_user_ns(ns));
> > +#endif
> > +#ifdef CONFIG_UTS_NS
> > +	case CLONE_NEWUTS:
> > +		return current_in_namespace(to_uts_ns(ns));
> > +#endif
> > +	default:
> > +		VFS_WARN_ON_ONCE(true);
> > +		return false;
> > +	}
> > +}
> > +
> >  static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
> >  					int fh_len, int fh_type)
> >  {
> > diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
> > index 88dce67e06e4..95b3e2aa177d 100644
> > --- a/include/linux/ns_common.h
> > +++ b/include/linux/ns_common.h
> > @@ -123,8 +123,10 @@ struct ns_common {
> >  				struct rb_node ns_tree_node;
> >  				struct list_head ns_list_node;
> >  			};
> > -			struct /* namespace ownership list */ {
> > +			struct /* namespace ownership rbtree and list */ {
> > +				struct rb_root ns_owner_tree; /* rbtree of namespaces owned by this namespace */
> >  				struct list_head ns_owner; /* list of namespaces owned by this namespace */
> > +				struct rb_node ns_owner_tree_node; /* node in the owner namespace's rbtree */
> 
> 
> The changelog doesn't mention why these fields are being added.
> 
> I'd prefer to see the parts of this patch that deal with the above
> fields added in a separate patch. I'm guessing that you added them here
> because this patch adds the need to check ns ownership for listns(),
> but I think it warrants a separate patch and changelog explaining
> what's going on.

Ok, no problem, I can do that.

> 
> >  				struct list_head ns_owner_entry; /* node in the owner namespace's ns_owned list */
> >  			};
> >  			atomic_t __ns_ref_active; /* do not use directly */
> > @@ -133,6 +135,7 @@ struct ns_common {
> >  	};
> >  };
> >  
> > +bool is_current_namespace(struct ns_common *ns);
> >  int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_operations *ops, int inum);
> >  void __ns_common_free(struct ns_common *ns);
> >  
> > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> > index 66c06fcdfe19..cf84d98964b2 100644
> > --- a/include/linux/syscalls.h
> > +++ b/include/linux/syscalls.h
> > @@ -77,6 +77,7 @@ struct cachestat_range;
> >  struct cachestat;
> >  struct statmount;
> >  struct mnt_id_req;
> > +struct ns_id_req;
> >  struct xattr_args;
> >  struct file_attr;
> >  
> > @@ -437,6 +438,9 @@ asmlinkage long sys_statmount(const struct mnt_id_req __user *req,
> >  asmlinkage long sys_listmount(const struct mnt_id_req __user *req,
> >  			      u64 __user *mnt_ids, size_t nr_mnt_ids,
> >  			      unsigned int flags);
> > +asmlinkage long sys_listns(const struct ns_id_req __user *req,
> > +			   u64 __user *ns_ids, size_t nr_ns_ids,
> > +			   unsigned int flags);
> >  asmlinkage long sys_truncate(const char __user *path, long length);
> >  asmlinkage long sys_ftruncate(unsigned int fd, off_t length);
> >  #if BITS_PER_LONG == 32
> > diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> > index 9a9aebbf96b9..9c3be157397e 100644
> > --- a/include/linux/user_namespace.h
> > +++ b/include/linux/user_namespace.h
> > @@ -166,13 +166,13 @@ static inline void set_userns_rlimit_max(struct user_namespace *ns,
> >  	ns->rlimit_max[type] = max <= LONG_MAX ? max : LONG_MAX;
> >  }
> >  
> > -#ifdef CONFIG_USER_NS
> > -
> >  static inline struct user_namespace *to_user_ns(struct ns_common *ns)
> >  {
> >  	return container_of(ns, struct user_namespace, ns);
> >  }
> >  
> > +#ifdef CONFIG_USER_NS
> > +
> >  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
> >  {
> >  	if (ns)
> > diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
> > index f8bc2aad74d6..a25e38d1c874 100644
> > --- a/include/uapi/linux/nsfs.h
> > +++ b/include/uapi/linux/nsfs.h
> > @@ -81,4 +81,48 @@ enum init_ns_id {
> >  #endif
> >  };
> >  
> > +enum ns_type {
> > +	TIME_NS    = (1ULL << 7),  /* CLONE_NEWTIME */
> > +	MNT_NS     = (1ULL << 17), /* CLONE_NEWNS */
> > +	CGROUP_NS  = (1ULL << 25), /* CLONE_NEWCGROUP */
> > +	UTS_NS     = (1ULL << 26), /* CLONE_NEWUTS */
> > +	IPC_NS     = (1ULL << 27), /* CLONE_NEWIPC */
> > +	USER_NS    = (1ULL << 28), /* CLONE_NEWUSER */
> > +	PID_NS     = (1ULL << 29), /* CLONE_NEWPID */
> > +	NET_NS     = (1ULL << 30), /* CLONE_NEWNET */
> > +};
> > +
> > +/**
> > + * struct ns_id_req - namespace ID request structure
> > + * @size: size of this structure
> > + * @spare: reserved for future use
> > + * @filter: filter mask
> > + * @ns_id: last namespace id
> > + * @user_ns_id: owning user namespace ID
> > + *
> > + * Structure for passing namespace ID and miscellaneous parameters to
> > + * statns(2) and listns(2).
> > + *
> > + * For statns(2) @param represents the request mask.
> > + * For listns(2) @param represents the last listed mount id (or zero).
> > + */
> > +struct ns_id_req {
> > +	__u32 size;
> > +	__u32 spare;
> > +	__u64 ns_id;
> > +	struct /* listns */ {
> > +		__u32 ns_type;
> > +		__u32 spare2;
> > +		__u64 user_ns_id;
> > +	};
> > +};
> > +
> > +/*
> > + * Special @user_ns_id value that can be passed to listns()
> > + */
> > +#define LISTNS_CURRENT_USER 0xffffffffffffffff /* Caller's userns */
> > +
> > +/* List of all ns_id_req versions. */
> > +#define NS_ID_REQ_SIZE_VER0 32 /* sizeof first published struct */
> > +
> >  #endif /* __LINUX_NSFS_H */
> > diff --git a/init/version-timestamp.c b/init/version-timestamp.c
> > index e5c278dabecf..cd6f435d5fde 100644
> > --- a/init/version-timestamp.c
> > +++ b/init/version-timestamp.c
> > @@ -22,6 +22,7 @@ struct uts_namespace init_uts_ns = {
> >  	.user_ns = &init_user_ns,
> >  	.ns.inum = ns_init_inum(&init_uts_ns),
> >  	.ns.ns_list_node = LIST_HEAD_INIT(init_uts_ns.ns.ns_list_node),
> > +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_uts_ns.ns.ns_unified_list_node),
> >  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_uts_ns.ns.ns_owner_entry),
> >  	.ns.ns_owner = LIST_HEAD_INIT(init_uts_ns.ns.ns_owner),
> >  #ifdef CONFIG_UTS_NS
> > diff --git a/ipc/msgutil.c b/ipc/msgutil.c
> > index ce1de73725c0..3708f325228d 100644
> > --- a/ipc/msgutil.c
> > +++ b/ipc/msgutil.c
> > @@ -32,6 +32,7 @@ struct ipc_namespace init_ipc_ns = {
> >  	.user_ns = &init_user_ns,
> >  	.ns.inum = ns_init_inum(&init_ipc_ns),
> >  	.ns.ns_list_node = LIST_HEAD_INIT(init_ipc_ns.ns.ns_list_node),
> > +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_ipc_ns.ns.ns_unified_list_node),
> >  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_ipc_ns.ns.ns_owner_entry),
> >  	.ns.ns_owner = LIST_HEAD_INIT(init_ipc_ns.ns.ns_owner),
> >  #ifdef CONFIG_IPC_NS
> > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > index 9fa082e2eb1a..a0eee0785080 100644
> > --- a/kernel/cgroup/cgroup.c
> > +++ b/kernel/cgroup/cgroup.c
> > @@ -258,6 +258,7 @@ struct cgroup_namespace init_cgroup_ns = {
> >  	.root_cset	= &init_css_set,
> >  	.ns.ns_type	= ns_common_type(&init_cgroup_ns),
> >  	.ns.ns_list_node = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_list_node),
> > +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_unified_list_node),
> >  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_owner_entry),
> >  	.ns.ns_owner = LIST_HEAD_INIT(init_cgroup_ns.ns.ns_owner),
> >  };
> > diff --git a/kernel/nscommon.c b/kernel/nscommon.c
> > index ba46de0637c3..def79b549c52 100644
> > --- a/kernel/nscommon.c
> > +++ b/kernel/nscommon.c
> > @@ -62,7 +62,10 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
> >  	ns->ns_type = ns_type;
> >  	RB_CLEAR_NODE(&ns->ns_tree_node);
> >  	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
> > +	RB_CLEAR_NODE(&ns->ns_owner_tree_node);
> >  	INIT_LIST_HEAD(&ns->ns_list_node);
> > +	INIT_LIST_HEAD(&ns->ns_unified_list_node);
> > +	ns->ns_owner_tree = RB_ROOT;
> >  	INIT_LIST_HEAD(&ns->ns_owner);
> >  	INIT_LIST_HEAD(&ns->ns_owner_entry);
> >  
> > diff --git a/kernel/nstree.c b/kernel/nstree.c
> > index 829682bb04a1..5fd50d73f0ae 100644
> > --- a/kernel/nstree.c
> > +++ b/kernel/nstree.c
> > @@ -2,11 +2,15 @@
> >  
> >  #include <linux/nstree.h>
> >  #include <linux/proc_ns.h>
> > +#include <linux/rculist.h>
> > +#include <linux/syscalls.h>
> >  #include <linux/vfsdebug.h>
> >  #include <linux/user_namespace.h>
> > +#include <linux/rcupdate_wait.h>
> >  
> >  __cacheline_aligned_in_smp DEFINE_SEQLOCK(ns_tree_lock);
> >  static struct rb_root ns_unified_tree = RB_ROOT; /* protected by ns_tree_lock */
> > +static LIST_HEAD(ns_unified_list); /* protected by ns_tree_lock */
> >  
> >  /**
> >   * struct ns_tree - Namespace tree
> > @@ -83,6 +87,13 @@ static inline struct ns_common *node_to_ns_unified(const struct rb_node *node)
> >  	return rb_entry(node, struct ns_common, ns_unified_tree_node);
> >  }
> >  
> > +static inline struct ns_common *node_to_ns_owner(const struct rb_node *node)
> > +{
> > +	if (!node)
> > +		return NULL;
> > +	return rb_entry(node, struct ns_common, ns_owner_tree_node);
> > +}
> > +
> >  static inline int ns_cmp(struct rb_node *a, const struct rb_node *b)
> >  {
> >  	struct ns_common *ns_a = node_to_ns(a);
> > @@ -111,6 +122,20 @@ static inline int ns_cmp_unified(struct rb_node *a, const struct rb_node *b)
> >  	return 0;
> >  }
> >  
> > +static inline int ns_cmp_owner(struct rb_node *a, const struct rb_node *b)
> > +{
> > +	struct ns_common *ns_a = node_to_ns_owner(a);
> > +	struct ns_common *ns_b = node_to_ns_owner(b);
> > +	u64 ns_id_a = ns_a->ns_id;
> > +	u64 ns_id_b = ns_b->ns_id;
> > +
> > +	if (ns_id_a < ns_id_b)
> > +		return -1;
> > +	if (ns_id_a > ns_id_b)
> > +		return 1;
> > +	return 0;
> > +}
> > +
> >  void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
> >  {
> >  	struct rb_node *node, *prev;
> > @@ -134,7 +159,13 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
> >  	else
> >  		list_add_rcu(&ns->ns_list_node, &node_to_ns(prev)->ns_list_node);
> >  
> > +	/* Add to unified tree and list */
> >  	rb_find_add_rcu(&ns->ns_unified_tree_node, &ns_unified_tree, ns_cmp_unified);
> > +	prev = rb_prev(&ns->ns_unified_tree_node);
> > +	if (!prev)
> > +		list_add_rcu(&ns->ns_unified_list_node, &ns_unified_list);
> > +	else
> > +		list_add_rcu(&ns->ns_unified_list_node, &node_to_ns_unified(prev)->ns_unified_list_node);
> >  
> >  	if (ops) {
> >  		struct user_namespace *user_ns;
> > @@ -144,7 +175,16 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
> >  		if (user_ns) {
> >  			struct ns_common *owner = &user_ns->ns;
> >  			VFS_WARN_ON_ONCE(owner->ns_type != CLONE_NEWUSER);
> > -			list_add_tail_rcu(&ns->ns_owner_entry, &owner->ns_owner);
> > +
> > +			/* Insert into owner's rbtree */
> > +			rb_find_add_rcu(&ns->ns_owner_tree_node, &owner->ns_owner_tree, ns_cmp_owner);
> > +
> > +			/* Insert into owner's list in sorted order */
> > +			prev = rb_prev(&ns->ns_owner_tree_node);
> > +			if (!prev)
> > +				list_add_rcu(&ns->ns_owner_entry, &owner->ns_owner);
> > +			else
> > +				list_add_rcu(&ns->ns_owner_entry, &node_to_ns_owner(prev)->ns_owner_entry);
> >  		} else {
> >  			/* Only the initial user namespace doesn't have an owner. */
> >  			VFS_WARN_ON_ONCE(ns != to_ns_common(&init_user_ns));
> > @@ -157,16 +197,36 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
> >  
> >  void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree)
> >  {
> > +	const struct proc_ns_operations *ops = ns->ops;
> > +	struct user_namespace *user_ns;
> > +
> >  	VFS_WARN_ON_ONCE(RB_EMPTY_NODE(&ns->ns_tree_node));
> >  	VFS_WARN_ON_ONCE(list_empty(&ns->ns_list_node));
> >  	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
> >  
> >  	write_seqlock(&ns_tree_lock);
> >  	rb_erase(&ns->ns_tree_node, &ns_tree->ns_tree);
> > -	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
> > -	list_bidir_del_rcu(&ns->ns_list_node);
> >  	RB_CLEAR_NODE(&ns->ns_tree_node);
> > -	list_bidir_del_rcu(&ns->ns_owner_entry);
> > +
> > +	list_bidir_del_rcu(&ns->ns_list_node);
> > +
> > +	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
> > +	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
> > +
> > +	list_bidir_del_rcu(&ns->ns_unified_list_node);
> > +
> > +	/* Remove from owner's rbtree if this namespace has an owner */
> > +	if (ops) {
> > +		user_ns = ops->owner(ns);
> > +		if (user_ns) {
> > +			struct ns_common *owner = &user_ns->ns;
> > +			rb_erase(&ns->ns_owner_tree_node, &owner->ns_owner_tree);
> > +			RB_CLEAR_NODE(&ns->ns_owner_tree_node);
> > +		}
> > +
> > +		list_bidir_del_rcu(&ns->ns_owner_entry);
> > +	}
> > +
> >  	write_sequnlock(&ns_tree_lock);
> >  }
> 
> The above changes in this file seem like they ought to be part of a
> different patch. This is adding 
> 
> >  EXPORT_SYMBOL_GPL(__ns_tree_remove);
> > @@ -312,3 +372,352 @@ u64 __ns_tree_gen_id(struct ns_common *ns, u64 id)
> >  		ns->ns_id = atomic64_inc_return(&namespace_cookie);
> >  	return ns->ns_id;
> >  }
> > +
> > +struct klistns {
> > +	u64 *kns_ids;
> > +	u32 nr_ns_ids;
> > +	u64 last_ns_id;
> > +	u64 user_ns_id;
> > +	u32 ns_type;
> > +	struct user_namespace *user_ns;
> > +	struct ns_common *first_ns;
> > +};
> > +
> > +static void __free_klistns_free(const struct klistns *kls)
> > +{
> > +	if (kls->user_ns_id != LISTNS_CURRENT_USER)
> > +		put_user_ns(kls->user_ns);
> > +	if (kls->first_ns)
> > +		kls->first_ns->ops->put(kls->first_ns);
> > +	kvfree(kls->kns_ids);
> > +}
> > +
> > +#define NS_ALL (PID_NS | USER_NS | MNT_NS | UTS_NS | IPC_NS | NET_NS | CGROUP_NS | TIME_NS)
> > +
> > +static int copy_ns_id_req(const struct ns_id_req __user *req,
> > +			  struct ns_id_req *kreq)
> > +{
> > +	int ret;
> > +	size_t usize;
> > +
> > +	BUILD_BUG_ON(sizeof(struct ns_id_req) != NS_ID_REQ_SIZE_VER0);
> > +
> > +	ret = get_user(usize, &req->size);
> > +	if (ret)
> > +		return -EFAULT;
> > +	if (unlikely(usize > PAGE_SIZE))
> > +		return -E2BIG;
> > +	if (unlikely(usize < NS_ID_REQ_SIZE_VER0))
> > +		return -EINVAL;
> > +	memset(kreq, 0, sizeof(*kreq));
> > +	ret = copy_struct_from_user(kreq, sizeof(*kreq), req, usize);
> > +	if (ret)
> > +		return ret;
> > +	if (kreq->spare != 0)
> > +		return -EINVAL;
> > +	if (kreq->ns_type & ~NS_ALL)
> > +		return -EOPNOTSUPP;
> > +	return 0;
> > +}
> > +
> > +static inline int prepare_klistns(struct klistns *kls, struct ns_id_req *kreq,
> > +				  size_t nr_ns_ids)
> > +{
> > +	kls->last_ns_id = kreq->ns_id;
> > +	kls->user_ns_id = kreq->user_ns_id;
> > +	kls->nr_ns_ids = nr_ns_ids;
> > +	kls->ns_type = kreq->ns_type;
> > +
> > +	kls->kns_ids = kvmalloc_array(nr_ns_ids, sizeof(*kls->kns_ids),
> > +				      GFP_KERNEL_ACCOUNT);
> > +	if (!kls->kns_ids)
> > +		return -ENOMEM;
> > +
> > +	return 0;
> > +}
> > +
> > +/*
> > + * Lookup a namespace owned by owner with id >= ns_id.
> > + * Returns the namespace with the smallest id that is >= ns_id.
> > + */
> > +static struct ns_common *lookup_ns_owner_at(u64 ns_id, struct ns_common *owner)
> > +{
> > +	struct ns_common *ret = NULL;
> > +	struct rb_node *node;
> > +
> > +	VFS_WARN_ON_ONCE(owner->ns_type != CLONE_NEWUSER);
> > +
> > +	read_seqlock_excl(&ns_tree_lock);
> > +	node = owner->ns_owner_tree.rb_node;
> > +
> > +	while (node) {
> > +		struct ns_common *ns = node_to_ns_owner(node);
> > +
> > +		if (ns_id <= ns->ns_id) {
> > +			ret = ns;
> > +			if (ns_id == ns->ns_id)
> > +				break;
> > +			node = node->rb_left;
> > +		} else {
> > +			node = node->rb_right;
> > +		}
> > +	}
> > +
> > +	if (ret && !ns_get_unless_inactive(ret))
> > +		ret = NULL;
> > +	read_sequnlock_excl(&ns_tree_lock);
> > +	return ret;
> > +}
> > +
> > +static struct ns_common *lookup_ns_id(u64 mnt_ns_id, int ns_type)
> > +{
> > +	struct ns_common *ns;
> > +
> > +	guard(rcu)();
> > +	ns = ns_tree_lookup_rcu(mnt_ns_id, ns_type);
> > +	if (!ns)
> > +		return NULL;
> > +
> > +	if (!ns_get_unless_inactive(ns))
> > +		return NULL;
> > +
> > +	return ns;
> > +}
> > +
> > +static ssize_t do_listns_userns(struct klistns *kls)
> > +{
> > +	u64 *ns_ids = kls->kns_ids;
> > +	size_t nr_ns_ids = kls->nr_ns_ids;
> > +	struct ns_common *ns = NULL, *first_ns = NULL;
> > +	const struct list_head *head;
> > +	bool userns_capable;
> > +	ssize_t ret;
> > +
> > +	VFS_WARN_ON_ONCE(!kls->user_ns_id);
> > +
> > +	if (kls->user_ns_id == LISTNS_CURRENT_USER)
> > +		ns = to_ns_common(current_user_ns());
> > +	else if (kls->user_ns_id)
> > +		ns = lookup_ns_id(kls->user_ns_id, CLONE_NEWUSER);
> > +	if (!ns)
> > +		return -EINVAL;
> > +	kls->user_ns = to_user_ns(ns);
> > +
> > +	/*
> > +	 * Use the rbtree to find the first namespace we care about and
> > +	 * then use it's list entry to iterate from there.
> > +	 */
> > +	if (kls->last_ns_id) {
> > +		kls->first_ns = lookup_ns_owner_at(kls->last_ns_id + 1, ns);
> > +		if (!kls->first_ns)
> > +			return -ENOENT;
> > +		first_ns = kls->first_ns;
> > +	}
> > +
> > +	ret = 0;
> > +	head = &to_ns_common(kls->user_ns)->ns_owner;
> > +	userns_capable = ns_capable_noaudit(kls->user_ns, CAP_SYS_ADMIN);
> > +	guard(rcu)();
> > +	if (!first_ns)
> > +		first_ns = list_entry_rcu(head->next, typeof(*ns), ns_owner_entry);
> > +	for (ns = first_ns; &ns->ns_owner_entry != head && nr_ns_ids;
> > +	     ns = list_entry_rcu(ns->ns_owner_entry.next, typeof(*ns), ns_owner_entry)) {
> > +		if (kls->ns_type && !(kls->ns_type & ns->ns_type))
> > +			continue;
> > +		if (!ns_get_unless_inactive(ns))
> > +			continue;
> > +		if (userns_capable || is_current_namespace(ns) ||
> > +		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
> > +			*ns_ids = ns->ns_id;
> > +			ns_ids++;
> > +			nr_ns_ids--;
> > +			ret++;
> > +		}
> > +		if (need_resched())
> > +			cond_resched_rcu();
> > +		/* doesn't sleep */
> > +		ns->ops->put(ns);
> > +	}
> > +
> > +	return ret;
> > +}
> > +
> > +/*
> > + * Lookup a namespace with id >= ns_id in either the unified tree or a type-specific tree.
> > + * Returns the namespace with the smallest id that is >= ns_id.
> > + */
> > +static struct ns_common *lookup_ns_id_at(u64 ns_id, int ns_type)
> > +{
> > +	struct ns_common *ret = NULL;
> > +	struct ns_tree *ns_tree = NULL;
> > +	struct rb_node *node;
> > +
> > +	if (ns_type) {
> > +		ns_tree = ns_tree_from_type(ns_type);
> > +		if (!ns_tree)
> > +			return NULL;
> > +	}
> > +
> > +	read_seqlock_excl(&ns_tree_lock);
> > +	if (ns_tree)
> > +		node = ns_tree->ns_tree.rb_node;
> > +	else
> > +		node = ns_unified_tree.rb_node;
> > +
> > +	while (node) {
> > +		struct ns_common *ns;
> > +
> > +		if (ns_type)
> > +			ns = node_to_ns(node);
> > +		else
> > +			ns = node_to_ns_unified(node);
> > +
> > +		if (ns_id <= ns->ns_id) {
> > +			if (ns_type)
> > +				ret = node_to_ns(node);
> > +			else
> > +				ret = node_to_ns_unified(node);
> > +			if (ns_id == ns->ns_id)
> > +				break;
> > +			node = node->rb_left;
> > +		} else {
> > +			node = node->rb_right;
> > +		}
> > +	}
> > +
> > +	if (ret && !ns_get_unless_inactive(ret))
> > +		ret = NULL;
> > +	read_sequnlock_excl(&ns_tree_lock);
> > +	return ret;
> > +}
> > +
> > +static inline struct ns_common *first_ns_common(const struct list_head *head,
> > +						struct ns_tree *ns_tree)
> > +{
> > +	if (ns_tree)
> > +		return list_entry_rcu(head->next, struct ns_common, ns_list_node);
> > +	return list_entry_rcu(head->next, struct ns_common, ns_unified_list_node);
> > +}
> > +
> > +static inline struct ns_common *next_ns_common(struct ns_common *ns,
> > +					       struct ns_tree *ns_tree)
> > +{
> > +	if (ns_tree)
> > +		return list_entry_rcu(ns->ns_list_node.next, struct ns_common, ns_list_node);
> > +	return list_entry_rcu(ns->ns_unified_list_node.next, struct ns_common, ns_unified_list_node);
> > +}
> > +
> > +static inline bool ns_common_is_head(struct ns_common *ns,
> > +				     const struct list_head *head,
> > +				     struct ns_tree *ns_tree)
> > +{
> > +	if (ns_tree)
> > +		return &ns->ns_list_node == head;
> > +	return &ns->ns_unified_list_node == head;
> > +}
> > +
> > +static ssize_t do_listns(struct klistns *kls)
> > +{
> > +	u64 *ns_ids = kls->kns_ids;
> > +	size_t nr_ns_ids = kls->nr_ns_ids;
> > +	struct ns_common *ns, *first_ns = NULL;
> > +	struct ns_tree *ns_tree = NULL;
> > +	const struct list_head *head;
> > +	struct user_namespace *user_ns;
> > +	u32 ns_type;
> > +	ssize_t ret;
> > +
> > +	if (hweight32(kls->ns_type) == 1)
> > +		ns_type = kls->ns_type;
> > +	else
> > +		ns_type = 0;
> > +
> > +	if (ns_type) {
> > +		ns_tree = ns_tree_from_type(ns_type);
> > +		if (!ns_tree)
> > +			return -EINVAL;
> > +	}
> > +
> > +	if (kls->last_ns_id) {
> > +		kls->first_ns = lookup_ns_id_at(kls->last_ns_id + 1, ns_type);
> > +		if (!kls->first_ns)
> > +			return -ENOENT;
> > +		first_ns = kls->first_ns;
> > +	}
> > +
> > +	ret = 0;
> > +	if (ns_tree)
> > +		head = &ns_tree->ns_list;
> > +	else
> > +		head = &ns_unified_list;
> > +
> > +	guard(rcu)();
> > +	if (!first_ns)
> > +		first_ns = first_ns_common(head, ns_tree);
> > +
> > +	for (ns = first_ns; !ns_common_is_head(ns, head, ns_tree) && nr_ns_ids;
> > +	     ns = next_ns_common(ns, ns_tree)) {
> > +		if (kls->ns_type && !(kls->ns_type & ns->ns_type))
> > +			continue;
> > +		if (!ns_get_unless_inactive(ns))
> > +			continue;
> > +		/* Check permissions */
> > +		if (!ns->ops)
> > +			user_ns = NULL;
> > +		else
> > +			user_ns = ns->ops->owner(ns);
> > +		if (!user_ns)
> > +			user_ns = &init_user_ns;
> > +		if (ns_capable_noaudit(user_ns, CAP_SYS_ADMIN) ||
> > +		    is_current_namespace(ns) ||
> > +		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
> > +			*ns_ids++ = ns->ns_id;
> > +			nr_ns_ids--;
> > +			ret++;
> > +		}
> > +		if (need_resched())
> > +			cond_resched_rcu();
> > +		/* doesn't sleep */
> > +		ns->ops->put(ns);
> > +	}
> > +
> > +	return ret;
> > +}
> > +
> > +SYSCALL_DEFINE4(listns, const struct ns_id_req __user *, req,
> > +		u64 __user *, ns_ids, size_t, nr_ns_ids, unsigned int, flags)
> > +{
> > +	struct klistns klns __free(klistns_free) = {};
> > +	const size_t maxcount = 1000000;
> > +	struct ns_id_req kreq;
> > +	ssize_t ret;
> > +
> > +	if (flags)
> > +		return -EINVAL;
> > +
> > +	if (unlikely(nr_ns_ids > maxcount))
> > +		return -EOVERFLOW;
> > +
> > +	if (!access_ok(ns_ids, nr_ns_ids * sizeof(*ns_ids)))
> > +		return -EFAULT;
> > +
> > +	ret = copy_ns_id_req(req, &kreq);
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = prepare_klistns(&klns, &kreq, nr_ns_ids);
> > +	if (ret)
> > +		return ret;
> > +
> > +	if (kreq.user_ns_id)
> > +		ret = do_listns_userns(&klns);
> > +	else
> > +		ret = do_listns(&klns);
> > +	if (ret <= 0)
> > +		return ret;
> > +
> > +	if (copy_to_user(ns_ids, klns.kns_ids, ret * sizeof(*ns_ids)))
> > +		return -EFAULT;
> > +
> > +	return ret;
> > +}
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index 8134c40b2584..22a0440a62fa 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -80,6 +80,7 @@ struct pid_namespace init_pid_ns = {
> >  	.user_ns = &init_user_ns,
> >  	.ns.inum = ns_init_inum(&init_pid_ns),
> >  	.ns.ns_list_node = LIST_HEAD_INIT(init_pid_ns.ns.ns_list_node),
> > +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_pid_ns.ns.ns_unified_list_node),
> >  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_pid_ns.ns.ns_owner_entry),
> >  	.ns.ns_owner = LIST_HEAD_INIT(init_pid_ns.ns.ns_owner),
> >  #ifdef CONFIG_PID_NS
> > diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
> > index 15cb74267c75..acbeec049263 100644
> > --- a/kernel/time/namespace.c
> > +++ b/kernel/time/namespace.c
> > @@ -489,6 +489,7 @@ struct time_namespace init_time_ns = {
> >  	.ns.ns_owner = LIST_HEAD_INIT(init_time_ns.ns.ns_owner),
> >  	.frozen_offsets	= true,
> >  	.ns.ns_list_node = LIST_HEAD_INIT(init_time_ns.ns.ns_list_node),
> > +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_time_ns.ns.ns_unified_list_node),
> >  };
> >  
> >  void __init time_ns_init(void)
> > diff --git a/kernel/user.c b/kernel/user.c
> > index e392768ccd44..68fe16617d38 100644
> > --- a/kernel/user.c
> > +++ b/kernel/user.c
> > @@ -72,6 +72,7 @@ struct user_namespace init_user_ns = {
> >  	.group = GLOBAL_ROOT_GID,
> >  	.ns.inum = ns_init_inum(&init_user_ns),
> >  	.ns.ns_list_node = LIST_HEAD_INIT(init_user_ns.ns.ns_list_node),
> > +	.ns.ns_unified_list_node = LIST_HEAD_INIT(init_user_ns.ns.ns_unified_list_node),
> >  	.ns.ns_owner_entry = LIST_HEAD_INIT(init_user_ns.ns.ns_owner_entry),
> >  	.ns.ns_owner = LIST_HEAD_INIT(init_user_ns.ns.ns_owner),
> >  #ifdef CONFIG_USER_NS
> 
> -- 
> Jeff Layton <jlayton@kernel.org>

Re: [PATCH v3 17/70] nstree: add listns()

[Simon Horman]

On Fri, Oct 24, 2025 at 12:52:46PM +0200, Christian Brauner wrote:

...

> diff --git a/kernel/nstree.c b/kernel/nstree.c

...

> +static ssize_t do_listns(struct klistns *kls)
> +{
> +	u64 *ns_ids = kls->kns_ids;
> +	size_t nr_ns_ids = kls->nr_ns_ids;
> +	struct ns_common *ns, *first_ns = NULL;
> +	struct ns_tree *ns_tree = NULL;
> +	const struct list_head *head;
> +	struct user_namespace *user_ns;
> +	u32 ns_type;
> +	ssize_t ret;
> +
> +	if (hweight32(kls->ns_type) == 1)
> +		ns_type = kls->ns_type;
> +	else
> +		ns_type = 0;
> +
> +	if (ns_type) {
> +		ns_tree = ns_tree_from_type(ns_type);
> +		if (!ns_tree)
> +			return -EINVAL;
> +	}
> +
> +	if (kls->last_ns_id) {
> +		kls->first_ns = lookup_ns_id_at(kls->last_ns_id + 1, ns_type);
> +		if (!kls->first_ns)
> +			return -ENOENT;
> +		first_ns = kls->first_ns;
> +	}
> +
> +	ret = 0;
> +	if (ns_tree)
> +		head = &ns_tree->ns_list;
> +	else
> +		head = &ns_unified_list;
> +
> +	guard(rcu)();
> +	if (!first_ns)
> +		first_ns = first_ns_common(head, ns_tree);
> +
> +	for (ns = first_ns; !ns_common_is_head(ns, head, ns_tree) && nr_ns_ids;
> +	     ns = next_ns_common(ns, ns_tree)) {
> +		if (kls->ns_type && !(kls->ns_type & ns->ns_type))
> +			continue;
> +		if (!ns_get_unless_inactive(ns))
> +			continue;
> +		/* Check permissions */
> +		if (!ns->ops)
> +			user_ns = NULL;

Hi Christian,

Here it is assumed that ns->ops may be NULL.

> +		else
> +			user_ns = ns->ops->owner(ns);
> +		if (!user_ns)
> +			user_ns = &init_user_ns;
> +		if (ns_capable_noaudit(user_ns, CAP_SYS_ADMIN) ||
> +		    is_current_namespace(ns) ||
> +		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
> +			*ns_ids++ = ns->ns_id;
> +			nr_ns_ids--;
> +			ret++;
> +		}
> +		if (need_resched())
> +			cond_resched_rcu();
> +		/* doesn't sleep */
> +		ns->ops->put(ns);

And, if so, it isn't clear to me why that wouldn't also be the case here.

Flagged by Smatch.

> +	}
> +
> +	return ret;
> +}

...

Re: [PATCH v3 17/70] nstree: add listns()

[Christian Brauner]

On Tue, Oct 28, 2025 at 10:36:30AM +0000, Simon Horman wrote:
> On Fri, Oct 24, 2025 at 12:52:46PM +0200, Christian Brauner wrote:
> 
> ...
> 
> > diff --git a/kernel/nstree.c b/kernel/nstree.c
> 
> ...
> 
> > +static ssize_t do_listns(struct klistns *kls)
> > +{
> > +	u64 *ns_ids = kls->kns_ids;
> > +	size_t nr_ns_ids = kls->nr_ns_ids;
> > +	struct ns_common *ns, *first_ns = NULL;
> > +	struct ns_tree *ns_tree = NULL;
> > +	const struct list_head *head;
> > +	struct user_namespace *user_ns;
> > +	u32 ns_type;
> > +	ssize_t ret;
> > +
> > +	if (hweight32(kls->ns_type) == 1)
> > +		ns_type = kls->ns_type;
> > +	else
> > +		ns_type = 0;
> > +
> > +	if (ns_type) {
> > +		ns_tree = ns_tree_from_type(ns_type);
> > +		if (!ns_tree)
> > +			return -EINVAL;
> > +	}
> > +
> > +	if (kls->last_ns_id) {
> > +		kls->first_ns = lookup_ns_id_at(kls->last_ns_id + 1, ns_type);
> > +		if (!kls->first_ns)
> > +			return -ENOENT;
> > +		first_ns = kls->first_ns;
> > +	}
> > +
> > +	ret = 0;
> > +	if (ns_tree)
> > +		head = &ns_tree->ns_list;
> > +	else
> > +		head = &ns_unified_list;
> > +
> > +	guard(rcu)();
> > +	if (!first_ns)
> > +		first_ns = first_ns_common(head, ns_tree);
> > +
> > +	for (ns = first_ns; !ns_common_is_head(ns, head, ns_tree) && nr_ns_ids;
> > +	     ns = next_ns_common(ns, ns_tree)) {
> > +		if (kls->ns_type && !(kls->ns_type & ns->ns_type))
> > +			continue;
> > +		if (!ns_get_unless_inactive(ns))
> > +			continue;
> > +		/* Check permissions */
> > +		if (!ns->ops)
> > +			user_ns = NULL;
> 
> Hi Christian,
> 
> Here it is assumed that ns->ops may be NULL.
> 
> > +		else
> > +			user_ns = ns->ops->owner(ns);
> > +		if (!user_ns)
> > +			user_ns = &init_user_ns;
> > +		if (ns_capable_noaudit(user_ns, CAP_SYS_ADMIN) ||
> > +		    is_current_namespace(ns) ||
> > +		    ((ns->ns_type == CLONE_NEWUSER) && ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))) {
> > +			*ns_ids++ = ns->ns_id;
> > +			nr_ns_ids--;
> > +			ret++;
> > +		}
> > +		if (need_resched())
> > +			cond_resched_rcu();
> > +		/* doesn't sleep */
> > +		ns->ops->put(ns);
> 
> And, if so, it isn't clear to me why that wouldn't also be the case here.

Right you are. Fixed.

[PATCH v3 18/70] arch: hookup listns() system call

[Christian Brauner]

Add the listns() system call to all architectures.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 arch/alpha/kernel/syscalls/syscall.tbl      | 1 +
 arch/arm/tools/syscall.tbl                  | 1 +
 arch/arm64/tools/syscall_32.tbl             | 1 +
 arch/m68k/kernel/syscalls/syscall.tbl       | 1 +
 arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl   | 1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl   | 1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl   | 1 +
 arch/parisc/kernel/syscalls/syscall.tbl     | 1 +
 arch/powerpc/kernel/syscalls/syscall.tbl    | 1 +
 arch/s390/kernel/syscalls/syscall.tbl       | 1 +
 arch/sh/kernel/syscalls/syscall.tbl         | 1 +
 arch/sparc/kernel/syscalls/syscall.tbl      | 1 +
 arch/x86/entry/syscalls/syscall_32.tbl      | 1 +
 arch/x86/entry/syscalls/syscall_64.tbl      | 1 +
 arch/xtensa/kernel/syscalls/syscall.tbl     | 1 +
 include/uapi/asm-generic/unistd.h           | 4 +++-
 scripts/syscall.tbl                         | 1 +
 18 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index 16dca28ebf17..3fed97478058 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -509,3 +509,4 @@
 577	common	open_tree_attr			sys_open_tree_attr
 578	common	file_getattr			sys_file_getattr
 579	common	file_setattr			sys_file_setattr
+580	common	listns				sys_listns
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index b07e699aaa3c..fd09afae72a2 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -484,3 +484,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/arm64/tools/syscall_32.tbl b/arch/arm64/tools/syscall_32.tbl
index 8d9088bc577d..8cdfe5d4dac9 100644
--- a/arch/arm64/tools/syscall_32.tbl
+++ b/arch/arm64/tools/syscall_32.tbl
@@ -481,3 +481,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index f41d38dfbf13..871a5d67bf41 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -469,3 +469,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index 580af574fe73..022fc85d94b3 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -475,3 +475,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index d824ffe9a014..8cedc83c3266 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -408,3 +408,4 @@
 467	n32	open_tree_attr			sys_open_tree_attr
 468	n32	file_getattr			sys_file_getattr
 469	n32	file_setattr			sys_file_setattr
+470	n32	listns				sys_listns
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index 7a7049c2c307..9b92bddf06b5 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -384,3 +384,4 @@
 467	n64	open_tree_attr			sys_open_tree_attr
 468	n64	file_getattr			sys_file_getattr
 469	n64	file_setattr			sys_file_setattr
+470	n64	listns				sys_listns
diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index d330274f0601..f810b8a55716 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -457,3 +457,4 @@
 467	o32	open_tree_attr			sys_open_tree_attr
 468	o32	file_getattr			sys_file_getattr
 469	o32	file_setattr			sys_file_setattr
+470	o32	listns				sys_listns
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index 88a788a7b18d..39bdacaa530b 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -468,3 +468,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index b453e80dfc00..ec4458cdb97b 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -560,3 +560,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index 8a6744d658db..5863787ab036 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -472,3 +472,4 @@
 467  common	open_tree_attr		sys_open_tree_attr		sys_open_tree_attr
 468  common	file_getattr		sys_file_getattr		sys_file_getattr
 469  common	file_setattr		sys_file_setattr		sys_file_setattr
+470  common	listns			sys_listns			sys_listns
diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index 5e9c9eff5539..969c11325ade 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -473,3 +473,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index ebb7d06d1044..39aa26b6a50b 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -515,3 +515,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 4877e16da69a..e979a3eac7a3 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -475,3 +475,4 @@
 467	i386	open_tree_attr		sys_open_tree_attr
 468	i386	file_getattr		sys_file_getattr
 469	i386	file_setattr		sys_file_setattr
+470	i386	listns			sys_listns
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index ced2a1deecd7..8a4ac4841be6 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -394,6 +394,7 @@
 467	common	open_tree_attr		sys_open_tree_attr
 468	common	file_getattr		sys_file_getattr
 469	common	file_setattr		sys_file_setattr
+470	common	listns			sys_listns
 
 #
 # Due to a historical design error, certain syscalls are numbered differently
diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 374e4cb788d8..438a3b170402 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -440,3 +440,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 04e0077fb4c9..942370b3f5d2 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -857,9 +857,11 @@ __SYSCALL(__NR_open_tree_attr, sys_open_tree_attr)
 __SYSCALL(__NR_file_getattr, sys_file_getattr)
 #define __NR_file_setattr 469
 __SYSCALL(__NR_file_setattr, sys_file_setattr)
+#define __NR_listns 470
+__SYSCALL(__NR_listns, sys_listns)
 
 #undef __NR_syscalls
-#define __NR_syscalls 470
+#define __NR_syscalls 471
 
 /*
  * 32 bit systems traditionally used different
diff --git a/scripts/syscall.tbl b/scripts/syscall.tbl
index d1ae5e92c615..e74868be513c 100644
--- a/scripts/syscall.tbl
+++ b/scripts/syscall.tbl
@@ -410,3 +410,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns

-- 
2.47.3

Re: [PATCH v3 18/70] arch: hookup listns() system call

[Arnd Bergmann]

On Fri, Oct 24, 2025, at 12:52, Christian Brauner wrote:
> Add the listns() system call to all architectures.
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>

This looks correct to me,

Reviewed-by: Arnd Bergmann <arnd@arndb.de>

> diff --git a/include/uapi/asm-generic/unistd.h 
> b/include/uapi/asm-generic/unistd.h
> index 04e0077fb4c9..942370b3f5d2 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -857,9 +857,11 @@ __SYSCALL(__NR_open_tree_attr, sys_open_tree_attr)
>  __SYSCALL(__NR_file_getattr, sys_file_getattr)
>  #define __NR_file_setattr 469
>  __SYSCALL(__NR_file_setattr, sys_file_setattr)
> +#define __NR_listns 470
> +__SYSCALL(__NR_listns, sys_listns)
> 
>  #undef __NR_syscalls
> -#define __NR_syscalls 470
> +#define __NR_syscalls 471
> 

I still need to remove this unused file, but that is my problem,
not yours. No need to add patch 71 to your series ;-)

    Arnd

[PATCH v3 19/70] nsfs: update tools header

[Christian Brauner]

Ensure all the new uapi bits are visible for the selftests.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/include/uapi/linux/nsfs.h | 70 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/tools/include/uapi/linux/nsfs.h b/tools/include/uapi/linux/nsfs.h
index 33c9b578b3b2..a25e38d1c874 100644
--- a/tools/include/uapi/linux/nsfs.h
+++ b/tools/include/uapi/linux/nsfs.h
@@ -53,6 +53,76 @@ enum init_ns_ino {
 	TIME_NS_INIT_INO	= 0xEFFFFFFAU,
 	NET_NS_INIT_INO		= 0xEFFFFFF9U,
 	MNT_NS_INIT_INO		= 0xEFFFFFF8U,
+#ifdef __KERNEL__
+	MNT_NS_ANON_INO		= 0xEFFFFFF7U,
+#endif
 };
 
+struct nsfs_file_handle {
+	__u64 ns_id;
+	__u32 ns_type;
+	__u32 ns_inum;
+};
+
+#define NSFS_FILE_HANDLE_SIZE_VER0 16 /* sizeof first published struct */
+#define NSFS_FILE_HANDLE_SIZE_LATEST sizeof(struct nsfs_file_handle) /* sizeof latest published struct */
+
+enum init_ns_id {
+	IPC_NS_INIT_ID		= 1ULL,
+	UTS_NS_INIT_ID		= 2ULL,
+	USER_NS_INIT_ID		= 3ULL,
+	PID_NS_INIT_ID		= 4ULL,
+	CGROUP_NS_INIT_ID	= 5ULL,
+	TIME_NS_INIT_ID		= 6ULL,
+	NET_NS_INIT_ID		= 7ULL,
+	MNT_NS_INIT_ID		= 8ULL,
+#ifdef __KERNEL__
+	NS_LAST_INIT_ID		= MNT_NS_INIT_ID,
+#endif
+};
+
+enum ns_type {
+	TIME_NS    = (1ULL << 7),  /* CLONE_NEWTIME */
+	MNT_NS     = (1ULL << 17), /* CLONE_NEWNS */
+	CGROUP_NS  = (1ULL << 25), /* CLONE_NEWCGROUP */
+	UTS_NS     = (1ULL << 26), /* CLONE_NEWUTS */
+	IPC_NS     = (1ULL << 27), /* CLONE_NEWIPC */
+	USER_NS    = (1ULL << 28), /* CLONE_NEWUSER */
+	PID_NS     = (1ULL << 29), /* CLONE_NEWPID */
+	NET_NS     = (1ULL << 30), /* CLONE_NEWNET */
+};
+
+/**
+ * struct ns_id_req - namespace ID request structure
+ * @size: size of this structure
+ * @spare: reserved for future use
+ * @filter: filter mask
+ * @ns_id: last namespace id
+ * @user_ns_id: owning user namespace ID
+ *
+ * Structure for passing namespace ID and miscellaneous parameters to
+ * statns(2) and listns(2).
+ *
+ * For statns(2) @param represents the request mask.
+ * For listns(2) @param represents the last listed mount id (or zero).
+ */
+struct ns_id_req {
+	__u32 size;
+	__u32 spare;
+	__u64 ns_id;
+	struct /* listns */ {
+		__u32 ns_type;
+		__u32 spare2;
+		__u64 user_ns_id;
+	};
+};
+
+/*
+ * Special @user_ns_id value that can be passed to listns()
+ */
+#define LISTNS_CURRENT_USER 0xffffffffffffffff /* Caller's userns */
+
+/* List of all ns_id_req versions. */
+#define NS_ID_REQ_SIZE_VER0 32 /* sizeof first published struct */
+
 #endif /* __LINUX_NSFS_H */

-- 
2.47.3

[PATCH v3 20/70] selftests/filesystems: remove CLONE_NEWPIDNS from setup_userns() helper

[Christian Brauner]

This is effectively unused and doesn't really server any purpose after
having reviewed all of the tests that rely on it.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/filesystems/utils.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/filesystems/utils.c b/tools/testing/selftests/filesystems/utils.c
index c43a69dffd83..a0c64f415a7f 100644
--- a/tools/testing/selftests/filesystems/utils.c
+++ b/tools/testing/selftests/filesystems/utils.c
@@ -487,7 +487,7 @@ int setup_userns(void)
 	uid_t uid = getuid();
 	gid_t gid = getgid();
 
-	ret = unshare(CLONE_NEWNS|CLONE_NEWUSER|CLONE_NEWPID);
+	ret = unshare(CLONE_NEWNS|CLONE_NEWUSER);
 	if (ret) {
 		ksft_exit_fail_msg("unsharing mountns and userns: %s\n",
 				   strerror(errno));

-- 
2.47.3

[PATCH v3 21/70] selftests/namespaces: first active reference count tests

[Christian Brauner]

Test that initial namespaces can be reopened via file handle. Initial
namespaces should always have a ref count of one from boot.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/.gitignore      |  1 +
 tools/testing/selftests/namespaces/Makefile        |  5 +-
 .../selftests/namespaces/ns_active_ref_test.c      | 74 ++++++++++++++++++++++
 3 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index ccfb40837a73..100cc5bfef04 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -1,3 +1,4 @@
 nsid_test
 file_handle_test
 init_ino_test
+ns_active_ref_test
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index 5fe4b3dc07d3..5cea938cdde8 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -1,7 +1,10 @@
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
+LDLIBS += -lcap
 
-TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test
+TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test
 
 include ../lib.mk
 
+$(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
+
diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
new file mode 100644
index 000000000000..21514a537b26
--- /dev/null
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/nsfs.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+
+#ifndef FD_NSFS_ROOT
+#define FD_NSFS_ROOT -10003 /* Root of the nsfs filesystem */
+#endif
+
+/*
+ * Test that initial namespaces can be reopened via file handle.
+ * Initial namespaces should have active ref count of 1 from boot.
+ */
+TEST(init_ns_always_active)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd1, fd2;
+	struct stat st1, st2;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open initial network namespace */
+	fd1 = open("/proc/1/ns/net", O_RDONLY);
+	ASSERT_GE(fd1, 0);
+
+	/* Get file handle for initial namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(fd1, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(fd1);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+
+	/* Close the namespace fd */
+	close(fd1);
+
+	/* Try to reopen via file handle - should succeed since init ns is always active */
+	fd2 = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd2 < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle);
+		     return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd2, 0);
+
+	/* Verify we opened the same namespace */
+	fd1 = open("/proc/1/ns/net", O_RDONLY);
+	ASSERT_GE(fd1, 0);
+	ASSERT_EQ(fstat(fd1, &st1), 0);
+	ASSERT_EQ(fstat(fd2, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	close(fd1);
+	close(fd2);
+	free(handle);
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 22/70] selftests/namespaces: second active reference count tests

[Christian Brauner]

Test namespace lifecycle: create a namespace in a child process, get a
file handle while it's active, then try to reopen after the process
exits (namespace becomes inactive).

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 81 ++++++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 21514a537b26..7cade298c754 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -71,4 +71,85 @@ TEST(init_ns_always_active)
 	free(handle);
 }
 
+/*
+ * Test namespace lifecycle: create a namespace in a child process,
+ * get a file handle while it's active, then try to reopen after
+ * the process exits (namespace becomes inactive).
+ */
+TEST(ns_inactive_after_exit)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	/* Create pipe for passing file handle from child */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* Create new network namespace */
+		ret = unshare(CLONE_NEWNET);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Open our new namespace */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get file handle for the namespace */
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(fd);
+
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Send handle to parent */
+		write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+		close(pipefd[1]);
+
+		/* Exit - namespace should become inactive */
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	/* Read file handle from child */
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	/* Try to reopen namespace - should fail with ENOENT since it's inactive */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd, 0);
+	/* Should fail with ENOENT (namespace inactive) or ESTALE */
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 23/70] selftests/namespaces: third active reference count tests

[Christian Brauner]

Test that a namespace remains active while a process is using it,
even after the creating process exits.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 124 +++++++++++++++++++++
 1 file changed, 124 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 7cade298c754..c2e34de7a3a9 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -152,4 +152,128 @@ TEST(ns_inactive_after_exit)
 	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
 }
 
+/*
+ * Test that a namespace remains active while a process is using it,
+ * even after the creating process exits.
+ */
+TEST(ns_active_with_multiple_processes)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	int syncpipe[2];
+	pid_t pid1, pid2;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+	char sync_byte;
+
+	/* Create pipes for communication */
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(pipe(syncpipe), 0);
+
+	pid1 = fork();
+	ASSERT_GE(pid1, 0);
+
+	if (pid1 == 0) {
+		/* First child - creates namespace */
+		close(pipefd[0]);
+		close(syncpipe[1]);
+
+		/* Create new network namespace */
+		ret = unshare(CLONE_NEWNET);
+		if (ret < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		/* Open and get handle */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(fd);
+
+		if (ret < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		/* Send handle to parent */
+		write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+		close(pipefd[1]);
+
+		/* Wait for signal before exiting */
+		read(syncpipe[0], &sync_byte, 1);
+		close(syncpipe[0]);
+		exit(0);
+	}
+
+	/* Parent reads handle */
+	close(pipefd[1]);
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+	ASSERT_GT(ret, 0);
+
+	handle = (struct file_handle *)buf;
+
+	/* Create second child that will keep namespace active */
+	pid2 = fork();
+	ASSERT_GE(pid2, 0);
+
+	if (pid2 == 0) {
+		/* Second child - reopens the namespace */
+		close(syncpipe[0]);
+		close(syncpipe[1]);
+
+		/* Open the namespace via handle */
+		fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+		if (fd < 0) {
+			exit(1);
+		}
+
+		/* Join the namespace */
+		ret = setns(fd, CLONE_NEWNET);
+		close(fd);
+		if (ret < 0) {
+			exit(1);
+		}
+
+		/* Sleep to keep namespace active */
+		sleep(1);
+		exit(0);
+	}
+
+	/* Let second child enter the namespace */
+	usleep(100000); /* 100ms */
+
+	/* Signal first child to exit */
+	close(syncpipe[0]);
+	sync_byte = 'X';
+	write(syncpipe[1], &sync_byte, 1);
+	close(syncpipe[1]);
+
+	/* Wait for first child */
+	waitpid(pid1, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+
+	/* Namespace should still be active because second child is using it */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(fd, 0);
+	close(fd);
+
+	/* Wait for second child */
+	waitpid(pid2, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 24/70] selftests/namespaces: fourth active reference count tests

[Christian Brauner]

Test user namespace active ref tracking via credential lifecycle.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 90 ++++++++++++++++++++++
 1 file changed, 90 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index c2e34de7a3a9..396066e641da 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -276,4 +276,94 @@ TEST(ns_active_with_multiple_processes)
 	ASSERT_TRUE(WIFEXITED(status));
 }
 
+/*
+ * Test user namespace active ref tracking via credential lifecycle
+ */
+TEST(userns_active_ref_lifecycle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* Create new user namespace */
+		ret = unshare(CLONE_NEWUSER);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Set up uid/gid mappings */
+		int uid_map_fd = open("/proc/self/uid_map", O_WRONLY);
+		int gid_map_fd = open("/proc/self/gid_map", O_WRONLY);
+		int setgroups_fd = open("/proc/self/setgroups", O_WRONLY);
+
+		if (uid_map_fd >= 0 && gid_map_fd >= 0 && setgroups_fd >= 0) {
+			write(setgroups_fd, "deny", 4);
+			close(setgroups_fd);
+
+			char mapping[64];
+			snprintf(mapping, sizeof(mapping), "0 %d 1", getuid());
+			write(uid_map_fd, mapping, strlen(mapping));
+			close(uid_map_fd);
+
+			snprintf(mapping, sizeof(mapping), "0 %d 1", getgid());
+			write(gid_map_fd, mapping, strlen(mapping));
+			close(gid_map_fd);
+		}
+
+		/* Get file handle */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(fd);
+
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Send handle to parent */
+		write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	/* Namespace should be inactive after all tasks exit */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd, 0);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 25/70] selftests/namespaces: fifth active reference count tests

[Christian Brauner]

Test PID namespace active ref tracking

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 82 ++++++++++++++++++++++
 1 file changed, 82 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 396066e641da..f4e92b772f70 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -366,4 +366,86 @@ TEST(userns_active_ref_lifecycle)
 	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
 }
 
+/*
+ * Test PID namespace active ref tracking
+ */
+TEST(pidns_active_ref_lifecycle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* Create new PID namespace */
+		ret = unshare(CLONE_NEWPID);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Fork to actually enter the PID namespace */
+		pid_t child = fork();
+		if (child < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (child == 0) {
+			/* Grandchild - in new PID namespace */
+			fd = open("/proc/self/ns/pid", O_RDONLY);
+			if (fd < 0) {
+				exit(1);
+			}
+
+			handle = (struct file_handle *)buf;
+			handle->handle_bytes = MAX_HANDLE_SZ;
+			ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+			close(fd);
+
+			if (ret < 0) {
+				exit(1);
+			}
+
+			/* Send handle to grandparent */
+			write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Wait for grandchild */
+		waitpid(child, NULL, 0);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	/* Namespace should be inactive after all processes exit */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd, 0);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 26/70] selftests/namespaces: sixth active reference count tests

[Christian Brauner]

Test that an open file descriptor keeps a namespace active.
Even after the creating process exits, the namespace should remain
active as long as an fd is held open.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 155 +++++++++++++++++++++
 1 file changed, 155 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index f4e92b772f70..50653096fcb6 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -448,4 +448,159 @@ TEST(pidns_active_ref_lifecycle)
 	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
 }
 
+/*
+ * Test that an open file descriptor keeps a namespace active.
+ * Even after the creating process exits, the namespace should remain
+ * active as long as an fd is held open.
+ */
+TEST(ns_fd_keeps_active)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int nsfd;
+	int pipe_child_ready[2];
+	int pipe_parent_ready[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+	char sync_byte;
+	char proc_path[64];
+
+	ASSERT_EQ(pipe(pipe_child_ready), 0);
+	ASSERT_EQ(pipe(pipe_parent_ready), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipe_child_ready[0]);
+		close(pipe_parent_ready[1]);
+
+		TH_LOG("Child: creating new network namespace");
+
+		/* Create new network namespace */
+		ret = unshare(CLONE_NEWNET);
+		if (ret < 0) {
+			TH_LOG("Child: unshare(CLONE_NEWNET) failed: %s", strerror(errno));
+			close(pipe_child_ready[1]);
+			close(pipe_parent_ready[0]);
+			exit(1);
+		}
+
+		TH_LOG("Child: network namespace created successfully");
+
+		/* Get file handle for the namespace */
+		nsfd = open("/proc/self/ns/net", O_RDONLY);
+		if (nsfd < 0) {
+			TH_LOG("Child: failed to open /proc/self/ns/net: %s", strerror(errno));
+			close(pipe_child_ready[1]);
+			close(pipe_parent_ready[0]);
+			exit(1);
+		}
+
+		TH_LOG("Child: opened namespace fd %d", nsfd);
+
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(nsfd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(nsfd);
+
+		if (ret < 0) {
+			TH_LOG("Child: name_to_handle_at failed: %s", strerror(errno));
+			close(pipe_child_ready[1]);
+			close(pipe_parent_ready[0]);
+			exit(1);
+		}
+
+		TH_LOG("Child: got file handle (bytes=%u)", handle->handle_bytes);
+
+		/* Send file handle to parent */
+		ret = write(pipe_child_ready[1], buf, sizeof(*handle) + handle->handle_bytes);
+		TH_LOG("Child: sent %d bytes of file handle to parent", ret);
+		close(pipe_child_ready[1]);
+
+		/* Wait for parent to open the fd */
+		TH_LOG("Child: waiting for parent to open fd");
+		ret = read(pipe_parent_ready[0], &sync_byte, 1);
+		close(pipe_parent_ready[0]);
+
+		TH_LOG("Child: parent signaled (read %d bytes), exiting now", ret);
+		/* Exit - namespace should stay active because parent holds fd */
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipe_child_ready[1]);
+	close(pipe_parent_ready[0]);
+
+	TH_LOG("Parent: reading file handle from child");
+
+	/* Read file handle from child */
+	ret = read(pipe_child_ready[0], buf, sizeof(buf));
+	close(pipe_child_ready[0]);
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	TH_LOG("Parent: received %d bytes, handle size=%u", ret, handle->handle_bytes);
+
+	/* Open the child's namespace while it's still alive */
+	snprintf(proc_path, sizeof(proc_path), "/proc/%d/ns/net", pid);
+	TH_LOG("Parent: opening child's namespace at %s", proc_path);
+	nsfd = open(proc_path, O_RDONLY);
+	if (nsfd < 0) {
+		TH_LOG("Parent: failed to open %s: %s", proc_path, strerror(errno));
+		close(pipe_parent_ready[1]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open child's namespace");
+	}
+
+	TH_LOG("Parent: opened child's namespace, got fd %d", nsfd);
+
+	/* Signal child that we have the fd */
+	sync_byte = 'G';
+	write(pipe_parent_ready[1], &sync_byte, 1);
+	close(pipe_parent_ready[1]);
+	TH_LOG("Parent: signaled child that we have the fd");
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	TH_LOG("Child exited, parent holds fd %d to namespace", nsfd);
+
+	/*
+	 * Namespace should still be ACTIVE because we hold an fd.
+	 * We should be able to reopen it via file handle.
+	 */
+	TH_LOG("Attempting to reopen namespace via file handle (should succeed - fd held)");
+	int fd2 = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(fd2, 0);
+
+	TH_LOG("Successfully reopened namespace via file handle, got fd %d", fd2);
+
+	/* Verify it's the same namespace */
+	struct stat st1, st2;
+	ASSERT_EQ(fstat(nsfd, &st1), 0);
+	ASSERT_EQ(fstat(fd2, &st2), 0);
+	TH_LOG("Namespace inodes: nsfd=%lu, fd2=%lu", st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	close(fd2);
+
+	/* Now close the fd - namespace should become inactive */
+	TH_LOG("Closing fd %d - namespace should become inactive", nsfd);
+	close(nsfd);
+
+	/* Now reopening should fail - namespace is inactive */
+	TH_LOG("Attempting to reopen namespace via file handle (should fail - inactive)");
+	fd2 = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd2, 0);
+	/* Should fail with ENOENT (inactive) or ESTALE (gone) */
+	TH_LOG("Reopen failed as expected: %s (errno=%d)", strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 27/70] selftests/namespaces: seventh active reference count tests

[Christian Brauner]

Test hierarchical active reference propagation.
When a child namespace is active, its owning user namespace should also
be active automatically due to hierarchical active reference propagation.
This ensures parents are always reachable when children are active.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 222 +++++++++++++++++++++
 1 file changed, 222 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 50653096fcb6..60876965dd71 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -20,6 +20,10 @@
 #define FD_NSFS_ROOT -10003 /* Root of the nsfs filesystem */
 #endif
 
+#ifndef FILEID_NSFS
+#define FILEID_NSFS 0xf1
+#endif
+
 /*
  * Test that initial namespaces can be reopened via file handle.
  * Initial namespaces should have active ref count of 1 from boot.
@@ -603,4 +607,222 @@ TEST(ns_fd_keeps_active)
 	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
 }
 
+/*
+ * Test hierarchical active reference propagation.
+ * When a child namespace is active, its owning user namespace should also
+ * be active automatically due to hierarchical active reference propagation.
+ * This ensures parents are always reachable when children are active.
+ */
+TEST(ns_parent_always_reachable)
+{
+	struct file_handle *parent_handle, *child_handle;
+	int ret;
+	int child_nsfd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 parent_id, child_id;
+	char parent_buf[sizeof(*parent_handle) + MAX_HANDLE_SZ];
+	char child_buf[sizeof(*child_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		TH_LOG("Child: creating parent user namespace and setting up mappings");
+
+		/* Create parent user namespace with mappings */
+		ret = setup_userns();
+		if (ret < 0) {
+			TH_LOG("Child: setup_userns() for parent failed: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: parent user namespace created, now uid=%d gid=%d", getuid(), getgid());
+
+		/* Get namespace ID for parent user namespace */
+		int parent_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (parent_fd < 0) {
+			TH_LOG("Child: failed to open parent /proc/self/ns/user: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: opened parent userns fd %d", parent_fd);
+
+		if (ioctl(parent_fd, NS_GET_ID, &parent_id) < 0) {
+			TH_LOG("Child: NS_GET_ID for parent failed: %s", strerror(errno));
+			close(parent_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(parent_fd);
+
+		TH_LOG("Child: got parent namespace ID %llu", (unsigned long long)parent_id);
+
+		/* Create child user namespace within parent */
+		TH_LOG("Child: creating nested child user namespace");
+		ret = setup_userns();
+		if (ret < 0) {
+			TH_LOG("Child: setup_userns() for child failed: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: nested child user namespace created, uid=%d gid=%d", getuid(), getgid());
+
+		/* Get namespace ID for child user namespace */
+		int child_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (child_fd < 0) {
+			TH_LOG("Child: failed to open child /proc/self/ns/user: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: opened child userns fd %d", child_fd);
+
+		if (ioctl(child_fd, NS_GET_ID, &child_id) < 0) {
+			TH_LOG("Child: NS_GET_ID for child failed: %s", strerror(errno));
+			close(child_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(child_fd);
+
+		TH_LOG("Child: got child namespace ID %llu", (unsigned long long)child_id);
+
+		/* Send both namespace IDs to parent */
+		TH_LOG("Child: sending both namespace IDs to parent");
+		write(pipefd[1], &parent_id, sizeof(parent_id));
+		write(pipefd[1], &child_id, sizeof(child_id));
+		close(pipefd[1]);
+
+		TH_LOG("Child: exiting - parent userns should become inactive");
+		/* Exit - parent user namespace should become inactive */
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	TH_LOG("Parent: reading both namespace IDs from child");
+
+	/* Read both namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &parent_id, sizeof(parent_id));
+	if (ret != sizeof(parent_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read parent namespace ID from child");
+	}
+
+	ret = read(pipefd[0], &child_id, sizeof(child_id));
+	close(pipefd[0]);
+	if (ret != sizeof(child_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read child namespace ID from child");
+	}
+
+	TH_LOG("Parent: received parent_id=%llu, child_id=%llu",
+	       (unsigned long long)parent_id, (unsigned long long)child_id);
+
+	/* Construct file handles from namespace IDs */
+	parent_handle = (struct file_handle *)parent_buf;
+	parent_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	parent_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *parent_fh = (struct nsfs_file_handle *)parent_handle->f_handle;
+	parent_fh->ns_id = parent_id;
+	parent_fh->ns_type = 0;
+	parent_fh->ns_inum = 0;
+
+	child_handle = (struct file_handle *)child_buf;
+	child_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	child_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *child_fh = (struct nsfs_file_handle *)child_handle->f_handle;
+	child_fh->ns_id = child_id;
+	child_fh->ns_type = 0;
+	child_fh->ns_inum = 0;
+
+	TH_LOG("Parent: opening child namespace BEFORE child exits");
+
+	/* Open child namespace while child is still alive to keep it active */
+	child_nsfd = open_by_handle_at(FD_NSFS_ROOT, child_handle, O_RDONLY);
+	if (child_nsfd < 0) {
+		TH_LOG("Failed to open child namespace: %s (errno=%d)", strerror(errno), errno);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open child namespace");
+	}
+
+	TH_LOG("Opened child namespace fd %d", child_nsfd);
+
+	/* Now wait for child to exit */
+	TH_LOG("Parent: waiting for child to exit");
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	TH_LOG("Child process exited, parent holds fd to child namespace");
+
+	/*
+	 * With hierarchical active reference propagation:
+	 * Since the child namespace is active (parent process holds fd),
+	 * the parent user namespace should ALSO be active automatically.
+	 * This is because when we took an active reference on the child,
+	 * it propagated up to the owning user namespace.
+	 */
+	TH_LOG("Attempting to reopen parent namespace (should SUCCEED - hierarchical propagation)");
+	int parent_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_GE(parent_fd, 0);
+
+	TH_LOG("SUCCESS: Parent namespace is active (fd=%d) due to active child", parent_fd);
+
+	/* Verify we can also get parent via NS_GET_USERNS */
+	TH_LOG("Verifying NS_GET_USERNS also works");
+	int parent_fd2 = ioctl(child_nsfd, NS_GET_USERNS);
+	if (parent_fd2 < 0) {
+		close(parent_fd);
+		close(child_nsfd);
+		TH_LOG("NS_GET_USERNS failed: %s (errno=%d)", strerror(errno), errno);
+		SKIP(return, "NS_GET_USERNS not supported or failed");
+	}
+
+	TH_LOG("NS_GET_USERNS succeeded, got parent fd %d", parent_fd2);
+
+	/* Verify both methods give us the same namespace */
+	struct stat st1, st2;
+	ASSERT_EQ(fstat(parent_fd, &st1), 0);
+	ASSERT_EQ(fstat(parent_fd2, &st2), 0);
+	TH_LOG("Parent namespace inodes: parent_fd=%lu, parent_fd2=%lu", st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	/*
+	 * Close child fd - parent should remain active because we still
+	 * hold direct references to it (parent_fd and parent_fd2).
+	 */
+	TH_LOG("Closing child fd - parent should remain active (direct refs held)");
+	close(child_nsfd);
+
+	/* Parent should still be openable */
+	TH_LOG("Verifying parent still active via file handle");
+	int parent_fd3 = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_GE(parent_fd3, 0);
+	close(parent_fd3);
+
+	TH_LOG("Closing all fds to parent namespace");
+	close(parent_fd);
+	close(parent_fd2);
+
+	/* Both should now be inactive */
+	TH_LOG("Attempting to reopen parent (should fail - inactive, no refs)");
+	parent_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_LT(parent_fd, 0);
+	TH_LOG("Parent inactive as expected: %s (errno=%d)", strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 28/70] selftests/namespaces: eigth active reference count tests

[Christian Brauner]

Test that bind mounts keep namespaces in the tree even when inactive

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 117 +++++++++++++++++++++
 1 file changed, 117 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 60876965dd71..55a741d32b08 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -825,4 +825,121 @@ TEST(ns_parent_always_reachable)
 	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
 }
 
+/*
+ * Test that bind mounts keep namespaces in the tree even when inactive
+ */
+TEST(ns_bind_mount_keeps_in_tree)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+	char tmpfile[] = "/tmp/ns-test-XXXXXX";
+	int tmpfd;
+
+	/* Create temporary file for bind mount */
+	tmpfd = mkstemp(tmpfile);
+	if (tmpfd < 0) {
+		SKIP(return, "Cannot create temporary file");
+	}
+	close(tmpfd);
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* Unshare mount namespace and make mounts private to avoid propagation */
+		ret = unshare(CLONE_NEWNS);
+		if (ret < 0) {
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+		ret = mount(NULL, "/", NULL, MS_PRIVATE | MS_REC, NULL);
+		if (ret < 0) {
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		/* Create new network namespace */
+		ret = unshare(CLONE_NEWNET);
+		if (ret < 0) {
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		/* Bind mount the namespace */
+		ret = mount("/proc/self/ns/net", tmpfile, NULL, MS_BIND, NULL);
+		if (ret < 0) {
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		/* Get file handle */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			umount(tmpfile);
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(fd);
+
+		if (ret < 0) {
+			umount(tmpfile);
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		/* Send handle to parent */
+		write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	/*
+	 * Namespace should be inactive but still in tree due to bind mount.
+	 * Reopening should fail with ENOENT (inactive) not ESTALE (not in tree).
+	 */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd, 0);
+	/* Should be ENOENT (inactive) since bind mount keeps it in tree */
+	if (errno != ENOENT && errno != ESTALE) {
+		TH_LOG("Unexpected error: %d", errno);
+	}
+
+	/* Cleanup */
+	umount(tmpfile);
+	unlink(tmpfile);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 29/70] selftests/namespaces: ninth active reference count tests

[Christian Brauner]

Test multi-level hierarchy (3+ levels deep).
Grandparent → Parent → Child
When child is active, both parent AND grandparent should be active.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 164 +++++++++++++++++++++
 1 file changed, 164 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 55a741d32b08..88189398aa35 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -942,4 +942,168 @@ TEST(ns_bind_mount_keeps_in_tree)
 	unlink(tmpfile);
 }
 
+/*
+ * Test multi-level hierarchy (3+ levels deep).
+ * Grandparent → Parent → Child
+ * When child is active, both parent AND grandparent should be active.
+ */
+TEST(ns_multilevel_hierarchy)
+{
+	struct file_handle *gp_handle, *p_handle, *c_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 gp_id, p_id, c_id;
+	char gp_buf[sizeof(*gp_handle) + MAX_HANDLE_SZ];
+	char p_buf[sizeof(*p_handle) + MAX_HANDLE_SZ];
+	char c_buf[sizeof(*c_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create grandparent user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int gp_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (gp_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(gp_fd, NS_GET_ID, &gp_id) < 0) {
+			close(gp_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(gp_fd);
+
+		/* Create parent user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int p_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (p_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(p_fd, NS_GET_ID, &p_id) < 0) {
+			close(p_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(p_fd);
+
+		/* Create child user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int c_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (c_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(c_fd, NS_GET_ID, &c_id) < 0) {
+			close(c_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(c_fd);
+
+		/* Send all three namespace IDs */
+		write(pipefd[1], &gp_id, sizeof(gp_id));
+		write(pipefd[1], &p_id, sizeof(p_id));
+		write(pipefd[1], &c_id, sizeof(c_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &gp_id, sizeof(gp_id));
+	if (ret != sizeof(gp_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read grandparent namespace ID from child");
+	}
+
+	ret = read(pipefd[0], &p_id, sizeof(p_id));
+	if (ret != sizeof(p_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read parent namespace ID from child");
+	}
+
+	ret = read(pipefd[0], &c_id, sizeof(c_id));
+	close(pipefd[0]);
+	if (ret != sizeof(c_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read child namespace ID from child");
+	}
+
+	/* Construct file handles from namespace IDs */
+	gp_handle = (struct file_handle *)gp_buf;
+	gp_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	gp_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *gp_fh = (struct nsfs_file_handle *)gp_handle->f_handle;
+	gp_fh->ns_id = gp_id;
+	gp_fh->ns_type = 0;
+	gp_fh->ns_inum = 0;
+
+	p_handle = (struct file_handle *)p_buf;
+	p_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	p_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *p_fh = (struct nsfs_file_handle *)p_handle->f_handle;
+	p_fh->ns_id = p_id;
+	p_fh->ns_type = 0;
+	p_fh->ns_inum = 0;
+
+	c_handle = (struct file_handle *)c_buf;
+	c_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	c_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *c_fh = (struct nsfs_file_handle *)c_handle->f_handle;
+	c_fh->ns_id = c_id;
+	c_fh->ns_type = 0;
+	c_fh->ns_inum = 0;
+
+	/* Open child before process exits */
+	int c_fd = open_by_handle_at(FD_NSFS_ROOT, c_handle, O_RDONLY);
+	if (c_fd < 0) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open child namespace");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/*
+	 * With 3-level hierarchy and child active:
+	 * - Child is active (we hold fd)
+	 * - Parent should be active (propagated from child)
+	 * - Grandparent should be active (propagated from parent)
+	 */
+	TH_LOG("Testing parent active when child is active");
+	int p_fd = open_by_handle_at(FD_NSFS_ROOT, p_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+
+	TH_LOG("Testing grandparent active when child is active");
+	int gp_fd = open_by_handle_at(FD_NSFS_ROOT, gp_handle, O_RDONLY);
+	ASSERT_GE(gp_fd, 0);
+
+	close(c_fd);
+	close(p_fd);
+	close(gp_fd);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 30/70] selftests/namespaces: tenth active reference count tests

[Christian Brauner]

Test multiple children sharing same parent.
Parent should stay active as long as ANY child is active.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 170 +++++++++++++++++++++
 1 file changed, 170 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 88189398aa35..a334792da982 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -1106,4 +1106,174 @@ TEST(ns_multilevel_hierarchy)
 	close(gp_fd);
 }
 
+/*
+ * Test multiple children sharing same parent.
+ * Parent should stay active as long as ANY child is active.
+ */
+TEST(ns_multiple_children_same_parent)
+{
+	struct file_handle *p_handle, *c1_handle, *c2_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 p_id, c1_id, c2_id;
+	char p_buf[sizeof(*p_handle) + MAX_HANDLE_SZ];
+	char c1_buf[sizeof(*c1_handle) + MAX_HANDLE_SZ];
+	char c2_buf[sizeof(*c2_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create parent user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int p_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (p_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(p_fd, NS_GET_ID, &p_id) < 0) {
+			close(p_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(p_fd);
+
+		/* Create first child user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int c1_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (c1_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(c1_fd, NS_GET_ID, &c1_id) < 0) {
+			close(c1_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(c1_fd);
+
+		/* Return to parent user namespace and create second child */
+		/* We can't actually do this easily, so let's create a sibling namespace
+		 * by creating a network namespace instead */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int c2_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (c2_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(c2_fd, NS_GET_ID, &c2_id) < 0) {
+			close(c2_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(c2_fd);
+
+		/* Send all namespace IDs */
+		write(pipefd[1], &p_id, sizeof(p_id));
+		write(pipefd[1], &c1_id, sizeof(c1_id));
+		write(pipefd[1], &c2_id, sizeof(c2_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &p_id, sizeof(p_id));
+	if (ret != sizeof(p_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read parent namespace ID");
+	}
+
+	ret = read(pipefd[0], &c1_id, sizeof(c1_id));
+	if (ret != sizeof(c1_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read first child namespace ID");
+	}
+
+	ret = read(pipefd[0], &c2_id, sizeof(c2_id));
+	close(pipefd[0]);
+	if (ret != sizeof(c2_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read second child namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	p_handle = (struct file_handle *)p_buf;
+	p_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	p_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *p_fh = (struct nsfs_file_handle *)p_handle->f_handle;
+	p_fh->ns_id = p_id;
+	p_fh->ns_type = 0;
+	p_fh->ns_inum = 0;
+
+	c1_handle = (struct file_handle *)c1_buf;
+	c1_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	c1_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *c1_fh = (struct nsfs_file_handle *)c1_handle->f_handle;
+	c1_fh->ns_id = c1_id;
+	c1_fh->ns_type = 0;
+	c1_fh->ns_inum = 0;
+
+	c2_handle = (struct file_handle *)c2_buf;
+	c2_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	c2_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *c2_fh = (struct nsfs_file_handle *)c2_handle->f_handle;
+	c2_fh->ns_id = c2_id;
+	c2_fh->ns_type = 0;
+	c2_fh->ns_inum = 0;
+
+	/* Open both children before process exits */
+	int c1_fd = open_by_handle_at(FD_NSFS_ROOT, c1_handle, O_RDONLY);
+	int c2_fd = open_by_handle_at(FD_NSFS_ROOT, c2_handle, O_RDONLY);
+
+	if (c1_fd < 0 || c2_fd < 0) {
+		if (c1_fd >= 0) close(c1_fd);
+		if (c2_fd >= 0) close(c2_fd);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open child namespaces");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Parent should be active (both children active) */
+	TH_LOG("Both children active - parent should be active");
+	int p_fd = open_by_handle_at(FD_NSFS_ROOT, p_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+	close(p_fd);
+
+	/* Close first child - parent should STILL be active */
+	TH_LOG("Closing first child - parent should still be active");
+	close(c1_fd);
+	p_fd = open_by_handle_at(FD_NSFS_ROOT, p_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+	close(p_fd);
+
+	/* Close second child - NOW parent should become inactive */
+	TH_LOG("Closing second child - parent should become inactive");
+	close(c2_fd);
+	p_fd = open_by_handle_at(FD_NSFS_ROOT, p_handle, O_RDONLY);
+	ASSERT_LT(p_fd, 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 31/70] selftests/namespaces: eleventh active reference count tests

[Christian Brauner]

Test that different namespace types with same owner all contribute
active references to the owning user namespace.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 171 +++++++++++++++++++++
 1 file changed, 171 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index a334792da982..885f58c81247 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -1276,4 +1276,175 @@ TEST(ns_multiple_children_same_parent)
 	ASSERT_LT(p_fd, 0);
 }
 
+/*
+ * Test that different namespace types with same owner all contribute
+ * active references to the owning user namespace.
+ */
+TEST(ns_different_types_same_owner)
+{
+	struct file_handle *u_handle, *n_handle, *ut_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 u_id, n_id, ut_id;
+	char u_buf[sizeof(*u_handle) + MAX_HANDLE_SZ];
+	char n_buf[sizeof(*n_handle) + MAX_HANDLE_SZ];
+	char ut_buf[sizeof(*ut_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int u_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (u_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(u_fd, NS_GET_ID, &u_id) < 0) {
+			close(u_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(u_fd);
+
+		/* Create network namespace (owned by user namespace) */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int n_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (n_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(n_fd, NS_GET_ID, &n_id) < 0) {
+			close(n_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(n_fd);
+
+		/* Create UTS namespace (also owned by user namespace) */
+		if (unshare(CLONE_NEWUTS) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ut_fd = open("/proc/self/ns/uts", O_RDONLY);
+		if (ut_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ut_fd, NS_GET_ID, &ut_id) < 0) {
+			close(ut_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ut_fd);
+
+		/* Send all namespace IDs */
+		write(pipefd[1], &u_id, sizeof(u_id));
+		write(pipefd[1], &n_id, sizeof(n_id));
+		write(pipefd[1], &ut_id, sizeof(ut_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &u_id, sizeof(u_id));
+	if (ret != sizeof(u_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user namespace ID");
+	}
+
+	ret = read(pipefd[0], &n_id, sizeof(n_id));
+	if (ret != sizeof(n_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read network namespace ID");
+	}
+
+	ret = read(pipefd[0], &ut_id, sizeof(ut_id));
+	close(pipefd[0]);
+	if (ret != sizeof(ut_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read UTS namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	u_handle = (struct file_handle *)u_buf;
+	u_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	u_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *u_fh = (struct nsfs_file_handle *)u_handle->f_handle;
+	u_fh->ns_id = u_id;
+	u_fh->ns_type = 0;
+	u_fh->ns_inum = 0;
+
+	n_handle = (struct file_handle *)n_buf;
+	n_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	n_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *n_fh = (struct nsfs_file_handle *)n_handle->f_handle;
+	n_fh->ns_id = n_id;
+	n_fh->ns_type = 0;
+	n_fh->ns_inum = 0;
+
+	ut_handle = (struct file_handle *)ut_buf;
+	ut_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ut_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ut_fh = (struct nsfs_file_handle *)ut_handle->f_handle;
+	ut_fh->ns_id = ut_id;
+	ut_fh->ns_type = 0;
+	ut_fh->ns_inum = 0;
+
+	/* Open both non-user namespaces before process exits */
+	int n_fd = open_by_handle_at(FD_NSFS_ROOT, n_handle, O_RDONLY);
+	int ut_fd = open_by_handle_at(FD_NSFS_ROOT, ut_handle, O_RDONLY);
+
+	if (n_fd < 0 || ut_fd < 0) {
+		if (n_fd >= 0) close(n_fd);
+		if (ut_fd >= 0) close(ut_fd);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open namespaces");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/*
+	 * Both network and UTS namespaces are active.
+	 * User namespace should be active (gets 2 active refs).
+	 */
+	TH_LOG("Both net and uts active - user namespace should be active");
+	int u_fd = open_by_handle_at(FD_NSFS_ROOT, u_handle, O_RDONLY);
+	ASSERT_GE(u_fd, 0);
+	close(u_fd);
+
+	/* Close network namespace - user namespace should STILL be active */
+	TH_LOG("Closing network ns - user ns should still be active (uts still active)");
+	close(n_fd);
+	u_fd = open_by_handle_at(FD_NSFS_ROOT, u_handle, O_RDONLY);
+	ASSERT_GE(u_fd, 0);
+	close(u_fd);
+
+	/* Close UTS namespace - user namespace should become inactive */
+	TH_LOG("Closing uts ns - user ns should become inactive");
+	close(ut_fd);
+	u_fd = open_by_handle_at(FD_NSFS_ROOT, u_handle, O_RDONLY);
+	ASSERT_LT(u_fd, 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 32/70] selftests/namespaces: twelth active reference count tests

[Christian Brauner]

Test hierarchical propagation with deep namespace hierarchy.
Create: init_user_ns -> user_A -> user_B -> net_ns
When net_ns is active, both user_A and user_B should be active.
This verifies the conditional recursion in __ns_ref_active_put() works.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 176 +++++++++++++++++++++
 1 file changed, 176 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 885f58c81247..b1a454dac9d0 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -1447,4 +1447,180 @@ TEST(ns_different_types_same_owner)
 	ASSERT_LT(u_fd, 0);
 }
 
+/*
+ * Test hierarchical propagation with deep namespace hierarchy.
+ * Create: init_user_ns -> user_A -> user_B -> net_ns
+ * When net_ns is active, both user_A and user_B should be active.
+ * This verifies the conditional recursion in __ns_ref_active_put() works.
+ */
+TEST(ns_deep_hierarchy_propagation)
+{
+	struct file_handle *ua_handle, *ub_handle, *net_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 ua_id, ub_id, net_id;
+	char ua_buf[sizeof(*ua_handle) + MAX_HANDLE_SZ];
+	char ub_buf[sizeof(*ub_handle) + MAX_HANDLE_SZ];
+	char net_buf[sizeof(*net_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create user_A -> user_B -> net hierarchy */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ua_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (ua_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ua_fd, NS_GET_ID, &ua_id) < 0) {
+			close(ua_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ua_fd);
+
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ub_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (ub_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ub_fd, NS_GET_ID, &ub_id) < 0) {
+			close(ub_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ub_fd);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int net_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (net_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(net_fd, NS_GET_ID, &net_id) < 0) {
+			close(net_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(net_fd);
+
+		/* Send all three namespace IDs */
+		write(pipefd[1], &ua_id, sizeof(ua_id));
+		write(pipefd[1], &ub_id, sizeof(ub_id));
+		write(pipefd[1], &net_id, sizeof(net_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &ua_id, sizeof(ua_id));
+	if (ret != sizeof(ua_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user_A namespace ID");
+	}
+
+	ret = read(pipefd[0], &ub_id, sizeof(ub_id));
+	if (ret != sizeof(ub_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user_B namespace ID");
+	}
+
+	ret = read(pipefd[0], &net_id, sizeof(net_id));
+	close(pipefd[0]);
+	if (ret != sizeof(net_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read network namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	ua_handle = (struct file_handle *)ua_buf;
+	ua_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ua_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ua_fh = (struct nsfs_file_handle *)ua_handle->f_handle;
+	ua_fh->ns_id = ua_id;
+	ua_fh->ns_type = 0;
+	ua_fh->ns_inum = 0;
+
+	ub_handle = (struct file_handle *)ub_buf;
+	ub_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ub_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ub_fh = (struct nsfs_file_handle *)ub_handle->f_handle;
+	ub_fh->ns_id = ub_id;
+	ub_fh->ns_type = 0;
+	ub_fh->ns_inum = 0;
+
+	net_handle = (struct file_handle *)net_buf;
+	net_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *net_fh = (struct nsfs_file_handle *)net_handle->f_handle;
+	net_fh->ns_id = net_id;
+	net_fh->ns_type = 0;
+	net_fh->ns_inum = 0;
+
+	/* Open net_ns before child exits to keep it active */
+	int net_fd = open_by_handle_at(FD_NSFS_ROOT, net_handle, O_RDONLY);
+	if (net_fd < 0) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open network namespace");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* With net_ns active, both user_A and user_B should be active */
+	TH_LOG("Testing user_B active (net_ns active causes propagation)");
+	int ub_fd = open_by_handle_at(FD_NSFS_ROOT, ub_handle, O_RDONLY);
+	ASSERT_GE(ub_fd, 0);
+
+	TH_LOG("Testing user_A active (propagated through user_B)");
+	int ua_fd = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_GE(ua_fd, 0);
+
+	/* Close net_ns - user_B should stay active (we hold direct ref) */
+	TH_LOG("Closing net_ns, user_B should remain active (direct ref held)");
+	close(net_fd);
+	int ub_fd2 = open_by_handle_at(FD_NSFS_ROOT, ub_handle, O_RDONLY);
+	ASSERT_GE(ub_fd2, 0);
+	close(ub_fd2);
+
+	/* Close user_B - user_A should stay active (we hold direct ref) */
+	TH_LOG("Closing user_B, user_A should remain active (direct ref held)");
+	close(ub_fd);
+	int ua_fd2 = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_GE(ua_fd2, 0);
+	close(ua_fd2);
+
+	/* Close user_A - everything should become inactive */
+	TH_LOG("Closing user_A, all should become inactive");
+	close(ua_fd);
+
+	/* All should now be inactive */
+	ua_fd = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_LT(ua_fd, 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 33/70] selftests/namespaces: thirteenth active reference count tests

[Christian Brauner]

Test that parent stays active as long as ANY child is active.
Create parent user namespace with two child net namespaces.
Parent should remain active until BOTH children are inactive.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 194 +++++++++++++++++++++
 1 file changed, 194 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index b1a454dac9d0..25f06e623064 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -1623,4 +1623,198 @@ TEST(ns_deep_hierarchy_propagation)
 	ASSERT_LT(ua_fd, 0);
 }
 
+/*
+ * Test that parent stays active as long as ANY child is active.
+ * Create parent user namespace with two child net namespaces.
+ * Parent should remain active until BOTH children are inactive.
+ */
+TEST(ns_parent_multiple_children_refcount)
+{
+	struct file_handle *parent_handle, *net1_handle, *net2_handle;
+	int ret, pipefd[2], syncpipe[2];
+	pid_t pid;
+	int status;
+	__u64 p_id, n1_id, n2_id;
+	char p_buf[sizeof(*parent_handle) + MAX_HANDLE_SZ];
+	char n1_buf[sizeof(*net1_handle) + MAX_HANDLE_SZ];
+	char n2_buf[sizeof(*net2_handle) + MAX_HANDLE_SZ];
+	char sync_byte;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(pipe(syncpipe), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+		close(syncpipe[1]);
+
+		/* Create parent user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int p_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (p_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(p_fd, NS_GET_ID, &p_id) < 0) {
+			close(p_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(p_fd);
+
+		/* Create first network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		int n1_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (n1_fd < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+		if (ioctl(n1_fd, NS_GET_ID, &n1_id) < 0) {
+			close(n1_fd);
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+		/* Keep n1_fd open so first namespace stays active */
+
+		/* Create second network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(n1_fd);
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		int n2_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (n2_fd < 0) {
+			close(n1_fd);
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+		if (ioctl(n2_fd, NS_GET_ID, &n2_id) < 0) {
+			close(n1_fd);
+			close(n2_fd);
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+		/* Keep both n1_fd and n2_fd open */
+
+		/* Send all namespace IDs */
+		write(pipefd[1], &p_id, sizeof(p_id));
+		write(pipefd[1], &n1_id, sizeof(n1_id));
+		write(pipefd[1], &n2_id, sizeof(n2_id));
+		close(pipefd[1]);
+
+		/* Wait for parent to signal before exiting */
+		read(syncpipe[0], &sync_byte, 1);
+		close(syncpipe[0]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+	close(syncpipe[0]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &p_id, sizeof(p_id));
+	if (ret != sizeof(p_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read parent namespace ID");
+	}
+
+	ret = read(pipefd[0], &n1_id, sizeof(n1_id));
+	if (ret != sizeof(n1_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read first network namespace ID");
+	}
+
+	ret = read(pipefd[0], &n2_id, sizeof(n2_id));
+	close(pipefd[0]);
+	if (ret != sizeof(n2_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read second network namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	parent_handle = (struct file_handle *)p_buf;
+	parent_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	parent_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *p_fh = (struct nsfs_file_handle *)parent_handle->f_handle;
+	p_fh->ns_id = p_id;
+	p_fh->ns_type = 0;
+	p_fh->ns_inum = 0;
+
+	net1_handle = (struct file_handle *)n1_buf;
+	net1_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net1_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *n1_fh = (struct nsfs_file_handle *)net1_handle->f_handle;
+	n1_fh->ns_id = n1_id;
+	n1_fh->ns_type = 0;
+	n1_fh->ns_inum = 0;
+
+	net2_handle = (struct file_handle *)n2_buf;
+	net2_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net2_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *n2_fh = (struct nsfs_file_handle *)net2_handle->f_handle;
+	n2_fh->ns_id = n2_id;
+	n2_fh->ns_type = 0;
+	n2_fh->ns_inum = 0;
+
+	/* Open both net namespaces while child is still alive */
+	int n1_fd = open_by_handle_at(FD_NSFS_ROOT, net1_handle, O_RDONLY);
+	int n2_fd = open_by_handle_at(FD_NSFS_ROOT, net2_handle, O_RDONLY);
+	if (n1_fd < 0 || n2_fd < 0) {
+		if (n1_fd >= 0) close(n1_fd);
+		if (n2_fd >= 0) close(n2_fd);
+		sync_byte = 'G';
+		write(syncpipe[1], &sync_byte, 1);
+		close(syncpipe[1]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open net namespaces");
+	}
+
+	/* Signal child that we have opened the namespaces */
+	sync_byte = 'G';
+	write(syncpipe[1], &sync_byte, 1);
+	close(syncpipe[1]);
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Parent should be active (has 2 active children) */
+	TH_LOG("Both net namespaces active - parent should be active");
+	int p_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+	close(p_fd);
+
+	/* Close first net namespace - parent should STILL be active */
+	TH_LOG("Closing first net ns - parent should still be active");
+	close(n1_fd);
+	p_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+	close(p_fd);
+
+	/* Close second net namespace - parent should become inactive */
+	TH_LOG("Closing second net ns - parent should become inactive");
+	close(n2_fd);
+	p_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_LT(p_fd, 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 34/70] selftests/namespaces: fourteenth active reference count tests

[Christian Brauner]

Test that user namespace as a child also propagates correctly.
Create user_A -> user_B, verify when user_B is active that user_A
is also active. This is different from non-user namespace children.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 132 +++++++++++++++++++++
 1 file changed, 132 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 25f06e623064..430702c041a9 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -1817,4 +1817,136 @@ TEST(ns_parent_multiple_children_refcount)
 	ASSERT_LT(p_fd, 0);
 }
 
+/*
+ * Test that user namespace as a child also propagates correctly.
+ * Create user_A -> user_B, verify when user_B is active that user_A
+ * is also active. This is different from non-user namespace children.
+ */
+TEST(ns_userns_child_propagation)
+{
+	struct file_handle *ua_handle, *ub_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 ua_id, ub_id;
+	char ua_buf[sizeof(*ua_handle) + MAX_HANDLE_SZ];
+	char ub_buf[sizeof(*ub_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create user_A */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ua_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (ua_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ua_fd, NS_GET_ID, &ua_id) < 0) {
+			close(ua_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ua_fd);
+
+		/* Create user_B (child of user_A) */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ub_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (ub_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ub_fd, NS_GET_ID, &ub_id) < 0) {
+			close(ub_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ub_fd);
+
+		/* Send both namespace IDs */
+		write(pipefd[1], &ua_id, sizeof(ua_id));
+		write(pipefd[1], &ub_id, sizeof(ub_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read both namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &ua_id, sizeof(ua_id));
+	if (ret != sizeof(ua_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user_A namespace ID");
+	}
+
+	ret = read(pipefd[0], &ub_id, sizeof(ub_id));
+	close(pipefd[0]);
+	if (ret != sizeof(ub_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user_B namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	ua_handle = (struct file_handle *)ua_buf;
+	ua_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ua_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ua_fh = (struct nsfs_file_handle *)ua_handle->f_handle;
+	ua_fh->ns_id = ua_id;
+	ua_fh->ns_type = 0;
+	ua_fh->ns_inum = 0;
+
+	ub_handle = (struct file_handle *)ub_buf;
+	ub_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ub_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ub_fh = (struct nsfs_file_handle *)ub_handle->f_handle;
+	ub_fh->ns_id = ub_id;
+	ub_fh->ns_type = 0;
+	ub_fh->ns_inum = 0;
+
+	/* Open user_B before child exits */
+	int ub_fd = open_by_handle_at(FD_NSFS_ROOT, ub_handle, O_RDONLY);
+	if (ub_fd < 0) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open user_B");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* With user_B active, user_A should also be active */
+	TH_LOG("Testing user_A active when child user_B is active");
+	int ua_fd = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_GE(ua_fd, 0);
+
+	/* Close user_B */
+	TH_LOG("Closing user_B");
+	close(ub_fd);
+
+	/* user_A should remain active (we hold direct ref) */
+	int ua_fd2 = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_GE(ua_fd2, 0);
+	close(ua_fd2);
+
+	/* Close user_A - should become inactive */
+	TH_LOG("Closing user_A - should become inactive");
+	close(ua_fd);
+
+	ua_fd = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_LT(ua_fd, 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 35/70] selftests/namespaces: fifteenth active reference count tests

[Christian Brauner]

Test different namespace types (net, uts, ipc) all contributing
active references to the same owning user namespace.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 164 +++++++++++++++++++++
 1 file changed, 164 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 430702c041a9..b7fa973a2572 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -1949,4 +1949,168 @@ TEST(ns_userns_child_propagation)
 	ASSERT_LT(ua_fd, 0);
 }
 
+/*
+ * Test different namespace types (net, uts, ipc) all contributing
+ * active references to the same owning user namespace.
+ */
+TEST(ns_mixed_types_same_owner)
+{
+	struct file_handle *user_handle, *net_handle, *uts_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 u_id, n_id, ut_id;
+	char u_buf[sizeof(*user_handle) + MAX_HANDLE_SZ];
+	char n_buf[sizeof(*net_handle) + MAX_HANDLE_SZ];
+	char ut_buf[sizeof(*uts_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int u_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (u_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(u_fd, NS_GET_ID, &u_id) < 0) {
+			close(u_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(u_fd);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int n_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (n_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(n_fd, NS_GET_ID, &n_id) < 0) {
+			close(n_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(n_fd);
+
+		if (unshare(CLONE_NEWUTS) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ut_fd = open("/proc/self/ns/uts", O_RDONLY);
+		if (ut_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ut_fd, NS_GET_ID, &ut_id) < 0) {
+			close(ut_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ut_fd);
+
+		/* Send all namespace IDs */
+		write(pipefd[1], &u_id, sizeof(u_id));
+		write(pipefd[1], &n_id, sizeof(n_id));
+		write(pipefd[1], &ut_id, sizeof(ut_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &u_id, sizeof(u_id));
+	if (ret != sizeof(u_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user namespace ID");
+	}
+
+	ret = read(pipefd[0], &n_id, sizeof(n_id));
+	if (ret != sizeof(n_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read network namespace ID");
+	}
+
+	ret = read(pipefd[0], &ut_id, sizeof(ut_id));
+	close(pipefd[0]);
+	if (ret != sizeof(ut_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read UTS namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	user_handle = (struct file_handle *)u_buf;
+	user_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	user_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *u_fh = (struct nsfs_file_handle *)user_handle->f_handle;
+	u_fh->ns_id = u_id;
+	u_fh->ns_type = 0;
+	u_fh->ns_inum = 0;
+
+	net_handle = (struct file_handle *)n_buf;
+	net_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *n_fh = (struct nsfs_file_handle *)net_handle->f_handle;
+	n_fh->ns_id = n_id;
+	n_fh->ns_type = 0;
+	n_fh->ns_inum = 0;
+
+	uts_handle = (struct file_handle *)ut_buf;
+	uts_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	uts_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ut_fh = (struct nsfs_file_handle *)uts_handle->f_handle;
+	ut_fh->ns_id = ut_id;
+	ut_fh->ns_type = 0;
+	ut_fh->ns_inum = 0;
+
+	/* Open both non-user namespaces */
+	int n_fd = open_by_handle_at(FD_NSFS_ROOT, net_handle, O_RDONLY);
+	int ut_fd = open_by_handle_at(FD_NSFS_ROOT, uts_handle, O_RDONLY);
+	if (n_fd < 0 || ut_fd < 0) {
+		if (n_fd >= 0) close(n_fd);
+		if (ut_fd >= 0) close(ut_fd);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open namespaces");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* User namespace should be active (2 active children) */
+	TH_LOG("Both net and uts active - user ns should be active");
+	int u_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_GE(u_fd, 0);
+	close(u_fd);
+
+	/* Close net - user ns should STILL be active (uts still active) */
+	TH_LOG("Closing net - user ns should still be active");
+	close(n_fd);
+	u_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_GE(u_fd, 0);
+	close(u_fd);
+
+	/* Close uts - user ns should become inactive */
+	TH_LOG("Closing uts - user ns should become inactive");
+	close(ut_fd);
+	u_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_LT(u_fd, 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 36/70] selftests/namespaces: add listns() wrapper

[Christian Brauner]

Add a wrapper for the listns() system call.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/wrappers.h | 35 +++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/tools/testing/selftests/namespaces/wrappers.h b/tools/testing/selftests/namespaces/wrappers.h
new file mode 100644
index 000000000000..9741a64a5b1d
--- /dev/null
+++ b/tools/testing/selftests/namespaces/wrappers.h
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/nsfs.h>
+#include <linux/types.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#ifndef __SELFTESTS_NAMESPACES_WRAPPERS_H__
+#define __SELFTESTS_NAMESPACES_WRAPPERS_H__
+
+#ifndef __NR_listns
+	#if defined __alpha__
+		#define __NR_listns 580
+	#elif defined _MIPS_SIM
+		#if _MIPS_SIM == _MIPS_SIM_ABI32	/* o32 */
+			#define __NR_listns 4470
+		#endif
+		#if _MIPS_SIM == _MIPS_SIM_NABI32	/* n32 */
+			#define __NR_listns 6470
+		#endif
+		#if _MIPS_SIM == _MIPS_SIM_ABI64	/* n64 */
+			#define __NR_listns 5470
+		#endif
+	#else
+		#define __NR_listns 470
+	#endif
+#endif
+
+static inline int sys_listns(const struct ns_id_req *req, __u64 *ns_ids,
+			     size_t nr_ns_ids, unsigned int flags)
+{
+	return syscall(__NR_listns, req, ns_ids, nr_ns_ids, flags);
+}
+
+#endif /* __SELFTESTS_NAMESPACES_WRAPPERS_H__ */

-- 
2.47.3

[PATCH v3 37/70] selftests/namespaces: first listns() test

[Christian Brauner]

Test basic listns() functionality with the unified namespace tree.
List all active namespaces globally.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/.gitignore    |  1 +
 tools/testing/selftests/namespaces/Makefile      |  3 +-
 tools/testing/selftests/namespaces/listns_test.c | 57 ++++++++++++++++++++++++
 3 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index 100cc5bfef04..5065f07e92c9 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -2,3 +2,4 @@ nsid_test
 file_handle_test
 init_ino_test
 ns_active_ref_test
+listns_test
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index 5cea938cdde8..de708f4df159 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -2,9 +2,10 @@
 CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 LDLIBS += -lcap
 
-TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test
+TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test listns_test
 
 include ../lib.mk
 
 $(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
+$(OUTPUT)/listns_test: ../filesystems/utils.c
 
diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
new file mode 100644
index 000000000000..cb42827d3dfe
--- /dev/null
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/nsfs.h>
+#include <sys/ioctl.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Test basic listns() functionality with the unified namespace tree.
+ * List all active namespaces globally.
+ */
+TEST(listns_basic_unified)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,  /* All types */
+		.spare2 = 0,
+		.user_ns_id = 0,  /* Global listing */
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+
+	/* Should find at least the initial namespaces */
+	ASSERT_GT(ret, 0);
+	TH_LOG("Found %zd active namespaces", ret);
+
+	/* Verify all returned IDs are non-zero */
+	for (ssize_t i = 0; i < ret; i++) {
+		ASSERT_NE(ns_ids[i], 0);
+		TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
+	}
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 38/70] selftests/namespaces: second listns() test

[Christian Brauner]

test listns() with type filtering.
List only network namespaces.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/listns_test.c | 61 ++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
index cb42827d3dfe..64249502ac49 100644
--- a/tools/testing/selftests/namespaces/listns_test.c
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -54,4 +54,65 @@ TEST(listns_basic_unified)
 	}
 }
 
+/*
+ * Test listns() with type filtering.
+ * List only network namespaces.
+ */
+TEST(listns_filter_by_type)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET,  /* Only network namespaces */
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret, 0);
+
+	/* Should find at least init_net */
+	ASSERT_GT(ret, 0);
+	TH_LOG("Found %zd active network namespaces", ret);
+
+	/* Verify we can open each namespace and it's actually a network namespace */
+	for (ssize_t i = 0; i < ret && i < 5; i++) {
+		struct nsfs_file_handle nsfh = {
+			.ns_id = ns_ids[i],
+			.ns_type = CLONE_NEWNET,
+			.ns_inum = 0,
+		};
+		struct file_handle *fh;
+		int fd;
+
+		fh = (struct file_handle *)malloc(sizeof(*fh) + sizeof(nsfh));
+		ASSERT_NE(fh, NULL);
+		fh->handle_bytes = sizeof(nsfh);
+		fh->handle_type = 0;
+		memcpy(fh->f_handle, &nsfh, sizeof(nsfh));
+
+		fd = open_by_handle_at(-10003, fh, O_RDONLY);
+		free(fh);
+
+		if (fd >= 0) {
+			int ns_type;
+			/* Verify it's a network namespace via ioctl */
+			ns_type = ioctl(fd, NS_GET_NSTYPE);
+			if (ns_type >= 0) {
+				ASSERT_EQ(ns_type, CLONE_NEWNET);
+			}
+			close(fd);
+		}
+	}
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 39/70] selftests/namespaces: third listns() test

[Christian Brauner]

Test listns() pagination.
List namespaces in batches.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/listns_test.c | 53 ++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
index 64249502ac49..7dff63a00263 100644
--- a/tools/testing/selftests/namespaces/listns_test.c
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -115,4 +115,57 @@ TEST(listns_filter_by_type)
 	}
 }
 
+/*
+ * Test listns() pagination.
+ * List namespaces in batches.
+ */
+TEST(listns_pagination)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 batch1[2], batch2[2];
+	ssize_t ret1, ret2;
+
+	/* Get first batch */
+	ret1 = sys_listns(&req, batch1, ARRAY_SIZE(batch1), 0);
+	if (ret1 < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret1, 0);
+
+	if (ret1 == 0)
+		SKIP(return, "No namespaces found");
+
+	TH_LOG("First batch: %zd namespaces", ret1);
+
+	/* Get second batch using last ID from first batch */
+	if (ret1 == ARRAY_SIZE(batch1)) {
+		req.ns_id = batch1[ret1 - 1];
+		ret2 = sys_listns(&req, batch2, ARRAY_SIZE(batch2), 0);
+		ASSERT_GE(ret2, 0);
+
+		TH_LOG("Second batch: %zd namespaces (after ns_id=%llu)",
+		       ret2, (unsigned long long)req.ns_id);
+
+		/* If we got more results, verify IDs are monotonically increasing */
+		if (ret2 > 0) {
+			ASSERT_GT(batch2[0], batch1[ret1 - 1]);
+			TH_LOG("Pagination working: %llu > %llu",
+			       (unsigned long long)batch2[0],
+			       (unsigned long long)batch1[ret1 - 1]);
+		}
+	} else {
+		TH_LOG("All namespaces fit in first batch");
+	}
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 40/70] selftests/namespaces: fourth listns() test

[Christian Brauner]

Test listns() with LISTNS_CURRENT_USER.
List namespaces owned by current user namespace.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/listns_test.c | 33 ++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
index 7dff63a00263..457298cb4c64 100644
--- a/tools/testing/selftests/namespaces/listns_test.c
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -168,4 +168,37 @@ TEST(listns_pagination)
 	}
 }
 
+/*
+ * Test listns() with LISTNS_CURRENT_USER.
+ * List namespaces owned by current user namespace.
+ */
+TEST(listns_current_user)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = LISTNS_CURRENT_USER,
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret, 0);
+
+	/* Should find at least the initial namespaces if we're in init_user_ns */
+	TH_LOG("Found %zd namespaces owned by current user namespace", ret);
+
+	for (ssize_t i = 0; i < ret; i++)
+		TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 41/70] selftests/namespaces: fifth listns() test

[Christian Brauner]

Test that listns() only returns active namespaces.
Create a namespace, let it become inactive, verify it's not listed.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/listns_test.c | 123 +++++++++++++++++++++++
 1 file changed, 123 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
index 457298cb4c64..a04ebe11ce2c 100644
--- a/tools/testing/selftests/namespaces/listns_test.c
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -201,4 +201,127 @@ TEST(listns_current_user)
 		TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
 }
 
+/*
+ * Test that listns() only returns active namespaces.
+ * Create a namespace, let it become inactive, verify it's not listed.
+ */
+TEST(listns_only_active)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[100], ns_ids_after[100];
+	ssize_t ret_before, ret_after;
+	int pipefd[2];
+	pid_t pid;
+	__u64 new_ns_id = 0;
+	int status;
+
+	/* Get initial list */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret_before, 0);
+
+	TH_LOG("Before: %zd active network namespaces", ret_before);
+
+	/* Create a new namespace in a child process and get its ID */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 ns_id;
+
+		close(pipefd[0]);
+
+		/* Create new network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get its ID */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &ns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send ID to parent */
+		write(pipefd[1], &ns_id, sizeof(ns_id));
+		close(pipefd[1]);
+
+		/* Keep namespace active briefly */
+		usleep(100000);
+		exit(0);
+	}
+
+	/* Parent reads the new namespace ID */
+	{
+		int bytes;
+
+		close(pipefd[1]);
+		bytes = read(pipefd[0], &new_ns_id, sizeof(new_ns_id));
+		close(pipefd[0]);
+
+		if (bytes == sizeof(new_ns_id)) {
+			__u64 ns_ids_during[100];
+			int ret_during;
+
+			TH_LOG("Child created namespace with ID %llu", (unsigned long long)new_ns_id);
+
+			/* List namespaces while child is still alive - should see new one */
+			ret_during = sys_listns(&req, ns_ids_during, ARRAY_SIZE(ns_ids_during), 0);
+			ASSERT_GE(ret_during, 0);
+			TH_LOG("During: %d active network namespaces", ret_during);
+
+			/* Should have more namespaces than before */
+			ASSERT_GE(ret_during, ret_before);
+		}
+	}
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+
+	/* Give time for namespace to become inactive */
+	usleep(100000);
+
+	/* List namespaces after child exits - should not see new one */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+	TH_LOG("After: %zd active network namespaces", ret_after);
+
+	/* Verify the new namespace ID is not in the after list */
+	if (new_ns_id != 0) {
+		bool found = false;
+
+		for (ssize_t i = 0; i < ret_after; i++) {
+			if (ns_ids_after[i] == new_ns_id) {
+				found = true;
+				break;
+			}
+		}
+		ASSERT_FALSE(found);
+	}
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 42/70] selftests/namespaces: sixth listns() test

[Christian Brauner]

Test listns() with specific user namespace ID.
Create a user namespace and list namespaces it owns.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/listns_test.c | 122 +++++++++++++++++++++++
 1 file changed, 122 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
index a04ebe11ce2c..f5b8bc5d111f 100644
--- a/tools/testing/selftests/namespaces/listns_test.c
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -9,6 +9,7 @@
 #include <string.h>
 #include <linux/nsfs.h>
 #include <sys/ioctl.h>
+#include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
@@ -324,4 +325,125 @@ TEST(listns_only_active)
 	}
 }
 
+/*
+ * Test listns() with specific user namespace ID.
+ * Create a user namespace and list namespaces it owns.
+ */
+TEST(listns_specific_userns)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,  /* Will be filled with created userns ID */
+	};
+	__u64 ns_ids[100];
+	int sv[2];
+	pid_t pid;
+	int status;
+	__u64 user_ns_id = 0;
+	int bytes;
+	ssize_t ret;
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 ns_id;
+		char buf;
+
+		close(sv[0]);
+
+		/* Create new user namespace */
+		if (setup_userns() < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &ns_id) < 0) {
+			close(fd);
+			close(sv[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send ID to parent */
+		if (write(sv[1], &ns_id, sizeof(ns_id)) != sizeof(ns_id)) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		/* Create some namespaces owned by this user namespace */
+		unshare(CLONE_NEWNET);
+		unshare(CLONE_NEWUTS);
+
+		/* Wait for parent signal */
+		if (read(sv[1], &buf, 1) != 1) {
+			close(sv[1]);
+			exit(1);
+		}
+		close(sv[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(sv[1]);
+	bytes = read(sv[0], &user_ns_id, sizeof(user_ns_id));
+
+	if (bytes != sizeof(user_ns_id)) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get user namespace ID from child");
+	}
+
+	TH_LOG("Child created user namespace with ID %llu", (unsigned long long)user_ns_id);
+
+	/* List namespaces owned by this user namespace */
+	req.user_ns_id = user_ns_id;
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+	if (ret < 0) {
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		if (errno == ENOSYS) {
+			SKIP(return, "listns() not supported");
+		}
+		ASSERT_GE(ret, 0);
+	}
+
+	TH_LOG("Found %zd namespaces owned by user namespace %llu", ret,
+	       (unsigned long long)user_ns_id);
+
+	/* Should find at least the network and UTS namespaces we created */
+	if (ret > 0) {
+		for (ssize_t i = 0; i < ret && i < 10; i++)
+			TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
+	}
+
+	/* Signal child to exit */
+	if (write(sv[0], "X", 1) != 1) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		ASSERT_TRUE(false);
+	}
+	close(sv[0]);
+	waitpid(pid, &status, 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 43/70] selftests/namespaces: seventh listns() test

[Christian Brauner]

Test listns() with multiple namespace types filter.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/listns_test.c | 31 ++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
index f5b8bc5d111f..d73c2a2898cf 100644
--- a/tools/testing/selftests/namespaces/listns_test.c
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -446,4 +446,35 @@ TEST(listns_specific_userns)
 	waitpid(pid, &status, 0);
 }
 
+/*
+ * Test listns() with multiple namespace types filter.
+ */
+TEST(listns_multiple_types)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET | CLONE_NEWUTS,  /* Network and UTS */
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret, 0);
+
+	TH_LOG("Found %zd active network/UTS namespaces", ret);
+
+	for (ssize_t i = 0; i < ret; i++)
+		TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 44/70] selftests/namespaces: eigth listns() test

[Christian Brauner]

Test that hierarchical active reference propagation keeps parent
user namespaces visible in listns().

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/listns_test.c | 150 +++++++++++++++++++++++
 1 file changed, 150 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
index d73c2a2898cf..d3be6f97d34e 100644
--- a/tools/testing/selftests/namespaces/listns_test.c
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -477,4 +477,154 @@ TEST(listns_multiple_types)
 		TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
 }
 
+/*
+ * Test that hierarchical active reference propagation keeps parent
+ * user namespaces visible in listns().
+ */
+TEST(listns_hierarchical_visibility)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 parent_ns_id = 0, child_ns_id = 0;
+	int sv[2];
+	pid_t pid;
+	int status;
+	int bytes;
+	__u64 ns_ids[100];
+	ssize_t ret;
+	bool found_parent, found_child;
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		char buf;
+
+		close(sv[0]);
+
+		/* Create parent user namespace */
+		if (setup_userns() < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &parent_ns_id) < 0) {
+			close(fd);
+			close(sv[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create child user namespace */
+		if (setup_userns() < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_ns_id) < 0) {
+			close(fd);
+			close(sv[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send both IDs to parent */
+		if (write(sv[1], &parent_ns_id, sizeof(parent_ns_id)) != sizeof(parent_ns_id)) {
+			close(sv[1]);
+			exit(1);
+		}
+		if (write(sv[1], &child_ns_id, sizeof(child_ns_id)) != sizeof(child_ns_id)) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		/* Wait for parent signal */
+		if (read(sv[1], &buf, 1) != 1) {
+			close(sv[1]);
+			exit(1);
+		}
+		close(sv[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(sv[1]);
+
+	/* Read both namespace IDs */
+	bytes = read(sv[0], &parent_ns_id, sizeof(parent_ns_id));
+	bytes += read(sv[0], &child_ns_id, sizeof(child_ns_id));
+
+	if (bytes != (int)(2 * sizeof(__u64))) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace IDs from child");
+	}
+
+	TH_LOG("Parent user namespace ID: %llu", (unsigned long long)parent_ns_id);
+	TH_LOG("Child user namespace ID: %llu", (unsigned long long)child_ns_id);
+
+	/* List all user namespaces */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+	if (ret < 0 && errno == ENOSYS) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "listns() not supported");
+	}
+
+	ASSERT_GE(ret, 0);
+	TH_LOG("Found %zd active user namespaces", ret);
+
+	/* Both parent and child should be visible (active due to child process) */
+	found_parent = false;
+	found_child = false;
+	for (ssize_t i = 0; i < ret; i++) {
+		if (ns_ids[i] == parent_ns_id)
+			found_parent = true;
+		if (ns_ids[i] == child_ns_id)
+			found_child = true;
+	}
+
+	TH_LOG("Parent namespace %s, child namespace %s",
+	       found_parent ? "found" : "NOT FOUND",
+	       found_child ? "found" : "NOT FOUND");
+
+	ASSERT_TRUE(found_child);
+	/* With hierarchical propagation, parent should also be active */
+	ASSERT_TRUE(found_parent);
+
+	/* Signal child to exit */
+	if (write(sv[0], "X", 1) != 1) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		ASSERT_TRUE(false);
+	}
+	close(sv[0]);
+	waitpid(pid, &status, 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 45/70] selftests/namespaces: ninth listns() test

[Christian Brauner]

Test error cases for listns().

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/listns_test.c | 49 ++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
index d3be6f97d34e..8a95789d6a87 100644
--- a/tools/testing/selftests/namespaces/listns_test.c
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -627,4 +627,53 @@ TEST(listns_hierarchical_visibility)
 	waitpid(pid, &status, 0);
 }
 
+/*
+ * Test error cases for listns().
+ */
+TEST(listns_error_cases)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[10];
+	int ret;
+
+	/* Test with invalid flags */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0xFFFF);
+	if (errno == ENOSYS) {
+		/* listns() not supported, skip this check */
+	} else {
+		ASSERT_LT(ret, 0);
+		ASSERT_EQ(errno, EINVAL);
+	}
+
+	/* Test with NULL ns_ids array */
+	ret = sys_listns(&req, NULL, 10, 0);
+	ASSERT_LT(ret, 0);
+
+	/* Test with invalid spare field */
+	req.spare = 1;
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (errno == ENOSYS) {
+		/* listns() not supported, skip this check */
+	} else {
+		ASSERT_LT(ret, 0);
+		ASSERT_EQ(errno, EINVAL);
+	}
+	req.spare = 0;
+
+	/* Test with huge nr_ns_ids */
+	ret = sys_listns(&req, ns_ids, 2000000, 0);
+	if (errno == ENOSYS) {
+		/* listns() not supported, skip this check */
+	} else {
+		ASSERT_LT(ret, 0);
+	}
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 46/70] selftests/namespaces: first listns() permission test

[Christian Brauner]

Test that unprivileged users can only see namespaces they're currently
in. Create a namespace, drop privileges, verify we can only see our own
namespaces.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/.gitignore      |   1 +
 tools/testing/selftests/namespaces/Makefile        |   3 +-
 .../selftests/namespaces/listns_permissions_test.c | 131 +++++++++++++++++++++
 3 files changed, 134 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index 5065f07e92c9..17f9c675a60b 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -3,3 +3,4 @@ file_handle_test
 init_ino_test
 ns_active_ref_test
 listns_test
+listns_permissions_test
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index de708f4df159..2dd22bc68b89 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -2,10 +2,11 @@
 CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 LDLIBS += -lcap
 
-TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test listns_test
+TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test listns_test listns_permissions_test
 
 include ../lib.mk
 
 $(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
 $(OUTPUT)/listns_test: ../filesystems/utils.c
+$(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
 
diff --git a/tools/testing/selftests/namespaces/listns_permissions_test.c b/tools/testing/selftests/namespaces/listns_permissions_test.c
new file mode 100644
index 000000000000..907fe419ec22
--- /dev/null
+++ b/tools/testing/selftests/namespaces/listns_permissions_test.c
@@ -0,0 +1,131 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/nsfs.h>
+#include <sys/capability.h>
+#include <sys/ioctl.h>
+#include <sys/prctl.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Test that unprivileged users can only see namespaces they're currently in.
+ * Create a namespace, drop privileges, verify we can only see our own namespaces.
+ */
+TEST(listns_unprivileged_current_only)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool found_ours;
+	int unexpected_count;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 our_netns_id;
+		bool found_ours;
+		int unexpected_count;
+
+		close(pipefd[0]);
+
+		/* Create user namespace to be unprivileged */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Create a network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get our network namespace ID */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &our_netns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Now we're unprivileged - list all network namespaces */
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* We should only see our own network namespace */
+		found_ours = false;
+		unexpected_count = 0;
+
+		for (ssize_t i = 0; i < ret; i++) {
+			if (ns_ids[i] == our_netns_id) {
+				found_ours = true;
+			} else {
+				/* This is either init_net (which we can see) or unexpected */
+				unexpected_count++;
+			}
+		}
+
+		/* Send results to parent */
+		write(pipefd[1], &found_ours, sizeof(found_ours));
+		write(pipefd[1], &unexpected_count, sizeof(unexpected_count));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	found_ours = false;
+	unexpected_count = 0;
+	read(pipefd[0], &found_ours, sizeof(found_ours));
+	read(pipefd[0], &unexpected_count, sizeof(unexpected_count));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Child should have seen its own namespace */
+	ASSERT_TRUE(found_ours);
+
+	TH_LOG("Unprivileged child saw its own namespace, plus %d others (likely init_net)",
+			unexpected_count);
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 47/70] selftests/namespaces: second listns() permission test

[Christian Brauner]

Test that users with CAP_SYS_ADMIN in a user namespace can see
all namespaces owned by that user namespace.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/listns_permissions_test.c | 100 +++++++++++++++++++++
 1 file changed, 100 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_permissions_test.c b/tools/testing/selftests/namespaces/listns_permissions_test.c
index 907fe419ec22..c99c87cca046 100644
--- a/tools/testing/selftests/namespaces/listns_permissions_test.c
+++ b/tools/testing/selftests/namespaces/listns_permissions_test.c
@@ -128,4 +128,104 @@ TEST(listns_unprivileged_current_only)
 			unexpected_count);
 }
 
+/*
+ * Test that users with CAP_SYS_ADMIN in a user namespace can see
+ * all namespaces owned by that user namespace.
+ */
+TEST(listns_cap_sys_admin_in_userns)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,  /* All types */
+		.spare2 = 0,
+		.user_ns_id = 0,  /* Will be set to our created user namespace */
+	};
+	__u64 ns_ids[100];
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool success;
+	ssize_t count;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 userns_id;
+		ssize_t ret;
+		int min_expected;
+		bool success;
+
+		close(pipefd[0]);
+
+		/* Create user namespace - we'll have CAP_SYS_ADMIN in it */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get the user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create several namespaces owned by this user namespace */
+		unshare(CLONE_NEWNET);
+		unshare(CLONE_NEWUTS);
+		unshare(CLONE_NEWIPC);
+
+		/* List namespaces owned by our user namespace */
+		req.user_ns_id = userns_id;
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/*
+		 * We have CAP_SYS_ADMIN in this user namespace,
+		 * so we should see all namespaces owned by it.
+		 * That includes: net, uts, ipc, and the user namespace itself.
+		 */
+		min_expected = 4;
+		success = (ret >= min_expected);
+
+		write(pipefd[1], &success, sizeof(success));
+		write(pipefd[1], &ret, sizeof(ret));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	success = false;
+	count = 0;
+	read(pipefd[0], &success, sizeof(success));
+	read(pipefd[0], &count, sizeof(count));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(success);
+	TH_LOG("User with CAP_SYS_ADMIN saw %zd namespaces owned by their user namespace",
+			count);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 48/70] selftests/namespaces: third listns() permission test

[Christian Brauner]

Test that users cannot see namespaces from unrelated user namespaces.
Create two sibling user namespaces, verify they can't see each other's
owned namespaces.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/listns_permissions_test.c | 138 +++++++++++++++++++++
 1 file changed, 138 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_permissions_test.c b/tools/testing/selftests/namespaces/listns_permissions_test.c
index c99c87cca046..aed7288c7eca 100644
--- a/tools/testing/selftests/namespaces/listns_permissions_test.c
+++ b/tools/testing/selftests/namespaces/listns_permissions_test.c
@@ -228,4 +228,142 @@ TEST(listns_cap_sys_admin_in_userns)
 			count);
 }
 
+/*
+ * Test that users cannot see namespaces from unrelated user namespaces.
+ * Create two sibling user namespaces, verify they can't see each other's
+ * owned namespaces.
+ */
+TEST(listns_cannot_see_sibling_userns_namespaces)
+{
+	int pipefd[2];
+	pid_t pid1, pid2;
+	int status;
+	__u64 netns_a_id;
+	int pipefd2[2];
+	bool found_sibling_netns;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	/* Fork first child - creates user namespace A */
+	pid1 = fork();
+	ASSERT_GE(pid1, 0);
+
+	if (pid1 == 0) {
+		int fd;
+		__u64 netns_a_id;
+		char buf;
+
+		close(pipefd[0]);
+
+		/* Create user namespace A */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Create network namespace owned by user namespace A */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get network namespace ID */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &netns_a_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send namespace ID to parent */
+		write(pipefd[1], &netns_a_id, sizeof(netns_a_id));
+
+		/* Keep alive for sibling to check */
+		read(pipefd[1], &buf, 1);
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent reads namespace A ID */
+	close(pipefd[1]);
+	netns_a_id = 0;
+	read(pipefd[0], &netns_a_id, sizeof(netns_a_id));
+
+	TH_LOG("User namespace A created network namespace with ID %llu",
+	       (unsigned long long)netns_a_id);
+
+	/* Fork second child - creates user namespace B */
+	ASSERT_EQ(pipe(pipefd2), 0);
+
+	pid2 = fork();
+	ASSERT_GE(pid2, 0);
+
+	if (pid2 == 0) {
+		struct ns_id_req req = {
+			.size = sizeof(req),
+			.spare = 0,
+			.ns_id = 0,
+			.ns_type = CLONE_NEWNET,
+			.spare2 = 0,
+			.user_ns_id = 0,
+		};
+		__u64 ns_ids[100];
+		ssize_t ret;
+		bool found_sibling_netns;
+
+		close(pipefd[0]);
+		close(pipefd2[0]);
+
+		/* Create user namespace B (sibling to A) */
+		if (setup_userns() < 0) {
+			close(pipefd2[1]);
+			exit(1);
+		}
+
+		/* Try to list all network namespaces */
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+		found_sibling_netns = false;
+		if (ret > 0) {
+			for (ssize_t i = 0; i < ret; i++) {
+				if (ns_ids[i] == netns_a_id) {
+					found_sibling_netns = true;
+					break;
+				}
+			}
+		}
+
+		/* We should NOT see the sibling's network namespace */
+		write(pipefd2[1], &found_sibling_netns, sizeof(found_sibling_netns));
+		close(pipefd2[1]);
+		exit(0);
+	}
+
+	/* Parent reads result from second child */
+	close(pipefd2[1]);
+	found_sibling_netns = false;
+	read(pipefd2[0], &found_sibling_netns, sizeof(found_sibling_netns));
+	close(pipefd2[0]);
+
+	/* Signal first child to exit */
+	close(pipefd[0]);
+
+	/* Wait for both children */
+	waitpid(pid2, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+
+	waitpid(pid1, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+
+	/* Second child should NOT have seen first child's namespace */
+	ASSERT_FALSE(found_sibling_netns);
+	TH_LOG("User namespace B correctly could not see sibling namespace A's network namespace");
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 49/70] selftests/namespaces: fourth listns() permission test

[Christian Brauner]

Test permission checking with LISTNS_CURRENT_USER.
Verify that listing with LISTNS_CURRENT_USER respects permissions.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/listns_permissions_test.c | 76 ++++++++++++++++++++++
 1 file changed, 76 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_permissions_test.c b/tools/testing/selftests/namespaces/listns_permissions_test.c
index aed7288c7eca..7727c5964104 100644
--- a/tools/testing/selftests/namespaces/listns_permissions_test.c
+++ b/tools/testing/selftests/namespaces/listns_permissions_test.c
@@ -366,4 +366,80 @@ TEST(listns_cannot_see_sibling_userns_namespaces)
 	TH_LOG("User namespace B correctly could not see sibling namespace A's network namespace");
 }
 
+/*
+ * Test permission checking with LISTNS_CURRENT_USER.
+ * Verify that listing with LISTNS_CURRENT_USER respects permissions.
+ */
+TEST(listns_current_user_permissions)
+{
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool success;
+	ssize_t count;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		struct ns_id_req req = {
+			.size = sizeof(req),
+			.spare = 0,
+			.ns_id = 0,
+			.ns_type = 0,
+			.spare2 = 0,
+			.user_ns_id = LISTNS_CURRENT_USER,
+		};
+		__u64 ns_ids[100];
+		ssize_t ret;
+		bool success;
+
+		close(pipefd[0]);
+
+		/* Create user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Create some namespaces owned by this user namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (unshare(CLONE_NEWUTS) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* List with LISTNS_CURRENT_USER - should see our owned namespaces */
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+		success = (ret >= 3);  /* At least user, net, uts */
+		write(pipefd[1], &success, sizeof(success));
+		write(pipefd[1], &ret, sizeof(ret));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	success = false;
+	count = 0;
+	read(pipefd[0], &success, sizeof(success));
+	read(pipefd[0], &count, sizeof(count));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(success);
+	TH_LOG("LISTNS_CURRENT_USER returned %zd namespaces", count);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 50/70] selftests/namespaces: fifth listns() permission test

[Christian Brauner]

Test that CAP_SYS_ADMIN in parent user namespace allows seeing
child user namespace's owned namespaces.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/listns_permissions_test.c | 119 +++++++++++++++++++++
 1 file changed, 119 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_permissions_test.c b/tools/testing/selftests/namespaces/listns_permissions_test.c
index 7727c5964104..b990b785dd7f 100644
--- a/tools/testing/selftests/namespaces/listns_permissions_test.c
+++ b/tools/testing/selftests/namespaces/listns_permissions_test.c
@@ -442,4 +442,123 @@ TEST(listns_current_user_permissions)
 	TH_LOG("LISTNS_CURRENT_USER returned %zd namespaces", count);
 }
 
+/*
+ * Test that CAP_SYS_ADMIN in parent user namespace allows seeing
+ * child user namespace's owned namespaces.
+ */
+TEST(listns_parent_userns_cap_sys_admin)
+{
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool found_child_userns;
+	ssize_t count;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 parent_userns_id;
+		__u64 child_userns_id;
+		struct ns_id_req req;
+		__u64 ns_ids[100];
+		ssize_t ret;
+		bool found_child_userns;
+
+		close(pipefd[0]);
+
+		/* Create parent user namespace - we have CAP_SYS_ADMIN in it */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get parent user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &parent_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create child user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get child user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create namespaces owned by child user namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* List namespaces owned by parent user namespace */
+		req.size = sizeof(req);
+		req.spare = 0;
+		req.ns_id = 0;
+		req.ns_type = 0;
+		req.spare2 = 0;
+		req.user_ns_id = parent_userns_id;
+
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+		/* Should see child user namespace in the list */
+		found_child_userns = false;
+		if (ret > 0) {
+			for (ssize_t i = 0; i < ret; i++) {
+				if (ns_ids[i] == child_userns_id) {
+					found_child_userns = true;
+					break;
+				}
+			}
+		}
+
+		write(pipefd[1], &found_child_userns, sizeof(found_child_userns));
+		write(pipefd[1], &ret, sizeof(ret));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	found_child_userns = false;
+	count = 0;
+	read(pipefd[0], &found_child_userns, sizeof(found_child_userns));
+	read(pipefd[0], &count, sizeof(count));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(found_child_userns);
+	TH_LOG("Process with CAP_SYS_ADMIN in parent user namespace saw child user namespace (total: %zd)",
+			count);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 51/70] selftests/namespaces: sixth listns() permission test

[Christian Brauner]

Test that we can see user namespaces we have CAP_SYS_ADMIN inside of.
This is different from seeing namespaces owned by a user namespace.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/listns_permissions_test.c | 87 ++++++++++++++++++++++
 1 file changed, 87 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_permissions_test.c b/tools/testing/selftests/namespaces/listns_permissions_test.c
index b990b785dd7f..9aa06ff76333 100644
--- a/tools/testing/selftests/namespaces/listns_permissions_test.c
+++ b/tools/testing/selftests/namespaces/listns_permissions_test.c
@@ -561,4 +561,91 @@ TEST(listns_parent_userns_cap_sys_admin)
 			count);
 }
 
+/*
+ * Test that we can see user namespaces we have CAP_SYS_ADMIN inside of.
+ * This is different from seeing namespaces owned by a user namespace.
+ */
+TEST(listns_cap_sys_admin_inside_userns)
+{
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool found_ours;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 our_userns_id;
+		struct ns_id_req req;
+		__u64 ns_ids[100];
+		ssize_t ret;
+		bool found_ours;
+
+		close(pipefd[0]);
+
+		/* Create user namespace - we have CAP_SYS_ADMIN inside it */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get our user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &our_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* List all user namespaces globally */
+		req.size = sizeof(req);
+		req.spare = 0;
+		req.ns_id = 0;
+		req.ns_type = CLONE_NEWUSER;
+		req.spare2 = 0;
+		req.user_ns_id = 0;
+
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+		/* We should be able to see our own user namespace */
+		found_ours = false;
+		if (ret > 0) {
+			for (ssize_t i = 0; i < ret; i++) {
+				if (ns_ids[i] == our_userns_id) {
+					found_ours = true;
+					break;
+				}
+			}
+		}
+
+		write(pipefd[1], &found_ours, sizeof(found_ours));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	found_ours = false;
+	read(pipefd[0], &found_ours, sizeof(found_ours));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(found_ours);
+	TH_LOG("Process can see user namespace it has CAP_SYS_ADMIN inside of");
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 52/70] selftests/namespaces: seventh listns() permission test

[Christian Brauner]

Test that dropping CAP_SYS_ADMIN restricts what we can see.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/listns_permissions_test.c | 108 +++++++++++++++++++++
 1 file changed, 108 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_permissions_test.c b/tools/testing/selftests/namespaces/listns_permissions_test.c
index 9aa06ff76333..82d818751a5f 100644
--- a/tools/testing/selftests/namespaces/listns_permissions_test.c
+++ b/tools/testing/selftests/namespaces/listns_permissions_test.c
@@ -648,4 +648,112 @@ TEST(listns_cap_sys_admin_inside_userns)
 	TH_LOG("Process can see user namespace it has CAP_SYS_ADMIN inside of");
 }
 
+/*
+ * Test that dropping CAP_SYS_ADMIN restricts what we can see.
+ */
+TEST(listns_drop_cap_sys_admin)
+{
+	cap_t caps;
+	cap_value_t cap_list[1] = { CAP_SYS_ADMIN };
+
+	/* This test needs to start with CAP_SYS_ADMIN */
+	caps = cap_get_proc();
+	if (!caps) {
+		SKIP(return, "Cannot get capabilities");
+	}
+
+	cap_flag_value_t cap_val;
+	if (cap_get_flag(caps, CAP_SYS_ADMIN, CAP_EFFECTIVE, &cap_val) < 0) {
+		cap_free(caps);
+		SKIP(return, "Cannot check CAP_SYS_ADMIN");
+	}
+
+	if (cap_val != CAP_SET) {
+		cap_free(caps);
+		SKIP(return, "Test needs CAP_SYS_ADMIN to start");
+	}
+	cap_free(caps);
+
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool correct;
+	ssize_t count_before, count_after;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		struct ns_id_req req = {
+			.size = sizeof(req),
+			.spare = 0,
+			.ns_id = 0,
+			.ns_type = CLONE_NEWNET,
+			.spare2 = 0,
+			.user_ns_id = LISTNS_CURRENT_USER,
+		};
+		__u64 ns_ids_before[100];
+		ssize_t count_before;
+		__u64 ns_ids_after[100];
+		ssize_t count_after;
+		bool correct;
+
+		close(pipefd[0]);
+
+		/* Create user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Count namespaces with CAP_SYS_ADMIN */
+		count_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+
+		/* Drop CAP_SYS_ADMIN */
+		caps = cap_get_proc();
+		if (caps) {
+			cap_set_flag(caps, CAP_EFFECTIVE, 1, cap_list, CAP_CLEAR);
+			cap_set_flag(caps, CAP_PERMITTED, 1, cap_list, CAP_CLEAR);
+			cap_set_proc(caps);
+			cap_free(caps);
+		}
+
+		/* Ensure we can't regain the capability */
+		prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+
+		/* Count namespaces without CAP_SYS_ADMIN */
+		count_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+
+		/* Without CAP_SYS_ADMIN, we should see same or fewer namespaces */
+		correct = (count_after <= count_before);
+
+		write(pipefd[1], &correct, sizeof(correct));
+		write(pipefd[1], &count_before, sizeof(count_before));
+		write(pipefd[1], &count_after, sizeof(count_after));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	correct = false;
+	count_before = 0;
+	count_after = 0;
+	read(pipefd[0], &correct, sizeof(correct));
+	read(pipefd[0], &count_before, sizeof(count_before));
+	read(pipefd[0], &count_after, sizeof(count_after));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(correct);
+	TH_LOG("With CAP_SYS_ADMIN: %zd namespaces, without: %zd namespaces",
+			count_before, count_after);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 53/70] selftests/namespaces: first inactive namespace resurrection test

[Christian Brauner]

Test basic SIOCGSKNS functionality. Create a socket and verify SIOCGSKNS
returns the correct network namespace.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/.gitignore      |  1 +
 tools/testing/selftests/namespaces/Makefile        |  9 ++-
 .../testing/selftests/namespaces/siocgskns_test.c  | 72 ++++++++++++++++++++++
 3 files changed, 81 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index 17f9c675a60b..aeb5f2711ff6 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -4,3 +4,4 @@ init_ino_test
 ns_active_ref_test
 listns_test
 listns_permissions_test
+siocgskns_test
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index 2dd22bc68b89..d456505189cd 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -2,11 +2,18 @@
 CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 LDLIBS += -lcap
 
-TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test listns_test listns_permissions_test
+TEST_GEN_PROGS := nsid_test \
+		  file_handle_test \
+		  init_ino_test \
+		  ns_active_ref_test \
+		  listns_test \
+		  listns_permissions_test \
+		  siocgskns_test
 
 include ../lib.mk
 
 $(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
 $(OUTPUT)/listns_test: ../filesystems/utils.c
 $(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
+$(OUTPUT)/siocgskns_test: ../filesystems/utils.c
 
diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
new file mode 100644
index 000000000000..0c9098624cd4
--- /dev/null
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <linux/if.h>
+#include <linux/sockios.h>
+#include <linux/nsfs.h>
+#include <arpa/inet.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+#ifndef SIOCGSKNS
+#define SIOCGSKNS 0x894C
+#endif
+
+#ifndef FD_NSFS_ROOT
+#define FD_NSFS_ROOT -10003
+#endif
+
+#ifndef FILEID_NSFS
+#define FILEID_NSFS 0xf1
+#endif
+
+/*
+ * Test basic SIOCGSKNS functionality.
+ * Create a socket and verify SIOCGSKNS returns the correct network namespace.
+ */
+TEST(siocgskns_basic)
+{
+	int sock_fd, netns_fd, current_netns_fd;
+	struct stat st1, st2;
+
+	/* Create a TCP socket */
+	sock_fd = socket(AF_INET, SOCK_STREAM, 0);
+	ASSERT_GE(sock_fd, 0);
+
+	/* Use SIOCGSKNS to get network namespace */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Get current network namespace */
+	current_netns_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(current_netns_fd, 0);
+
+	/* Verify they match */
+	ASSERT_EQ(fstat(netns_fd, &st1), 0);
+	ASSERT_EQ(fstat(current_netns_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	close(sock_fd);
+	close(netns_fd);
+	close(current_netns_fd);
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 54/70] selftests/namespaces: second inactive namespace resurrection test

[Christian Brauner]

Test that socket file descriptors keep network namespaces active. Create
a network namespace, create a socket in it, then exit the namespace. The
namespace should remain active while the socket FD is held.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 126 +++++++++++++++++++++
 1 file changed, 126 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 0c9098624cd4..0ad5e39b7e16 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -69,4 +69,130 @@ TEST(siocgskns_basic)
 	close(current_netns_fd);
 }
 
+/*
+ * Test that socket file descriptors keep network namespaces active.
+ * Create a network namespace, create a socket in it, then exit the namespace.
+ * The namespace should remain active while the socket FD is held.
+ */
+TEST(siocgskns_keeps_netns_active)
+{
+	int sock_fd, netns_fd, test_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	struct stat st;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new netns and socket */
+		close(ipc_sockets[0]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			TH_LOG("unshare(CLONE_NEWNET) failed: %s", strerror(errno));
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Create a socket in the new network namespace */
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			TH_LOG("socket() failed: %s", strerror(errno));
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	ASSERT_EQ(cmsg->cmsg_type, SCM_RIGHTS);
+
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	if (WEXITSTATUS(status) != 0)
+		SKIP(close(sock_fd); return, "Child failed to create namespace");
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	ASSERT_EQ(fstat(netns_fd, &st), 0);
+
+	/*
+	 * Namespace should still be active because socket FD keeps it alive.
+	 * Try to access it via /proc/self/fd/<fd>.
+	 */
+	char path[64];
+	snprintf(path, sizeof(path), "/proc/self/fd/%d", netns_fd);
+	test_fd = open(path, O_RDONLY);
+	ASSERT_GE(test_fd, 0);
+	close(test_fd);
+	close(netns_fd);
+
+	/* Close socket - namespace should become inactive */
+	close(sock_fd);
+
+	/* Try SIOCGSKNS again - should fail since socket is closed */
+	ASSERT_LT(ioctl(sock_fd, SIOCGSKNS), 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 55/70] selftests/namespaces: third inactive namespace resurrection test

[Christian Brauner]

Test SIOCGSKNS with different socket types (TCP, UDP, RAW).

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 65 +++++++++++++++++++++-
 1 file changed, 63 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 0ad5e39b7e16..02798e59fc11 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -163,8 +163,7 @@ TEST(siocgskns_keeps_netns_active)
 	/* Wait for child to exit */
 	waitpid(pid, &status, 0);
 	ASSERT_TRUE(WIFEXITED(status));
-	if (WEXITSTATUS(status) != 0)
-		SKIP(close(sock_fd); return, "Child failed to create namespace");
+	ASSERT_EQ(WEXITSTATUS(status), 0);
 
 	/* Get network namespace from socket */
 	netns_fd = ioctl(sock_fd, SIOCGSKNS);
@@ -195,4 +194,66 @@ TEST(siocgskns_keeps_netns_active)
 	ASSERT_LT(ioctl(sock_fd, SIOCGSKNS), 0);
 }
 
+/*
+ * Test SIOCGSKNS with different socket types (TCP, UDP, RAW).
+ */
+TEST(siocgskns_socket_types)
+{
+	int sock_tcp, sock_udp, sock_raw;
+	int netns_tcp, netns_udp, netns_raw;
+	struct stat st_tcp, st_udp, st_raw;
+
+	/* TCP socket */
+	sock_tcp = socket(AF_INET, SOCK_STREAM, 0);
+	ASSERT_GE(sock_tcp, 0);
+
+	/* UDP socket */
+	sock_udp = socket(AF_INET, SOCK_DGRAM, 0);
+	ASSERT_GE(sock_udp, 0);
+
+	/* RAW socket (may require privileges) */
+	sock_raw = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
+	if (sock_raw < 0 && (errno == EPERM || errno == EACCES)) {
+		sock_raw = -1; /* Skip raw socket test */
+	}
+
+	/* Test SIOCGSKNS on TCP */
+	netns_tcp = ioctl(sock_tcp, SIOCGSKNS);
+	if (netns_tcp < 0) {
+		close(sock_tcp);
+		close(sock_udp);
+		if (sock_raw >= 0) close(sock_raw);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_tcp, 0);
+	}
+
+	/* Test SIOCGSKNS on UDP */
+	netns_udp = ioctl(sock_udp, SIOCGSKNS);
+	ASSERT_GE(netns_udp, 0);
+
+	/* Test SIOCGSKNS on RAW (if available) */
+	if (sock_raw >= 0) {
+		netns_raw = ioctl(sock_raw, SIOCGSKNS);
+		ASSERT_GE(netns_raw, 0);
+	}
+
+	/* Verify all return the same network namespace */
+	ASSERT_EQ(fstat(netns_tcp, &st_tcp), 0);
+	ASSERT_EQ(fstat(netns_udp, &st_udp), 0);
+	ASSERT_EQ(st_tcp.st_ino, st_udp.st_ino);
+
+	if (sock_raw >= 0) {
+		ASSERT_EQ(fstat(netns_raw, &st_raw), 0);
+		ASSERT_EQ(st_tcp.st_ino, st_raw.st_ino);
+		close(netns_raw);
+		close(sock_raw);
+	}
+
+	close(netns_tcp);
+	close(netns_udp);
+	close(sock_tcp);
+	close(sock_udp);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 56/70] selftests/namespaces: fourth inactive namespace resurrection test

[Christian Brauner]

Test SIOCGSKNS across setns. Create a socket in netns A, switch to netns
B, verify SIOCGSKNS still returns netns A.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 51 ++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 02798e59fc11..28e45954c4fa 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -256,4 +256,55 @@ TEST(siocgskns_socket_types)
 	close(sock_udp);
 }
 
+/*
+ * Test SIOCGSKNS across setns.
+ * Create a socket in netns A, switch to netns B, verify SIOCGSKNS still
+ * returns netns A.
+ */
+TEST(siocgskns_across_setns)
+{
+	int sock_fd, netns_a_fd, netns_b_fd, result_fd;
+	struct stat st_a;
+
+	/* Get current netns (A) */
+	netns_a_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(netns_a_fd, 0);
+	ASSERT_EQ(fstat(netns_a_fd, &st_a), 0);
+
+	/* Create socket in netns A */
+	sock_fd = socket(AF_INET, SOCK_STREAM, 0);
+	ASSERT_GE(sock_fd, 0);
+
+	/* Create new netns (B) */
+	ASSERT_EQ(unshare(CLONE_NEWNET), 0);
+
+	netns_b_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(netns_b_fd, 0);
+
+	/* Get netns from socket created in A */
+	result_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (result_fd < 0) {
+		close(sock_fd);
+		setns(netns_a_fd, CLONE_NEWNET);
+		close(netns_a_fd);
+		close(netns_b_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(result_fd, 0);
+	}
+
+	/* Verify it still points to netns A */
+	struct stat st_result_stat;
+	ASSERT_EQ(fstat(result_fd, &st_result_stat), 0);
+	ASSERT_EQ(st_a.st_ino, st_result_stat.st_ino);
+
+	close(result_fd);
+	close(sock_fd);
+	close(netns_b_fd);
+
+	/* Restore original netns */
+	ASSERT_EQ(setns(netns_a_fd, CLONE_NEWNET), 0);
+	close(netns_a_fd);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 57/70] selftests/namespaces: fifth inactive namespace resurrection test

[Christian Brauner]

Test SIOCGSKNS fails on non-socket file descriptors.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 26 ++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 28e45954c4fa..bbfef3c51ac1 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -307,4 +307,30 @@ TEST(siocgskns_across_setns)
 	close(netns_a_fd);
 }
 
+/*
+ * Test SIOCGSKNS fails on non-socket file descriptors.
+ */
+TEST(siocgskns_non_socket)
+{
+	int fd;
+	int pipefd[2];
+
+	/* Test on regular file */
+	fd = open("/dev/null", O_RDONLY);
+	ASSERT_GE(fd, 0);
+
+	ASSERT_LT(ioctl(fd, SIOCGSKNS), 0);
+	ASSERT_TRUE(errno == ENOTTY || errno == EINVAL);
+	close(fd);
+
+	/* Test on pipe */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	ASSERT_LT(ioctl(pipefd[0], SIOCGSKNS), 0);
+	ASSERT_TRUE(errno == ENOTTY || errno == EINVAL);
+
+	close(pipefd[0]);
+	close(pipefd[1]);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 58/70] selftests/namespaces: sixth inactive namespace resurrection test

[Christian Brauner]

Test multiple sockets keep the same network namespace active. Create
multiple sockets, verify closing some doesn't affect others.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 68 ++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index bbfef3c51ac1..231830daf5dc 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -333,4 +333,72 @@ TEST(siocgskns_non_socket)
 	close(pipefd[1]);
 }
 
+/*
+ * Test multiple sockets keep the same network namespace active.
+ * Create multiple sockets, verify closing some doesn't affect others.
+ */
+TEST(siocgskns_multiple_sockets)
+{
+	int socks[5];
+	int netns_fds[5];
+	int i;
+	struct stat st;
+	ino_t netns_ino;
+
+	/* Create new network namespace */
+	ASSERT_EQ(unshare(CLONE_NEWNET), 0);
+
+	/* Create multiple sockets */
+	for (i = 0; i < 5; i++) {
+		socks[i] = socket(AF_INET, SOCK_STREAM, 0);
+		ASSERT_GE(socks[i], 0);
+	}
+
+	/* Get netns from all sockets */
+	for (i = 0; i < 5; i++) {
+		netns_fds[i] = ioctl(socks[i], SIOCGSKNS);
+		if (netns_fds[i] < 0) {
+			int j;
+			for (j = 0; j <= i; j++) {
+				close(socks[j]);
+				if (j < i && netns_fds[j] >= 0)
+					close(netns_fds[j]);
+			}
+			if (errno == ENOTTY || errno == EINVAL)
+				SKIP(return, "SIOCGSKNS not supported");
+			ASSERT_GE(netns_fds[i], 0);
+		}
+	}
+
+	/* Verify all point to same netns */
+	ASSERT_EQ(fstat(netns_fds[0], &st), 0);
+	netns_ino = st.st_ino;
+
+	for (i = 1; i < 5; i++) {
+		ASSERT_EQ(fstat(netns_fds[i], &st), 0);
+		ASSERT_EQ(st.st_ino, netns_ino);
+	}
+
+	/* Close some sockets */
+	for (i = 0; i < 3; i++) {
+		close(socks[i]);
+	}
+
+	/* Remaining netns FDs should still be valid */
+	for (i = 3; i < 5; i++) {
+		char path[64];
+		snprintf(path, sizeof(path), "/proc/self/fd/%d", netns_fds[i]);
+		int test_fd = open(path, O_RDONLY);
+		ASSERT_GE(test_fd, 0);
+		close(test_fd);
+	}
+
+	/* Cleanup */
+	for (i = 0; i < 5; i++) {
+		if (i >= 3)
+			close(socks[i]);
+		close(netns_fds[i]);
+	}
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 59/70] selftests/namespaces: seventh inactive namespace resurrection test

[Christian Brauner]

Test socket keeps netns active after creating process exits. Verify that
as long as the socket FD exists, the namespace remains active.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 141 +++++++++++++++++++++
 1 file changed, 141 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 231830daf5dc..60028eeecde0 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -401,4 +401,145 @@ TEST(siocgskns_multiple_sockets)
 	}
 }
 
+/*
+ * Test socket keeps netns active after creating process exits.
+ * Verify that as long as the socket FD exists, the namespace remains active.
+ */
+TEST(siocgskns_netns_lifecycle)
+{
+	int sock_fd, netns_fd;
+	int ipc_sockets[2];
+	int syncpipe[2];
+	pid_t pid;
+	int status;
+	char sync_byte;
+	struct stat st;
+	ino_t netns_ino;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	ASSERT_EQ(pipe(syncpipe), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child */
+		close(ipc_sockets[0]);
+		close(syncpipe[1]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(ipc_sockets[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		sock_fd = socket(AF_INET, SOCK_STREAM, 0);
+		if (sock_fd < 0) {
+			close(ipc_sockets[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		/* Send socket to parent */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+
+		/* Wait for parent signal */
+		read(syncpipe[0], &sync_byte, 1);
+		close(syncpipe[0]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(ipc_sockets[1]);
+	close(syncpipe[0]);
+
+	/* Receive socket FD */
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Get netns from socket while child is alive */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		sync_byte = 'G';
+		write(syncpipe[1], &sync_byte, 1);
+		close(syncpipe[1]);
+		close(sock_fd);
+		waitpid(pid, NULL, 0);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+	ASSERT_EQ(fstat(netns_fd, &st), 0);
+	netns_ino = st.st_ino;
+
+	/* Signal child to exit */
+	sync_byte = 'G';
+	write(syncpipe[1], &sync_byte, 1);
+	close(syncpipe[1]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+
+	/*
+	 * Socket FD should still keep namespace active even after
+	 * the creating process exited.
+	 */
+	int test_fd = ioctl(sock_fd, SIOCGSKNS);
+	ASSERT_GE(test_fd, 0);
+
+	struct stat st_test;
+	ASSERT_EQ(fstat(test_fd, &st_test), 0);
+	ASSERT_EQ(st_test.st_ino, netns_ino);
+
+	close(test_fd);
+	close(netns_fd);
+
+	/* Close socket - namespace should become inactive */
+	close(sock_fd);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 60/70] selftests/namespaces: eigth inactive namespace resurrection test

[Christian Brauner]

Test IPv6 sockets also work with SIOCGSKNS.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 34 ++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 60028eeecde0..47c1524a8648 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -542,4 +542,38 @@ TEST(siocgskns_netns_lifecycle)
 	close(sock_fd);
 }
 
+/*
+ * Test IPv6 sockets also work with SIOCGSKNS.
+ */
+TEST(siocgskns_ipv6)
+{
+	int sock_fd, netns_fd, current_netns_fd;
+	struct stat st1, st2;
+
+	/* Create an IPv6 TCP socket */
+	sock_fd = socket(AF_INET6, SOCK_STREAM, 0);
+	ASSERT_GE(sock_fd, 0);
+
+	/* Use SIOCGSKNS */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Verify it matches current namespace */
+	current_netns_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(current_netns_fd, 0);
+
+	ASSERT_EQ(fstat(netns_fd, &st1), 0);
+	ASSERT_EQ(fstat(current_netns_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	close(sock_fd);
+	close(netns_fd);
+	close(current_netns_fd);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 61/70] selftests/namespaces: ninth inactive namespace resurrection test

[Christian Brauner]

Test that socket-kept netns appears in listns() output.
Verify that a network namespace kept alive by a socket FD appears in
listns() output even after the creating process exits, and that it
disappears when the socket is closed.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 203 +++++++++++++++++++++
 1 file changed, 203 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 47c1524a8648..98f6a0e1b9dd 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -576,4 +576,207 @@ TEST(siocgskns_ipv6)
 	close(current_netns_fd);
 }
 
+/*
+ * Test that socket-kept netns appears in listns() output.
+ * Verify that a network namespace kept alive by a socket FD appears in
+ * listns() output even after the creating process exits, and that it
+ * disappears when the socket is closed.
+ */
+TEST(siocgskns_listns_visibility)
+{
+	int sock_fd, netns_fd, owner_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	__u64 netns_id, owner_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	int ret, i;
+	bool found_netns = false;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new netns and socket */
+		close(ipc_sockets[0]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Get namespace ID */
+	ret = ioctl(netns_fd, NS_GET_ID, &netns_id);
+	if (ret < 0) {
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_ID not supported");
+		ASSERT_EQ(ret, 0);
+	}
+
+	/* Get owner user namespace */
+	owner_fd = ioctl(netns_fd, NS_GET_USERNS);
+	if (owner_fd < 0) {
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_USERNS not supported");
+		ASSERT_GE(owner_fd, 0);
+	}
+
+	/* Get owner namespace ID */
+	ret = ioctl(owner_fd, NS_GET_ID, &owner_id);
+	if (ret < 0) {
+		close(owner_fd);
+		close(sock_fd);
+		close(netns_fd);
+		ASSERT_EQ(ret, 0);
+	}
+	close(owner_fd);
+
+	/* Namespace should appear in listns() output */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s", strerror(errno));
+		ASSERT_GE(ret, 0);
+	}
+
+	/* Search for our network namespace in the list */
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id) {
+			found_netns = true;
+			break;
+		}
+	}
+
+	ASSERT_TRUE(found_netns);
+	TH_LOG("Found netns %llu in listns() output (kept alive by socket)", netns_id);
+
+	/* Now verify with owner filtering */
+	req.user_ns_id = owner_id;
+	found_netns = false;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id) {
+			found_netns = true;
+			break;
+		}
+	}
+
+	ASSERT_TRUE(found_netns);
+	TH_LOG("Found netns %llu owned by userns %llu", netns_id, owner_id);
+
+	/* Close socket - namespace should become inactive and disappear from listns() */
+	close(sock_fd);
+	close(netns_fd);
+
+	/* Verify it's no longer in listns() output */
+	req.user_ns_id = 0;
+	found_netns = false;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id) {
+			found_netns = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found_netns);
+	TH_LOG("Netns %llu correctly disappeared from listns() after socket closed", netns_id);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 62/70] selftests/namespaces: tenth inactive namespace resurrection test

[Christian Brauner]

Test that socket-kept netns can be reopened via file handle.
Verify that a network namespace kept alive by a socket FD can be
reopened using file handles even after the creating process exits.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 195 +++++++++++++++++++++
 1 file changed, 195 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 98f6a0e1b9dd..a909232dba36 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -779,4 +779,199 @@ TEST(siocgskns_listns_visibility)
 	TH_LOG("Netns %llu correctly disappeared from listns() after socket closed", netns_id);
 }
 
+/*
+ * Test that socket-kept netns can be reopened via file handle.
+ * Verify that a network namespace kept alive by a socket FD can be
+ * reopened using file handles even after the creating process exits.
+ */
+TEST(siocgskns_file_handle)
+{
+	int sock_fd, netns_fd, reopened_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	struct stat st1, st2;
+	ino_t netns_ino;
+	__u64 netns_id;
+	struct file_handle *handle;
+	struct nsfs_file_handle *nsfs_fh;
+	int ret;
+
+	/* Allocate file_handle structure for nsfs */
+	handle = malloc(sizeof(struct file_handle) + sizeof(struct nsfs_file_handle));
+	ASSERT_NE(handle, NULL);
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new netns and socket */
+		close(ipc_sockets[0]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	ASSERT_EQ(fstat(netns_fd, &st1), 0);
+	netns_ino = st1.st_ino;
+
+	/* Get namespace ID */
+	ret = ioctl(netns_fd, NS_GET_ID, &netns_id);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_ID not supported");
+		ASSERT_EQ(ret, 0);
+	}
+
+	/* Construct file handle from namespace ID */
+	nsfs_fh = (struct nsfs_file_handle *)handle->f_handle;
+	nsfs_fh->ns_id = netns_id;
+	nsfs_fh->ns_type = 0;  /* Type field not needed for reopening */
+	nsfs_fh->ns_inum = 0;  /* Inum field not needed for reopening */
+
+	TH_LOG("Constructed file handle for netns %lu (id=%llu)", netns_ino, netns_id);
+
+	/* Reopen namespace using file handle (while socket still keeps it alive) */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	/* Verify it's the same namespace */
+	ASSERT_EQ(fstat(reopened_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	TH_LOG("Successfully reopened netns %lu via file handle", netns_ino);
+
+	close(reopened_fd);
+
+	/* Close the netns FD */
+	close(netns_fd);
+
+	/* Try to reopen via file handle - should fail since namespace is now inactive */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(reopened_fd, 0);
+	TH_LOG("Correctly failed to reopen inactive netns: %s", strerror(errno));
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Reopen namespace using file handle (while socket still keeps it alive) */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	/* Verify it's the same namespace */
+	ASSERT_EQ(fstat(reopened_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	TH_LOG("Successfully reopened netns %lu via file handle", netns_ino);
+
+	/* Close socket - namespace should become inactive */
+	close(sock_fd);
+	free(handle);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 63/70] selftests/namespaces: eleventh inactive namespace resurrection test

[Christian Brauner]

Test combined listns() and file handle operations with socket-kept
netns. Create a netns, keep it alive with a socket, verify it appears in
listns(), then reopen it via file handle obtained from listns() entry.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 283 +++++++++++++++++++++
 1 file changed, 283 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index a909232dba36..706049768d52 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -974,4 +974,287 @@ TEST(siocgskns_file_handle)
 	free(handle);
 }
 
+/*
+ * Test combined listns() and file handle operations with socket-kept netns.
+ * Create a netns, keep it alive with a socket, verify it appears in listns(),
+ * then reopen it via file handle obtained from listns() entry.
+ */
+TEST(siocgskns_listns_and_file_handle)
+{
+	int sock_fd, netns_fd, userns_fd, reopened_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	struct stat st;
+	ino_t netns_ino;
+	__u64 netns_id, userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET | CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	int ret, i;
+	bool found_netns = false, found_userns = false;
+	struct file_handle *handle;
+	struct nsfs_file_handle *nsfs_fh;
+
+	/* Allocate file_handle structure for nsfs */
+	handle = malloc(sizeof(struct file_handle) + sizeof(struct nsfs_file_handle));
+	ASSERT_NE(handle, NULL);
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new userns and netns with socket */
+		close(ipc_sockets[0]);
+
+		if (setup_userns() < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	ASSERT_EQ(fstat(netns_fd, &st), 0);
+	netns_ino = st.st_ino;
+
+	/* Get namespace ID */
+	ret = ioctl(netns_fd, NS_GET_ID, &netns_id);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_ID not supported");
+		ASSERT_EQ(ret, 0);
+	}
+
+	/* Get owner user namespace */
+	userns_fd = ioctl(netns_fd, NS_GET_USERNS);
+	if (userns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_USERNS not supported");
+		ASSERT_GE(userns_fd, 0);
+	}
+
+	/* Get owner namespace ID */
+	ret = ioctl(userns_fd, NS_GET_ID, &userns_id);
+	if (ret < 0) {
+		close(userns_fd);
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		ASSERT_EQ(ret, 0);
+	}
+	close(userns_fd);
+
+	TH_LOG("Testing netns %lu (id=%llu) owned by userns id=%llu", netns_ino, netns_id, userns_id);
+
+	/* Verify namespace appears in listns() */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s", strerror(errno));
+		ASSERT_GE(ret, 0);
+	}
+
+	found_netns = false;
+	found_userns = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id)
+			found_netns = true;
+		if (ns_ids[i] == userns_id)
+			found_userns = true;
+	}
+	ASSERT_TRUE(found_netns);
+	ASSERT_TRUE(found_userns);
+	TH_LOG("Found netns %llu in listns() output", netns_id);
+
+	/* Construct file handle from namespace ID */
+	nsfs_fh = (struct nsfs_file_handle *)handle->f_handle;
+	nsfs_fh->ns_id = netns_id;
+	nsfs_fh->ns_type = 0;
+	nsfs_fh->ns_inum = 0;
+
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	struct stat reopened_st;
+	ASSERT_EQ(fstat(reopened_fd, &reopened_st), 0);
+	ASSERT_EQ(reopened_st.st_ino, netns_ino);
+
+	TH_LOG("Successfully reopened netns %lu via file handle (socket-kept)", netns_ino);
+
+	close(reopened_fd);
+	close(netns_fd);
+
+	/* Try to reopen via file handle - should fail since namespace is now inactive */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(reopened_fd, 0);
+	TH_LOG("Correctly failed to reopen inactive netns: %s", strerror(errno));
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Verify namespace appears in listns() */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s", strerror(errno));
+		ASSERT_GE(ret, 0);
+	}
+
+	found_netns = false;
+	found_userns = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id)
+			found_netns = true;
+		if (ns_ids[i] == userns_id)
+			found_userns = true;
+	}
+	ASSERT_TRUE(found_netns);
+	ASSERT_TRUE(found_userns);
+	TH_LOG("Found netns %llu in listns() output", netns_id);
+
+	close(netns_fd);
+
+	/* Verify namespace appears in listns() */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s", strerror(errno));
+		ASSERT_GE(ret, 0);
+	}
+
+	found_netns = false;
+	found_userns = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id)
+			found_netns = true;
+		if (ns_ids[i] == userns_id)
+			found_userns = true;
+	}
+	ASSERT_FALSE(found_netns);
+	ASSERT_FALSE(found_userns);
+	TH_LOG("Netns %llu correctly disappeared from listns() after socket closed", netns_id);
+
+	close(sock_fd);
+	free(handle);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 64/70] selftests/namespaces: twelth inactive namespace resurrection test

[Christian Brauner]

Test multi-level namespace resurrection across three user namespace levels.

This test creates a complex namespace hierarchy with three levels of user
namespaces and a network namespace at the deepest level. It verifies that
the resurrection semantics work correctly when SIOCGSKNS is called on a
socket from an inactive namespace tree, and that listns() and
open_by_handle_at() correctly respect visibility rules.

Hierarchy after child processes exit (all with 0 active refcount):

         net_L3A (0)                <- Level 3 network namespace
             |
             +
         userns_L3 (0)              <- Level 3 user namespace
             |
             +
         userns_L2 (0)              <- Level 2 user namespace
             |
             +
         userns_L1 (0)              <- Level 1 user namespace
             |
             x
         init_user_ns

The test verifies:
1. SIOCGSKNS on a socket from inactive net_L3A resurrects the entire chain
2. After resurrection, all namespaces are visible in listns()
3. Resurrected namespaces can be reopened via file handles
4. Closing the netns FD cascades down: the entire ownership chain
   (userns_L3 -> userns_L2 -> userns_L1) becomes inactive again
5. Inactive namespaces disappear from listns() and cannot be reopened
6. Calling SIOCGSKNS again on the same socket resurrects the tree again
7. After second resurrection, namespaces are visible and can be reopened

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 564 +++++++++++++++++++++
 1 file changed, 564 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 706049768d52..ba689a22d82f 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -1257,4 +1257,568 @@ TEST(siocgskns_listns_and_file_handle)
 	free(handle);
 }
 
+/*
+ * Test multi-level namespace resurrection across three user namespace levels.
+ *
+ * This test creates a complex namespace hierarchy with three levels of user
+ * namespaces and a network namespace at the deepest level. It verifies that
+ * the resurrection semantics work correctly when SIOCGSKNS is called on a
+ * socket from an inactive namespace tree, and that listns() and
+ * open_by_handle_at() correctly respect visibility rules.
+ *
+ * Hierarchy after child processes exit (all with 0 active refcount):
+ *
+ *          net_L3A (0)                <- Level 3 network namespace
+ *              |
+ *              +
+ *          userns_L3 (0)              <- Level 3 user namespace
+ *              |
+ *              +
+ *          userns_L2 (0)              <- Level 2 user namespace
+ *              |
+ *              +
+ *          userns_L1 (0)              <- Level 1 user namespace
+ *              |
+ *              x
+ *          init_user_ns
+ *
+ * The test verifies:
+ * 1. SIOCGSKNS on a socket from inactive net_L3A resurrects the entire chain
+ * 2. After resurrection, all namespaces are visible in listns()
+ * 3. Resurrected namespaces can be reopened via file handles
+ * 4. Closing the netns FD cascades down: the entire ownership chain
+ *    (userns_L3 -> userns_L2 -> userns_L1) becomes inactive again
+ * 5. Inactive namespaces disappear from listns() and cannot be reopened
+ * 6. Calling SIOCGSKNS again on the same socket resurrects the tree again
+ * 7. After second resurrection, namespaces are visible and can be reopened
+ */
+TEST(siocgskns_multilevel_resurrection)
+{
+	int ipc_sockets[2];
+	pid_t pid_l1, pid_l2, pid_l3;
+	int status;
+
+	/* Namespace file descriptors to be received from child */
+	int sock_L3A_fd = -1;
+	int netns_L3A_fd = -1;
+	__u64 netns_L3A_id;
+	__u64 userns_L1_id, userns_L2_id, userns_L3_id;
+
+	/* For listns() and file handle testing */
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET | CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	int ret, i;
+	struct file_handle *handle;
+	struct nsfs_file_handle *nsfs_fh;
+	int reopened_fd;
+
+	/* Allocate file handle for testing */
+	handle = malloc(sizeof(struct file_handle) + sizeof(struct nsfs_file_handle));
+	ASSERT_NE(handle, NULL);
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	/*
+	 * Fork level 1 child that creates userns_L1
+	 */
+	pid_l1 = fork();
+	ASSERT_GE(pid_l1, 0);
+
+	if (pid_l1 == 0) {
+		/* Level 1 child */
+		int ipc_L2[2];
+		close(ipc_sockets[0]);
+
+		/* Create userns_L1 */
+		if (setup_userns() < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Create socketpair for communicating with L2 child */
+		if (socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_L2) < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/*
+		 * Fork level 2 child that creates userns_L2
+		 */
+		pid_l2 = fork();
+		if (pid_l2 < 0) {
+			close(ipc_sockets[1]);
+			close(ipc_L2[0]);
+			close(ipc_L2[1]);
+			exit(1);
+		}
+
+		if (pid_l2 == 0) {
+			/* Level 2 child */
+			int ipc_L3[2];
+			close(ipc_L2[0]);
+
+			/* Create userns_L2 (nested inside userns_L1) */
+			if (setup_userns() < 0) {
+				close(ipc_L2[1]);
+				exit(1);
+			}
+
+			/* Create socketpair for communicating with L3 child */
+			if (socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_L3) < 0) {
+				close(ipc_L2[1]);
+				exit(1);
+			}
+
+			/*
+			 * Fork level 3 child that creates userns_L3 and network namespaces
+			 */
+			pid_l3 = fork();
+			if (pid_l3 < 0) {
+				close(ipc_L2[1]);
+				close(ipc_L3[0]);
+				close(ipc_L3[1]);
+				exit(1);
+			}
+
+			if (pid_l3 == 0) {
+				/* Level 3 child - the deepest level */
+				int sock_fd;
+				close(ipc_L3[0]);
+
+				/* Create userns_L3 (nested inside userns_L2) */
+				if (setup_userns() < 0) {
+					close(ipc_L3[1]);
+					exit(1);
+				}
+
+				/* Create network namespace at level 3 */
+				if (unshare(CLONE_NEWNET) < 0) {
+					close(ipc_L3[1]);
+					exit(1);
+				}
+
+				/* Create socket in net_L3A */
+				sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+				if (sock_fd < 0) {
+					close(ipc_L3[1]);
+					exit(1);
+				}
+
+				/* Send socket FD to L2 parent */
+				struct msghdr msg = {0};
+				struct iovec iov = {0};
+				char buf[1] = {'X'};
+				char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+				iov.iov_base = buf;
+				iov.iov_len = 1;
+				msg.msg_iov = &iov;
+				msg.msg_iovlen = 1;
+				msg.msg_control = cmsg_buf;
+				msg.msg_controllen = sizeof(cmsg_buf);
+
+				struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+				cmsg->cmsg_level = SOL_SOCKET;
+				cmsg->cmsg_type = SCM_RIGHTS;
+				cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+				memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+				if (sendmsg(ipc_L3[1], &msg, 0) < 0) {
+					close(sock_fd);
+					close(ipc_L3[1]);
+					exit(1);
+				}
+
+				close(sock_fd);
+				close(ipc_L3[1]);
+				exit(0);
+			}
+
+			/* Level 2 child - receive from L3 and forward to L1 */
+			close(ipc_L3[1]);
+
+			struct msghdr msg = {0};
+			struct iovec iov = {0};
+			char buf[1];
+			char cmsg_buf[CMSG_SPACE(sizeof(int))];
+			int received_fd;
+
+			iov.iov_base = buf;
+			iov.iov_len = 1;
+			msg.msg_iov = &iov;
+			msg.msg_iovlen = 1;
+			msg.msg_control = cmsg_buf;
+			msg.msg_controllen = sizeof(cmsg_buf);
+
+			ssize_t n = recvmsg(ipc_L3[0], &msg, 0);
+			close(ipc_L3[0]);
+
+			if (n != 1) {
+				close(ipc_L2[1]);
+				waitpid(pid_l3, NULL, 0);
+				exit(1);
+			}
+
+			struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+			if (!cmsg) {
+				close(ipc_L2[1]);
+				waitpid(pid_l3, NULL, 0);
+				exit(1);
+			}
+			memcpy(&received_fd, CMSG_DATA(cmsg), sizeof(int));
+
+			/* Wait for L3 child */
+			waitpid(pid_l3, NULL, 0);
+
+			/* Forward the socket FD to L1 parent */
+			memset(&msg, 0, sizeof(msg));
+			buf[0] = 'Y';
+			iov.iov_base = buf;
+			iov.iov_len = 1;
+			msg.msg_iov = &iov;
+			msg.msg_iovlen = 1;
+			msg.msg_control = cmsg_buf;
+			msg.msg_controllen = sizeof(cmsg_buf);
+
+			cmsg = CMSG_FIRSTHDR(&msg);
+			cmsg->cmsg_level = SOL_SOCKET;
+			cmsg->cmsg_type = SCM_RIGHTS;
+			cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+			memcpy(CMSG_DATA(cmsg), &received_fd, sizeof(int));
+
+			if (sendmsg(ipc_L2[1], &msg, 0) < 0) {
+				close(received_fd);
+				close(ipc_L2[1]);
+				exit(1);
+			}
+
+			close(received_fd);
+			close(ipc_L2[1]);
+			exit(0);
+		}
+
+		/* Level 1 child - receive from L2 and forward to parent */
+		close(ipc_L2[1]);
+
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1];
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+		int received_fd;
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		ssize_t n = recvmsg(ipc_L2[0], &msg, 0);
+		close(ipc_L2[0]);
+
+		if (n != 1) {
+			close(ipc_sockets[1]);
+			waitpid(pid_l2, NULL, 0);
+			exit(1);
+		}
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		if (!cmsg) {
+			close(ipc_sockets[1]);
+			waitpid(pid_l2, NULL, 0);
+			exit(1);
+		}
+		memcpy(&received_fd, CMSG_DATA(cmsg), sizeof(int));
+
+		/* Wait for L2 child */
+		waitpid(pid_l2, NULL, 0);
+
+		/* Forward the socket FD to parent */
+		memset(&msg, 0, sizeof(msg));
+		buf[0] = 'Z';
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &received_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(received_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(received_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent - receive the socket from the deepest level */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+
+	if (n != 1) {
+		free(handle);
+		waitpid(pid_l1, NULL, 0);
+		SKIP(return, "Failed to receive socket from child");
+	}
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	if (!cmsg) {
+		free(handle);
+		waitpid(pid_l1, NULL, 0);
+		SKIP(return, "Failed to receive socket from child");
+	}
+	memcpy(&sock_L3A_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for L1 child */
+	waitpid(pid_l1, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/*
+	 * At this point, all child processes have exited. The socket itself
+	 * doesn't keep the namespace active - we need to call SIOCGSKNS which
+	 * will resurrect the entire namespace tree by taking active references.
+	 */
+
+	/* Get network namespace from socket - this resurrects the tree */
+	netns_L3A_fd = ioctl(sock_L3A_fd, SIOCGSKNS);
+	if (netns_L3A_fd < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_L3A_fd, 0);
+	}
+
+	/* Get namespace ID for net_L3A */
+	ret = ioctl(netns_L3A_fd, NS_GET_ID, &netns_L3A_id);
+	if (ret < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_ID not supported");
+		ASSERT_EQ(ret, 0);
+	}
+
+	/* Get owner user namespace chain: userns_L3 -> userns_L2 -> userns_L1 */
+	int userns_L3_fd = ioctl(netns_L3A_fd, NS_GET_USERNS);
+	if (userns_L3_fd < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_USERNS not supported");
+		ASSERT_GE(userns_L3_fd, 0);
+	}
+
+	ret = ioctl(userns_L3_fd, NS_GET_ID, &userns_L3_id);
+	ASSERT_EQ(ret, 0);
+
+	int userns_L2_fd = ioctl(userns_L3_fd, NS_GET_USERNS);
+	ASSERT_GE(userns_L2_fd, 0);
+	ret = ioctl(userns_L2_fd, NS_GET_ID, &userns_L2_id);
+	ASSERT_EQ(ret, 0);
+
+	int userns_L1_fd = ioctl(userns_L2_fd, NS_GET_USERNS);
+	ASSERT_GE(userns_L1_fd, 0);
+	ret = ioctl(userns_L1_fd, NS_GET_ID, &userns_L1_id);
+	ASSERT_EQ(ret, 0);
+
+	close(userns_L1_fd);
+	close(userns_L2_fd);
+	close(userns_L3_fd);
+
+	TH_LOG("Multi-level hierarchy: net_L3A (id=%llu) -> userns_L3 (id=%llu) -> userns_L2 (id=%llu) -> userns_L1 (id=%llu)",
+	       netns_L3A_id, userns_L3_id, userns_L2_id, userns_L1_id);
+
+	/*
+	 * Test 1: Verify net_L3A is visible in listns() after resurrection.
+	 * The entire ownership chain should be resurrected and visible.
+	 */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	bool found_netns_L3A = false;
+	bool found_userns_L1 = false;
+	bool found_userns_L2 = false;
+	bool found_userns_L3 = false;
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_L3A_id)
+			found_netns_L3A = true;
+		if (ns_ids[i] == userns_L1_id)
+			found_userns_L1 = true;
+		if (ns_ids[i] == userns_L2_id)
+			found_userns_L2 = true;
+		if (ns_ids[i] == userns_L3_id)
+			found_userns_L3 = true;
+	}
+
+	ASSERT_TRUE(found_netns_L3A);
+	ASSERT_TRUE(found_userns_L1);
+	ASSERT_TRUE(found_userns_L2);
+	ASSERT_TRUE(found_userns_L3);
+	TH_LOG("Resurrection verified: all namespaces in hierarchy visible in listns()");
+
+	/*
+	 * Test 2: Verify net_L3A can be reopened via file handle.
+	 */
+	nsfs_fh = (struct nsfs_file_handle *)handle->f_handle;
+	nsfs_fh->ns_id = netns_L3A_id;
+	nsfs_fh->ns_type = 0;
+	nsfs_fh->ns_inum = 0;
+
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	close(reopened_fd);
+	TH_LOG("File handle test passed: net_L3A can be reopened");
+
+	/*
+	 * Test 3: Verify that when we close the netns FD (dropping the last
+	 * active reference), the entire tree becomes inactive and disappears
+	 * from listns(). The cascade goes: net_L3A drops -> userns_L3 drops ->
+	 * userns_L2 drops -> userns_L1 drops.
+	 */
+	close(netns_L3A_fd);
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	found_netns_L3A = false;
+	found_userns_L1 = false;
+	found_userns_L2 = false;
+	found_userns_L3 = false;
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_L3A_id)
+			found_netns_L3A = true;
+		if (ns_ids[i] == userns_L1_id)
+			found_userns_L1 = true;
+		if (ns_ids[i] == userns_L2_id)
+			found_userns_L2 = true;
+		if (ns_ids[i] == userns_L3_id)
+			found_userns_L3 = true;
+	}
+
+	ASSERT_FALSE(found_netns_L3A);
+	ASSERT_FALSE(found_userns_L1);
+	ASSERT_FALSE(found_userns_L2);
+	ASSERT_FALSE(found_userns_L3);
+	TH_LOG("Cascade test passed: all namespaces disappeared after netns FD closed");
+
+	/*
+	 * Test 4: Verify file handle no longer works for inactive namespace.
+	 */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd >= 0) {
+		close(reopened_fd);
+		free(handle);
+		ASSERT_TRUE(false); /* Should have failed */
+	}
+	TH_LOG("Inactive namespace correctly cannot be reopened via file handle");
+
+	/*
+	 * Test 5: Verify that calling SIOCGSKNS again resurrects the tree again.
+	 * The socket is still valid, so we can call SIOCGSKNS on it to resurrect
+	 * the namespace tree once more.
+	 */
+	netns_L3A_fd = ioctl(sock_L3A_fd, SIOCGSKNS);
+	ASSERT_GE(netns_L3A_fd, 0);
+
+	TH_LOG("Called SIOCGSKNS again to resurrect the namespace tree");
+
+	/* Verify the namespace tree is resurrected and visible in listns() */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	found_netns_L3A = false;
+	found_userns_L1 = false;
+	found_userns_L2 = false;
+	found_userns_L3 = false;
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_L3A_id)
+			found_netns_L3A = true;
+		if (ns_ids[i] == userns_L1_id)
+			found_userns_L1 = true;
+		if (ns_ids[i] == userns_L2_id)
+			found_userns_L2 = true;
+		if (ns_ids[i] == userns_L3_id)
+			found_userns_L3 = true;
+	}
+
+	ASSERT_TRUE(found_netns_L3A);
+	ASSERT_TRUE(found_userns_L1);
+	ASSERT_TRUE(found_userns_L2);
+	ASSERT_TRUE(found_userns_L3);
+	TH_LOG("Second resurrection verified: all namespaces in hierarchy visible in listns() again");
+
+	/* Verify we can reopen via file handle again */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		TH_LOG("open_by_handle_at failed after second resurrection: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	close(reopened_fd);
+	TH_LOG("File handle test passed: net_L3A can be reopened after second resurrection");
+
+	/* Final cleanup */
+	close(sock_L3A_fd);
+	close(netns_L3A_fd);
+	free(handle);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 65/70] selftests/namespace: first threaded active reference count test

[Christian Brauner]

Test that namespace becomes inactive after thread exits. This verifies
active reference counting works with threads, not just processes.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 138 +++++++++++++++++++++
 1 file changed, 138 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index b7fa973a2572..0c6c4869bb16 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -12,9 +12,12 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <sys/syscall.h>
 #include <unistd.h>
+#include <pthread.h>
 #include "../kselftest_harness.h"
 #include "../filesystems/utils.h"
+#include "wrappers.h"
 
 #ifndef FD_NSFS_ROOT
 #define FD_NSFS_ROOT -10003 /* Root of the nsfs filesystem */
@@ -2113,4 +2116,139 @@ TEST(ns_mixed_types_same_owner)
 	ASSERT_LT(u_fd, 0);
 }
 
+/* Thread test helpers and structures */
+struct thread_ns_info {
+	__u64 ns_id;
+	int pipefd;
+	int syncfd_read;
+	int syncfd_write;
+	int exit_code;
+};
+
+static void *thread_create_namespace(void *arg)
+{
+	struct thread_ns_info *info = (struct thread_ns_info *)arg;
+	int ret;
+
+	/* Create new network namespace */
+	ret = unshare(CLONE_NEWNET);
+	if (ret < 0) {
+		info->exit_code = 1;
+		return NULL;
+	}
+
+	/* Get namespace ID */
+	int fd = open("/proc/thread-self/ns/net", O_RDONLY);
+	if (fd < 0) {
+		info->exit_code = 2;
+		return NULL;
+	}
+
+	ret = ioctl(fd, NS_GET_ID, &info->ns_id);
+	close(fd);
+	if (ret < 0) {
+		info->exit_code = 3;
+		return NULL;
+	}
+
+	/* Send namespace ID to main thread */
+	if (write(info->pipefd, &info->ns_id, sizeof(info->ns_id)) != sizeof(info->ns_id)) {
+		info->exit_code = 4;
+		return NULL;
+	}
+
+	/* Wait for signal to exit */
+	char sync_byte;
+	if (read(info->syncfd_read, &sync_byte, 1) != 1) {
+		info->exit_code = 5;
+		return NULL;
+	}
+
+	info->exit_code = 0;
+	return NULL;
+}
+
+/*
+ * Test that namespace becomes inactive after thread exits.
+ * This verifies active reference counting works with threads, not just processes.
+ */
+TEST(thread_ns_inactive_after_exit)
+{
+	pthread_t thread;
+	struct thread_ns_info info;
+	struct file_handle *handle;
+	int pipefd[2];
+	int syncpipe[2];
+	int ret;
+	char sync_byte;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(pipe(syncpipe), 0);
+
+	info.pipefd = pipefd[1];
+	info.syncfd_read = syncpipe[0];
+	info.syncfd_write = -1;
+	info.exit_code = -1;
+
+	/* Create thread that will create a namespace */
+	ret = pthread_create(&thread, NULL, thread_create_namespace, &info);
+	ASSERT_EQ(ret, 0);
+
+	/* Read namespace ID from thread */
+	__u64 ns_id;
+	ret = read(pipefd[0], &ns_id, sizeof(ns_id));
+	if (ret != sizeof(ns_id)) {
+		sync_byte = 'X';
+		write(syncpipe[1], &sync_byte, 1);
+		pthread_join(thread, NULL);
+		close(pipefd[0]);
+		close(pipefd[1]);
+		close(syncpipe[0]);
+		close(syncpipe[1]);
+		SKIP(return, "Failed to read namespace ID from thread");
+	}
+
+	TH_LOG("Thread created namespace with ID %llu", (unsigned long long)ns_id);
+
+	/* Construct file handle */
+	handle = (struct file_handle *)buf;
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *fh = (struct nsfs_file_handle *)handle->f_handle;
+	fh->ns_id = ns_id;
+	fh->ns_type = 0;
+	fh->ns_inum = 0;
+
+	/* Namespace should be active while thread is alive */
+	TH_LOG("Attempting to open namespace while thread is alive (should succeed)");
+	int nsfd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(nsfd, 0);
+	close(nsfd);
+
+	/* Signal thread to exit */
+	TH_LOG("Signaling thread to exit");
+	sync_byte = 'X';
+	ASSERT_EQ(write(syncpipe[1], &sync_byte, 1), 1);
+	close(syncpipe[1]);
+
+	/* Wait for thread to exit */
+	ASSERT_EQ(pthread_join(thread, NULL), 0);
+	close(pipefd[0]);
+	close(pipefd[1]);
+	close(syncpipe[0]);
+
+	if (info.exit_code != 0)
+		SKIP(return, "Thread failed to create namespace");
+
+	TH_LOG("Thread exited, namespace should be inactive");
+
+	/* Namespace should now be inactive */
+	nsfd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(nsfd, 0);
+	/* Should fail with ENOENT (inactive) or ESTALE (gone) */
+	TH_LOG("Namespace inactive as expected: %s (errno=%d)", strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 66/70] selftests/namespace: second threaded active reference count test

[Christian Brauner]

Test that a namespace remains active while a thread holds an fd to it.
Even after the thread exits, the namespace should remain active as long
as another thread holds a file descriptor to it.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 99 ++++++++++++++++++++++
 1 file changed, 99 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 0c6c4869bb16..24dc8ef106b9 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -2251,4 +2251,103 @@ TEST(thread_ns_inactive_after_exit)
 	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
 }
 
+/*
+ * Test that a namespace remains active while a thread holds an fd to it.
+ * Even after the thread exits, the namespace should remain active as long as
+ * another thread holds a file descriptor to it.
+ */
+TEST(thread_ns_fd_keeps_active)
+{
+	pthread_t thread;
+	struct thread_ns_info info;
+	struct file_handle *handle;
+	int pipefd[2];
+	int syncpipe[2];
+	int ret;
+	char sync_byte;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(pipe(syncpipe), 0);
+
+	info.pipefd = pipefd[1];
+	info.syncfd_read = syncpipe[0];
+	info.syncfd_write = -1;
+	info.exit_code = -1;
+
+	/* Create thread that will create a namespace */
+	ret = pthread_create(&thread, NULL, thread_create_namespace, &info);
+	ASSERT_EQ(ret, 0);
+
+	/* Read namespace ID from thread */
+	__u64 ns_id;
+	ret = read(pipefd[0], &ns_id, sizeof(ns_id));
+	if (ret != sizeof(ns_id)) {
+		sync_byte = 'X';
+		write(syncpipe[1], &sync_byte, 1);
+		pthread_join(thread, NULL);
+		close(pipefd[0]);
+		close(pipefd[1]);
+		close(syncpipe[0]);
+		close(syncpipe[1]);
+		SKIP(return, "Failed to read namespace ID from thread");
+	}
+
+	TH_LOG("Thread created namespace with ID %llu", (unsigned long long)ns_id);
+
+	/* Construct file handle */
+	handle = (struct file_handle *)buf;
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *fh = (struct nsfs_file_handle *)handle->f_handle;
+	fh->ns_id = ns_id;
+	fh->ns_type = 0;
+	fh->ns_inum = 0;
+
+	/* Open namespace while thread is alive */
+	TH_LOG("Opening namespace while thread is alive");
+	int nsfd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(nsfd, 0);
+
+	/* Signal thread to exit */
+	TH_LOG("Signaling thread to exit");
+	sync_byte = 'X';
+	write(syncpipe[1], &sync_byte, 1);
+	close(syncpipe[1]);
+
+	/* Wait for thread to exit */
+	pthread_join(thread, NULL);
+	close(pipefd[0]);
+	close(pipefd[1]);
+	close(syncpipe[0]);
+
+	if (info.exit_code != 0) {
+		close(nsfd);
+		SKIP(return, "Thread failed to create namespace");
+	}
+
+	TH_LOG("Thread exited, but main thread holds fd - namespace should remain active");
+
+	/* Namespace should still be active because we hold an fd */
+	int nsfd2 = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(nsfd2, 0);
+
+	/* Verify it's the same namespace */
+	struct stat st1, st2;
+	ASSERT_EQ(fstat(nsfd, &st1), 0);
+	ASSERT_EQ(fstat(nsfd2, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	close(nsfd2);
+
+	TH_LOG("Closing fd - namespace should become inactive");
+	close(nsfd);
+
+	/* Now namespace should be inactive */
+	nsfd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(nsfd, 0);
+	/* Should fail with ENOENT (inactive) or ESTALE (gone) */
+	TH_LOG("Namespace inactive as expected: %s (errno=%d)", strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 67/70] selftests/namespace: third threaded active reference count test

[Christian Brauner]

Test that namespaces become inactive after subprocess with multiple
threads exits. Create a subprocess that unshares user and network
namespaces, then creates two threads that share those namespaces. Verify
that after all threads and subprocess exit, the namespaces are no longer
listed by listns() and cannot be opened by open_by_handle_at().

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 319 +++++++++++++++++++++
 1 file changed, 319 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index 24dc8ef106b9..093268f0efaa 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -9,6 +9,7 @@
 #include <string.h>
 #include <linux/nsfs.h>
 #include <sys/mount.h>
+#include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
@@ -2350,4 +2351,322 @@ TEST(thread_ns_fd_keeps_active)
 	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
 }
 
+/* Structure for thread data in subprocess */
+struct thread_sleep_data {
+	int syncfd_read;
+};
+
+static void *thread_sleep_and_wait(void *arg)
+{
+	struct thread_sleep_data *data = (struct thread_sleep_data *)arg;
+	char sync_byte;
+
+	/* Wait for signal to exit - read will unblock when pipe is closed */
+	(void)read(data->syncfd_read, &sync_byte, 1);
+	return NULL;
+}
+
+/*
+ * Test that namespaces become inactive after subprocess with multiple threads exits.
+ * Create a subprocess that unshares user and network namespaces, then creates two
+ * threads that share those namespaces. Verify that after all threads and subprocess
+ * exit, the namespaces are no longer listed by listns() and cannot be opened by
+ * open_by_handle_at().
+ */
+TEST(thread_subprocess_ns_inactive_after_all_exit)
+{
+	int pipefd[2];
+	int sv[2];
+	pid_t pid;
+	int status;
+	__u64 user_id, net_id;
+	struct file_handle *user_handle, *net_handle;
+	char user_buf[sizeof(*user_handle) + MAX_HANDLE_SZ];
+	char net_buf[sizeof(*net_handle) + MAX_HANDLE_SZ];
+	char sync_byte;
+	int ret;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+		close(sv[0]);
+
+		/* Create user namespace with mappings */
+		if (setup_userns() < 0) {
+			fprintf(stderr, "Child: setup_userns() failed: %s\n", strerror(errno));
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+		fprintf(stderr, "Child: setup_userns() succeeded\n");
+
+		/* Get user namespace ID */
+		int user_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (user_fd < 0) {
+			fprintf(stderr, "Child: open(/proc/self/ns/user) failed: %s\n", strerror(errno));
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(user_fd, NS_GET_ID, &user_id) < 0) {
+			fprintf(stderr, "Child: ioctl(NS_GET_ID) for user ns failed: %s\n", strerror(errno));
+			close(user_fd);
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+		close(user_fd);
+		fprintf(stderr, "Child: user ns ID = %llu\n", (unsigned long long)user_id);
+
+		/* Unshare network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			fprintf(stderr, "Child: unshare(CLONE_NEWNET) failed: %s\n", strerror(errno));
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+		fprintf(stderr, "Child: unshare(CLONE_NEWNET) succeeded\n");
+
+		/* Get network namespace ID */
+		int net_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (net_fd < 0) {
+			fprintf(stderr, "Child: open(/proc/self/ns/net) failed: %s\n", strerror(errno));
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(net_fd, NS_GET_ID, &net_id) < 0) {
+			fprintf(stderr, "Child: ioctl(NS_GET_ID) for net ns failed: %s\n", strerror(errno));
+			close(net_fd);
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+		close(net_fd);
+		fprintf(stderr, "Child: net ns ID = %llu\n", (unsigned long long)net_id);
+
+		/* Send namespace IDs to parent */
+		if (write(pipefd[1], &user_id, sizeof(user_id)) != sizeof(user_id)) {
+			fprintf(stderr, "Child: write(user_id) failed: %s\n", strerror(errno));
+			exit(1);
+		}
+		if (write(pipefd[1], &net_id, sizeof(net_id)) != sizeof(net_id)) {
+			fprintf(stderr, "Child: write(net_id) failed: %s\n", strerror(errno));
+			exit(1);
+		}
+		close(pipefd[1]);
+		fprintf(stderr, "Child: sent namespace IDs to parent\n");
+
+		/* Create two threads that share the namespaces */
+		pthread_t thread1, thread2;
+		struct thread_sleep_data data;
+		data.syncfd_read = sv[1];
+
+		int ret_thread = pthread_create(&thread1, NULL, thread_sleep_and_wait, &data);
+		if (ret_thread != 0) {
+			fprintf(stderr, "Child: pthread_create(thread1) failed: %s\n", strerror(ret_thread));
+			close(sv[1]);
+			exit(1);
+		}
+		fprintf(stderr, "Child: created thread1\n");
+
+		ret_thread = pthread_create(&thread2, NULL, thread_sleep_and_wait, &data);
+		if (ret_thread != 0) {
+			fprintf(stderr, "Child: pthread_create(thread2) failed: %s\n", strerror(ret_thread));
+			close(sv[1]);
+			pthread_cancel(thread1);
+			exit(1);
+		}
+		fprintf(stderr, "Child: created thread2\n");
+
+		/* Wait for threads to complete - they will unblock when parent writes */
+		fprintf(stderr, "Child: waiting for threads to exit\n");
+		pthread_join(thread1, NULL);
+		fprintf(stderr, "Child: thread1 exited\n");
+		pthread_join(thread2, NULL);
+		fprintf(stderr, "Child: thread2 exited\n");
+
+		close(sv[1]);
+
+		/* Exit - namespaces should become inactive */
+		fprintf(stderr, "Child: all threads joined, exiting with success\n");
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+	close(sv[1]);
+
+	TH_LOG("Parent: waiting to read namespace IDs from child");
+
+	/* Read namespace IDs from child */
+	ret = read(pipefd[0], &user_id, sizeof(user_id));
+	if (ret != sizeof(user_id)) {
+		TH_LOG("Parent: failed to read user_id, ret=%d, errno=%s", ret, strerror(errno));
+		close(pipefd[0]);
+		sync_byte = 'X';
+		(void)write(sv[0], &sync_byte, 1);
+		close(sv[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user namespace ID from child");
+	}
+
+	ret = read(pipefd[0], &net_id, sizeof(net_id));
+	close(pipefd[0]);
+	if (ret != sizeof(net_id)) {
+		TH_LOG("Parent: failed to read net_id, ret=%d, errno=%s", ret, strerror(errno));
+		sync_byte = 'X';
+		(void)write(sv[0], &sync_byte, 1);
+		close(sv[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read network namespace ID from child");
+	}
+
+	TH_LOG("Child created user ns %llu and net ns %llu with 2 threads",
+	       (unsigned long long)user_id, (unsigned long long)net_id);
+
+	/* Construct file handles */
+	user_handle = (struct file_handle *)user_buf;
+	user_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	user_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *user_fh = (struct nsfs_file_handle *)user_handle->f_handle;
+	user_fh->ns_id = user_id;
+	user_fh->ns_type = 0;
+	user_fh->ns_inum = 0;
+
+	net_handle = (struct file_handle *)net_buf;
+	net_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *net_fh = (struct nsfs_file_handle *)net_handle->f_handle;
+	net_fh->ns_id = net_id;
+	net_fh->ns_type = 0;
+	net_fh->ns_inum = 0;
+
+	/* Verify namespaces are active while subprocess and threads are alive */
+	TH_LOG("Verifying namespaces are active while subprocess with threads is running");
+	int user_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_GE(user_fd, 0);
+
+	int net_fd = open_by_handle_at(FD_NSFS_ROOT, net_handle, O_RDONLY);
+	ASSERT_GE(net_fd, 0);
+
+	close(user_fd);
+	close(net_fd);
+
+	/* Also verify they appear in listns() */
+	TH_LOG("Verifying namespaces appear in listns() while active");
+	struct ns_id_req req = {
+		.size = sizeof(struct ns_id_req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	int nr_ids = sys_listns(&req, ns_ids, 256, 0);
+	if (nr_ids < 0) {
+		TH_LOG("listns() not available, skipping listns verification");
+	} else {
+		/* Check if user_id is in the list */
+		int found_user = 0;
+		for (int i = 0; i < nr_ids; i++) {
+			if (ns_ids[i] == user_id) {
+				found_user = 1;
+				break;
+			}
+		}
+		ASSERT_TRUE(found_user);
+		TH_LOG("User namespace found in listns() as expected");
+
+		/* Check network namespace */
+		req.ns_type = CLONE_NEWNET;
+		nr_ids = sys_listns(&req, ns_ids, 256, 0);
+		if (nr_ids >= 0) {
+			int found_net = 0;
+			for (int i = 0; i < nr_ids; i++) {
+				if (ns_ids[i] == net_id) {
+					found_net = 1;
+					break;
+				}
+			}
+			ASSERT_TRUE(found_net);
+			TH_LOG("Network namespace found in listns() as expected");
+		}
+	}
+
+	/* Signal threads to exit */
+	TH_LOG("Signaling threads to exit");
+	sync_byte = 'X';
+	/* Write two bytes - one for each thread */
+	ASSERT_EQ(write(sv[0], &sync_byte, 1), 1);
+	ASSERT_EQ(write(sv[0], &sync_byte, 1), 1);
+	close(sv[0]);
+
+	/* Wait for child process to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	if (WEXITSTATUS(status) != 0) {
+		TH_LOG("Child process failed with exit code %d", WEXITSTATUS(status));
+		SKIP(return, "Child process failed");
+	}
+
+	TH_LOG("Subprocess and all threads have exited successfully");
+
+	/* Verify namespaces are now inactive - open_by_handle_at should fail */
+	TH_LOG("Verifying namespaces are inactive after subprocess and threads exit");
+	user_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_LT(user_fd, 0);
+	TH_LOG("User namespace inactive as expected: %s (errno=%d)",
+	       strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+
+	net_fd = open_by_handle_at(FD_NSFS_ROOT, net_handle, O_RDONLY);
+	ASSERT_LT(net_fd, 0);
+	TH_LOG("Network namespace inactive as expected: %s (errno=%d)",
+	       strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+
+	/* Verify namespaces do NOT appear in listns() */
+	TH_LOG("Verifying namespaces do NOT appear in listns() when inactive");
+	memset(&req, 0, sizeof(req));
+	req.size = sizeof(struct ns_id_req);
+	req.ns_type = CLONE_NEWUSER;
+	nr_ids = sys_listns(&req, ns_ids, 256, 0);
+	if (nr_ids >= 0) {
+		int found_user = 0;
+		for (int i = 0; i < nr_ids; i++) {
+			if (ns_ids[i] == user_id) {
+				found_user = 1;
+				break;
+			}
+		}
+		ASSERT_FALSE(found_user);
+		TH_LOG("User namespace correctly not listed in listns()");
+
+		/* Check network namespace */
+		req.ns_type = CLONE_NEWNET;
+		nr_ids = sys_listns(&req, ns_ids, 256, 0);
+		if (nr_ids >= 0) {
+			int found_net = 0;
+			for (int i = 0; i < nr_ids; i++) {
+				if (ns_ids[i] == net_id) {
+					found_net = 1;
+					break;
+				}
+			}
+			ASSERT_FALSE(found_net);
+			TH_LOG("Network namespace correctly not listed in listns()");
+		}
+	}
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 68/70] selftests/namespace: commit_creds() active reference tests

[Christian Brauner]

Test credential changes and their impact on namespace active references.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/.gitignore      |   1 +
 tools/testing/selftests/namespaces/Makefile        |   4 +-
 .../selftests/namespaces/cred_change_test.c        | 814 +++++++++++++++++++++
 3 files changed, 818 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index aeb5f2711ff6..0091a7dfff20 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -5,3 +5,4 @@ ns_active_ref_test
 listns_test
 listns_permissions_test
 siocgskns_test
+cred_change_test
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index d456505189cd..5d73f8dde6a0 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -8,7 +8,8 @@ TEST_GEN_PROGS := nsid_test \
 		  ns_active_ref_test \
 		  listns_test \
 		  listns_permissions_test \
-		  siocgskns_test
+		  siocgskns_test \
+		  cred_change_test
 
 include ../lib.mk
 
@@ -16,4 +17,5 @@ $(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
 $(OUTPUT)/listns_test: ../filesystems/utils.c
 $(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
 $(OUTPUT)/siocgskns_test: ../filesystems/utils.c
+$(OUTPUT)/cred_change_test: ../filesystems/utils.c
 
diff --git a/tools/testing/selftests/namespaces/cred_change_test.c b/tools/testing/selftests/namespaces/cred_change_test.c
new file mode 100644
index 000000000000..7b4f5ad3f725
--- /dev/null
+++ b/tools/testing/selftests/namespaces/cred_change_test.c
@@ -0,0 +1,814 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/capability.h>
+#include <sys/ioctl.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <linux/nsfs.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Test credential changes and their impact on namespace active references.
+ */
+
+/*
+ * Test setuid() in a user namespace properly swaps active references.
+ * Create a user namespace with multiple UIDs mapped, then setuid() between them.
+ * Verify that the user namespace remains active throughout.
+ */
+TEST(setuid_preserves_active_refs)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int setuid_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace with multiple UIDs mapped (0-9) */
+		userns_fd = get_userns_fd(0, orig_uid, 10);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send namespace ID to parent */
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/*
+		 * Perform multiple setuid() calls.
+		 * Each setuid() triggers commit_creds() which should properly
+		 * swap active references via switch_cred_namespaces().
+		 */
+		for (setuid_count = 0; setuid_count < 50; setuid_count++) {
+			uid_t target_uid = (setuid_count % 10);
+			if (setuid(target_uid) < 0) {
+				if (errno != EPERM) {
+					close(pipefd[1]);
+					exit(1);
+				}
+			}
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	TH_LOG("Child user namespace ID: %llu", (unsigned long long)userns_id);
+
+	/* Verify namespace is active while child is running */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+	ASSERT_TRUE(found);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive after child exits */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	found = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("setuid() correctly preserved active references (no leak)");
+}
+
+/*
+ * Test setgid() in a user namespace properly handles active references.
+ */
+TEST(setgid_preserves_active_refs)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int setgid_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace with multiple GIDs mapped */
+		userns_fd = get_userns_fd(0, orig_uid, 10);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/* Perform multiple setgid() calls */
+		for (setgid_count = 0; setgid_count < 50; setgid_count++) {
+			gid_t target_gid = (setgid_count % 10);
+			if (setgid(target_gid) < 0) {
+				if (errno != EPERM) {
+					close(pipefd[1]);
+					exit(1);
+				}
+			}
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("setgid() correctly preserved active references (no leak)");
+}
+
+/*
+ * Test setresuid() which changes real, effective, and saved UIDs.
+ * This should properly swap active references via commit_creds().
+ */
+TEST(setresuid_preserves_active_refs)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int setres_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace */
+		userns_fd = get_userns_fd(0, orig_uid, 10);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/* Perform multiple setresuid() calls */
+		for (setres_count = 0; setres_count < 30; setres_count++) {
+			uid_t uid1 = (setres_count % 5);
+			uid_t uid2 = ((setres_count + 1) % 5);
+			uid_t uid3 = ((setres_count + 2) % 5);
+
+			if (setresuid(uid1, uid2, uid3) < 0) {
+				if (errno != EPERM) {
+					close(pipefd[1]);
+					exit(1);
+				}
+			}
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("setresuid() correctly preserved active references (no leak)");
+}
+
+/*
+ * Test credential changes across multiple user namespaces.
+ * Create nested user namespaces and verify active reference tracking.
+ */
+TEST(cred_change_nested_userns)
+{
+	pid_t pid;
+	int status;
+	__u64 parent_userns_id, child_userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found_parent = false, found_child = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 parent_id, child_id;
+		uid_t orig_uid = getuid();
+
+		close(pipefd[0]);
+
+		/* Create first user namespace */
+		userns_fd = get_userns_fd(0, orig_uid, 1);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get first namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &parent_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create nested user namespace */
+		userns_fd = get_userns_fd(0, 0, 1);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get nested namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send both IDs to parent */
+		write(pipefd[1], &parent_id, sizeof(parent_id));
+		write(pipefd[1], &child_id, sizeof(child_id));
+
+		/* Perform some credential changes in nested namespace */
+		setuid(0);
+		setgid(0);
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	/* Read both namespace IDs */
+	if (read(pipefd[0], &parent_userns_id, sizeof(parent_userns_id)) != sizeof(parent_userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get parent namespace ID");
+	}
+
+	if (read(pipefd[0], &child_userns_id, sizeof(child_userns_id)) != sizeof(child_userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get child namespace ID");
+	}
+	close(pipefd[0]);
+
+	TH_LOG("Parent userns: %llu, Child userns: %llu",
+	       (unsigned long long)parent_userns_id,
+	       (unsigned long long)child_userns_id);
+
+	/* Verify both namespaces are active */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == parent_userns_id)
+			found_parent = true;
+		if (ns_ids[i] == child_userns_id)
+			found_child = true;
+	}
+
+	ASSERT_TRUE(found_parent);
+	ASSERT_TRUE(found_child);
+
+	/* Wait for child */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify both namespaces become inactive */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	found_parent = false;
+	found_child = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == parent_userns_id)
+			found_parent = true;
+		if (ns_ids[i] == child_userns_id)
+			found_child = true;
+	}
+
+	ASSERT_FALSE(found_parent);
+	ASSERT_FALSE(found_child);
+	TH_LOG("Nested user namespace credential changes preserved active refs (no leak)");
+}
+
+/*
+ * Test rapid credential changes don't cause refcount imbalances.
+ * This stress-tests the switch_cred_namespaces() logic.
+ */
+TEST(rapid_cred_changes_no_leak)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int change_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace with wider range of UIDs/GIDs */
+		userns_fd = get_userns_fd(0, orig_uid, 100);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/*
+		 * Perform many rapid credential changes.
+		 * Mix setuid, setgid, setreuid, setregid, setresuid, setresgid.
+		 */
+		for (change_count = 0; change_count < 200; change_count++) {
+			switch (change_count % 6) {
+			case 0:
+				setuid(change_count % 50);
+				break;
+			case 1:
+				setgid(change_count % 50);
+				break;
+			case 2:
+				setreuid(change_count % 50, (change_count + 1) % 50);
+				break;
+			case 3:
+				setregid(change_count % 50, (change_count + 1) % 50);
+				break;
+			case 4:
+				setresuid(change_count % 50, (change_count + 1) % 50, (change_count + 2) % 50);
+				break;
+			case 5:
+				setresgid(change_count % 50, (change_count + 1) % 50, (change_count + 2) % 50);
+				break;
+			}
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	TH_LOG("Testing with user namespace ID: %llu", (unsigned long long)userns_id);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive (no leaked active refs) */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("200 rapid credential changes completed with no active ref leak");
+}
+
+/*
+ * Test setfsuid/setfsgid which change filesystem UID/GID.
+ * These also trigger credential changes but may have different code paths.
+ */
+TEST(setfsuid_preserves_active_refs)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int change_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace */
+		userns_fd = get_userns_fd(0, orig_uid, 10);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/* Perform multiple setfsuid/setfsgid calls */
+		for (change_count = 0; change_count < 50; change_count++) {
+			setfsuid(change_count % 10);
+			setfsgid(change_count % 10);
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("setfsuid/setfsgid correctly preserved active references (no leak)");
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 69/70] selftests/namespace: add stress test

[Christian Brauner]

Stress tests for namespace active reference counting.

These tests validate that the active reference counting system can
handle high load scenarios including rapid namespace
creation/destruction, large numbers of concurrent namespaces, and
various edge cases under stress.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/.gitignore    |   1 +
 tools/testing/selftests/namespaces/Makefile      |   4 +-
 tools/testing/selftests/namespaces/stress_test.c | 626 +++++++++++++++++++++++
 3 files changed, 630 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index 0091a7dfff20..f6dcf769f150 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -6,3 +6,4 @@ listns_test
 listns_permissions_test
 siocgskns_test
 cred_change_test
+stress_test
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index 5d73f8dde6a0..3c776740f3ac 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -9,7 +9,8 @@ TEST_GEN_PROGS := nsid_test \
 		  listns_test \
 		  listns_permissions_test \
 		  siocgskns_test \
-		  cred_change_test
+		  cred_change_test \
+		  stress_test
 
 include ../lib.mk
 
@@ -18,4 +19,5 @@ $(OUTPUT)/listns_test: ../filesystems/utils.c
 $(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
 $(OUTPUT)/siocgskns_test: ../filesystems/utils.c
 $(OUTPUT)/cred_change_test: ../filesystems/utils.c
+$(OUTPUT)/stress_test: ../filesystems/utils.c
 
diff --git a/tools/testing/selftests/namespaces/stress_test.c b/tools/testing/selftests/namespaces/stress_test.c
new file mode 100644
index 000000000000..dd7df7d6cb27
--- /dev/null
+++ b/tools/testing/selftests/namespaces/stress_test.c
@@ -0,0 +1,626 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <linux/nsfs.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Stress tests for namespace active reference counting.
+ *
+ * These tests validate that the active reference counting system can handle
+ * high load scenarios including rapid namespace creation/destruction, large
+ * numbers of concurrent namespaces, and various edge cases under stress.
+ */
+
+/*
+ * Test rapid creation and destruction of user namespaces.
+ * Create and destroy namespaces in quick succession to stress the
+ * active reference tracking and ensure no leaks occur.
+ */
+TEST(rapid_namespace_creation_destruction)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[256], ns_ids_after[256];
+	ssize_t ret_before, ret_after;
+	int i;
+
+	/* Get baseline count of active user namespaces */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active user namespaces", ret_before);
+
+	/* Rapidly create and destroy 100 user namespaces */
+	for (i = 0; i < 100; i++) {
+		pid_t pid = fork();
+		ASSERT_GE(pid, 0);
+
+		if (pid == 0) {
+			/* Child: create user namespace and immediately exit */
+			if (setup_userns() < 0)
+				exit(1);
+			exit(0);
+		}
+
+		/* Parent: wait for child */
+		int status;
+		waitpid(pid, &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+		ASSERT_EQ(WEXITSTATUS(status), 0);
+	}
+
+	/* Verify we're back to baseline (no leaked namespaces) */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 100 rapid create/destroy cycles: %zd active user namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test creating many concurrent namespaces.
+ * Verify that listns() correctly tracks all of them and that they all
+ * become inactive after processes exit.
+ */
+TEST(many_concurrent_namespaces)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_during[512], ns_ids_after[512];
+	ssize_t ret_before, ret_during, ret_after;
+	pid_t pids[50];
+	int num_children = 50;
+	int i;
+	int sv[2];
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active user namespaces", ret_before);
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	/* Create many children, each with their own user namespace */
+	for (i = 0; i < num_children; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			/* Child: create user namespace and wait for parent signal */
+			char c;
+
+			close(sv[0]);
+
+			if (setup_userns() < 0) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Signal parent we're ready */
+			if (write(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Wait for parent signal to exit */
+			if (read(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			close(sv[1]);
+			exit(0);
+		}
+	}
+
+	close(sv[1]);
+
+	/* Wait for all children to signal ready */
+	for (i = 0; i < num_children; i++) {
+		char c;
+		if (read(sv[0], &c, 1) != 1) {
+			/* If we fail to read, kill all children and exit */
+			close(sv[0]);
+			for (int j = 0; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	/* List namespaces while all children are running */
+	ret_during = sys_listns(&req, ns_ids_during, ARRAY_SIZE(ns_ids_during), 0);
+	ASSERT_GE(ret_during, 0);
+
+	TH_LOG("With %d children running: %zd active user namespaces", num_children, ret_during);
+
+	/* Should have at least num_children more namespaces than baseline */
+	ASSERT_GE(ret_during, ret_before + num_children);
+
+	/* Signal all children to exit */
+	for (i = 0; i < num_children; i++) {
+		char c = 'X';
+		if (write(sv[0], &c, 1) != 1) {
+			/* If we fail to write, kill remaining children */
+			close(sv[0]);
+			for (int j = i; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	close(sv[0]);
+
+	/* Wait for all children */
+	for (i = 0; i < num_children; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After all children exit: %zd active user namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test rapid namespace creation with different namespace types.
+ * Create multiple types of namespaces rapidly to stress the tracking system.
+ */
+TEST(rapid_mixed_namespace_creation)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,  /* All types */
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	int i;
+
+	/* Get baseline count */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active namespaces (all types)", ret_before);
+
+	/* Rapidly create and destroy namespaces with multiple types */
+	for (i = 0; i < 50; i++) {
+		pid_t pid = fork();
+		ASSERT_GE(pid, 0);
+
+		if (pid == 0) {
+			/* Child: create multiple namespace types */
+			if (setup_userns() < 0)
+				exit(1);
+
+			/* Create additional namespace types */
+			if (unshare(CLONE_NEWNET) < 0)
+				exit(1);
+			if (unshare(CLONE_NEWUTS) < 0)
+				exit(1);
+			if (unshare(CLONE_NEWIPC) < 0)
+				exit(1);
+
+			exit(0);
+		}
+
+		/* Parent: wait for child */
+		int status;
+		waitpid(pid, &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 50 rapid mixed namespace cycles: %zd active namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test nested namespace creation under stress.
+ * Create deeply nested namespace hierarchies and verify proper cleanup.
+ */
+TEST(nested_namespace_stress)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	int i;
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active user namespaces", ret_before);
+
+	/* Create 20 processes, each with nested user namespaces */
+	for (i = 0; i < 20; i++) {
+		pid_t pid = fork();
+		ASSERT_GE(pid, 0);
+
+		if (pid == 0) {
+			int userns_fd;
+			uid_t orig_uid = getuid();
+			int depth;
+
+			/* Create nested user namespaces (up to 5 levels) */
+			for (depth = 0; depth < 5; depth++) {
+				userns_fd = get_userns_fd(0, (depth == 0) ? orig_uid : 0, 1);
+				if (userns_fd < 0)
+					exit(1);
+
+				if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+					close(userns_fd);
+					exit(1);
+				}
+				close(userns_fd);
+			}
+
+			exit(0);
+		}
+
+		/* Parent: wait for child */
+		int status;
+		waitpid(pid, &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 20 nested namespace hierarchies: %zd active user namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test listns() pagination under stress.
+ * Create many namespaces and verify pagination works correctly.
+ */
+TEST(listns_pagination_stress)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	pid_t pids[30];
+	int num_children = 30;
+	int i;
+	int sv[2];
+	__u64 all_ns_ids[512];
+	int total_found = 0;
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	/* Create many children with user namespaces */
+	for (i = 0; i < num_children; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			char c;
+			close(sv[0]);
+
+			if (setup_userns() < 0) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Signal parent we're ready */
+			if (write(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Wait for parent signal to exit */
+			if (read(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			close(sv[1]);
+			exit(0);
+		}
+	}
+
+	close(sv[1]);
+
+	/* Wait for all children to signal ready */
+	for (i = 0; i < num_children; i++) {
+		char c;
+		if (read(sv[0], &c, 1) != 1) {
+			/* If we fail to read, kill all children and exit */
+			close(sv[0]);
+			for (int j = 0; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	/* Paginate through all namespaces using small batch sizes */
+	req.ns_id = 0;
+	while (1) {
+		__u64 batch[5];  /* Small batch size to force pagination */
+		ssize_t ret;
+
+		ret = sys_listns(&req, batch, ARRAY_SIZE(batch), 0);
+		if (ret < 0) {
+			if (errno == ENOSYS) {
+				close(sv[0]);
+				for (i = 0; i < num_children; i++)
+					kill(pids[i], SIGKILL);
+				for (i = 0; i < num_children; i++)
+					waitpid(pids[i], NULL, 0);
+				SKIP(return, "listns() not supported");
+			}
+			ASSERT_GE(ret, 0);
+		}
+
+		if (ret == 0)
+			break;
+
+		/* Store results */
+		for (i = 0; i < ret && total_found < 512; i++) {
+			all_ns_ids[total_found++] = batch[i];
+		}
+
+		/* Update cursor for next batch */
+		if (ret == ARRAY_SIZE(batch))
+			req.ns_id = batch[ret - 1];
+		else
+			break;
+	}
+
+	TH_LOG("Paginated through %d user namespaces", total_found);
+
+	/* Verify no duplicates in pagination */
+	for (i = 0; i < total_found; i++) {
+		for (int j = i + 1; j < total_found; j++) {
+			if (all_ns_ids[i] == all_ns_ids[j]) {
+				TH_LOG("Found duplicate ns_id: %llu at positions %d and %d",
+				       (unsigned long long)all_ns_ids[i], i, j);
+				ASSERT_TRUE(false);
+			}
+		}
+	}
+
+	/* Signal all children to exit */
+	for (i = 0; i < num_children; i++) {
+		char c = 'X';
+		if (write(sv[0], &c, 1) != 1) {
+			close(sv[0]);
+			for (int j = i; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	close(sv[0]);
+
+	/* Wait for all children */
+	for (i = 0; i < num_children; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+	}
+}
+
+/*
+ * Test concurrent namespace operations.
+ * Multiple processes creating, querying, and destroying namespaces concurrently.
+ */
+TEST(concurrent_namespace_operations)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	pid_t pids[20];
+	int num_workers = 20;
+	int i;
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active namespaces", ret_before);
+
+	/* Create worker processes that do concurrent operations */
+	for (i = 0; i < num_workers; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			/* Each worker: create namespaces, list them, repeat */
+			int iterations;
+
+			for (iterations = 0; iterations < 10; iterations++) {
+				int userns_fd;
+				__u64 temp_ns_ids[100];
+				ssize_t ret;
+
+				/* Create a user namespace */
+				userns_fd = get_userns_fd(0, getuid(), 1);
+				if (userns_fd < 0)
+					continue;
+
+				/* List namespaces */
+				ret = sys_listns(&req, temp_ns_ids, ARRAY_SIZE(temp_ns_ids), 0);
+				(void)ret;
+
+				close(userns_fd);
+
+				/* Small delay */
+				usleep(1000);
+			}
+
+			exit(0);
+		}
+	}
+
+	/* Wait for all workers */
+	for (i = 0; i < num_workers; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+		ASSERT_EQ(WEXITSTATUS(status), 0);
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After concurrent operations: %zd active namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test namespace churn - continuous creation and destruction.
+ * Simulates high-churn scenarios like container orchestration.
+ */
+TEST(namespace_churn)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWUTS,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	int cycle;
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active namespaces", ret_before);
+
+	/* Simulate churn: batches of namespaces created and destroyed */
+	for (cycle = 0; cycle < 10; cycle++) {
+		pid_t batch_pids[10];
+		int i;
+
+		/* Create batch */
+		for (i = 0; i < 10; i++) {
+			batch_pids[i] = fork();
+			ASSERT_GE(batch_pids[i], 0);
+
+			if (batch_pids[i] == 0) {
+				/* Create multiple namespace types */
+				if (setup_userns() < 0)
+					exit(1);
+				if (unshare(CLONE_NEWNET) < 0)
+					exit(1);
+				if (unshare(CLONE_NEWUTS) < 0)
+					exit(1);
+
+				/* Keep namespaces alive briefly */
+				usleep(10000);
+				exit(0);
+			}
+		}
+
+		/* Wait for batch to complete */
+		for (i = 0; i < 10; i++) {
+			int status;
+			waitpid(batch_pids[i], &status, 0);
+		}
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 10 churn cycles (100 namespace sets): %zd active namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v3 70/70] selftests/namespace: test listns() pagination

[Christian Brauner]

Minimal test case to reproduce KASAN out-of-bounds in listns pagination.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/.gitignore      |   1 +
 tools/testing/selftests/namespaces/Makefile        |   4 +-
 .../selftests/namespaces/listns_pagination_bug.c   | 138 +++++++++++++++++++++
 3 files changed, 142 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index f6dcf769f150..f4d2209ca4e4 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -7,3 +7,4 @@ listns_permissions_test
 siocgskns_test
 cred_change_test
 stress_test
+listns_pagination_bug
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index 3c776740f3ac..01569e0abbdb 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -10,7 +10,8 @@ TEST_GEN_PROGS := nsid_test \
 		  listns_permissions_test \
 		  siocgskns_test \
 		  cred_change_test \
-		  stress_test
+		  stress_test \
+		  listns_pagination_bug
 
 include ../lib.mk
 
@@ -20,4 +21,5 @@ $(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
 $(OUTPUT)/siocgskns_test: ../filesystems/utils.c
 $(OUTPUT)/cred_change_test: ../filesystems/utils.c
 $(OUTPUT)/stress_test: ../filesystems/utils.c
+$(OUTPUT)/listns_pagination_bug: ../filesystems/utils.c
 
diff --git a/tools/testing/selftests/namespaces/listns_pagination_bug.c b/tools/testing/selftests/namespaces/listns_pagination_bug.c
new file mode 100644
index 000000000000..da7d33f96397
--- /dev/null
+++ b/tools/testing/selftests/namespaces/listns_pagination_bug.c
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/socket.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Minimal test case to reproduce KASAN out-of-bounds in listns pagination.
+ *
+ * The bug occurs when:
+ * 1. Filtering by a specific namespace type (e.g., CLONE_NEWUSER)
+ * 2. Using pagination (req.ns_id != 0)
+ * 3. The lookup_ns_id_at() call in do_listns() passes ns_type=0 instead of
+ *    the filtered type, causing it to search the unified tree and potentially
+ *    return a namespace of the wrong type.
+ */
+TEST(pagination_with_type_filter)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,  /* Filter by user namespace */
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	pid_t pids[10];
+	int num_children = 10;
+	int i;
+	int sv[2];
+	__u64 first_batch[3];
+	ssize_t ret;
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	/* Create children with user namespaces */
+	for (i = 0; i < num_children; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			char c;
+			close(sv[0]);
+
+			if (setup_userns() < 0) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Signal parent we're ready */
+			if (write(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Wait for parent signal to exit */
+			if (read(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			close(sv[1]);
+			exit(0);
+		}
+	}
+
+	close(sv[1]);
+
+	/* Wait for all children to signal ready */
+	for (i = 0; i < num_children; i++) {
+		char c;
+		if (read(sv[0], &c, 1) != 1) {
+			close(sv[0]);
+			for (int j = 0; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	/* First batch - this should work */
+	ret = sys_listns(&req, first_batch, 3, 0);
+	if (ret < 0) {
+		if (errno == ENOSYS) {
+			close(sv[0]);
+			for (i = 0; i < num_children; i++)
+				kill(pids[i], SIGKILL);
+			for (i = 0; i < num_children; i++)
+				waitpid(pids[i], NULL, 0);
+			SKIP(return, "listns() not supported");
+		}
+		ASSERT_GE(ret, 0);
+	}
+
+	TH_LOG("First batch returned %zd entries", ret);
+
+	if (ret == 3) {
+		__u64 second_batch[3];
+
+		/* Second batch - pagination triggers the bug */
+		req.ns_id = first_batch[2];  /* Continue from last ID */
+		ret = sys_listns(&req, second_batch, 3, 0);
+
+		TH_LOG("Second batch returned %zd entries", ret);
+		ASSERT_GE(ret, 0);
+	}
+
+	/* Signal all children to exit */
+	for (i = 0; i < num_children; i++) {
+		char c = 'X';
+		if (write(sv[0], &c, 1) != 1) {
+			close(sv[0]);
+			for (int j = i; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	close(sv[0]);
+
+	/* Cleanup */
+	for (i = 0; i < num_children; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+	}
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3

Re: [PATCH v3 00/70] nstree: listns()

[Jeff Layton]

On Fri, 2025-10-24 at 12:52 +0200, Christian Brauner wrote:
> Hey,
> 
> As announced a while ago this is the next step building on the nstree
> work from prior cycles. There's a bunch of fixes and semantic cleanups
> in here and a ton of tests.
> 
> Currently listns() is relying on active namespace reference counts which
> are introduced alongside this series.
> 
> While a namespace is on the namespace trees with a valid reference count
> it is possible to reopen it through a namespace file handle. This is all
> fine but has some issues that should be addressed.
> 
> On current kernels a namespace is visible to userspace in the
> following cases:
> 
> (1) The namespace is in use by a task.
> (2) The namespace is persisted through a VFS object (namespace file
>     descriptor or bind-mount).
>     Note that (2) only cares about direct persistence of the namespace
>     itself not indirectly via e.g., file->f_cred file references or
>     similar.
> (3) The namespace is a hierarchical namespace type and is the parent of
>     a single or multiple child namespaces.
> 
> Case (3) is interesting because it is possible that a parent namespace
> might not fulfill any of (1) or (2), i.e., is invisible to userspace but
> it may still be resurrected through the NS_GET_PARENT ioctl().
> 
> Currently namespace file handles allow much broader access to namespaces
> than what is currently possible via (1)-(3). The reason is that
> namespaces may remain pinned for completely internal reasons yet are
> inaccessible to userspace.
> 
> For example, a user namespace my remain pinned by get_cred() calls to
> stash the opener's credentials into file->f_cred. As it stands file
> handles allow to resurrect such a users namespace even though this
> should not be possible via (1)-(3). This is a fundamental uapi change
> that we shouldn't do if we don't have to.
> 
> Consider the following insane case: Various architectures support the
> CONFIG_MMU_LAZY_TLB_REFCOUNT option which uses lazy TLB destruction.
> When this option is set a userspace task's struct mm_struct may be used
> for kernel threads such as the idle task and will only be destroyed once
> the cpu's runqueue switches back to another task. But because of ptrace()
> permission checks struct mm_struct stashes the user namespace of the
> task that struct mm_struct originally belonged to. The kernel thread
> will take a reference on the struct mm_struct and thus pin it.
> 
> So on an idle system user namespaces can be persisted for arbitrary
> amounts of time which also means that they can be resurrected using
> namespace file handles. That makes no sense whatsoever. The problem is
> of course excarabted on large systems with a huge number of cpus.
> 
> To handle this nicely we introduce an active reference count which
> tracks (1)-(3). This is easy to do as all of these things are already
> managed centrally. Only (1)-(3) will count towards the active reference
> count and only namespaces which are active may be opened via namespace
> file handles.
> 
> The problem is that namespaces may be resurrected. Which means that they
> can become temporarily inactive and will be reactived some time later.
> Currently the only example of this is the SIOGCSKNS socket ioctl. The
> SIOCGSKNS ioctl allows to open a network namespace file descriptor based
> on a socket file descriptor.
> 
> If a socket is tied to a network namespace that subsequently becomes
> inactive but that socket is persisted by another process in another
> network namespace (e.g., via SCM_RIGHTS of pidfd_getfd()) then the
> SIOCGSKNS ioctl will resurrect this network namespace.
> 
> So calls to open_related_ns() and open_namespace() will end up
> resurrecting the corresponding namespace tree.
> 
> Note that the active reference count does not regulate the lifetime of
> the namespace itself. This is still done by the normal reference count.
> The active reference count can only be elevated if the regular reference
> count is elevated.
> 
> The active reference count also doesn't regulate the presence of a
> namespace on the namespace trees. It only regulates its visiblity to
> namespace file handles (and in later patches to listns()).
> 
> A namespace remains on the namespace trees from creation until its
> actual destruction. This will allow the kernel to always reach any
> namespace trivially and it will also enable subsystems like bpf to walk
> the namespace lists on the system for tracing or general introspection
> purposes.
> 
> Note that different namespaces have different visibility lifetimes on
> current kernels. While most namespace are immediately released when the
> last task using them exits, the user- and pid namespace are persisted
> and thus both remain accessible via /proc/<pid>/ns/<ns_type>.
> 
> The user namespace lifetime is aliged with struct cred and is only
> released through exit_creds(). However, it becomes inaccessible to
> userspace once the last task using it is reaped, i.e., when
> release_task() is called and all proc entries are flushed. Similarly,
> the pid namespace is also visible until the last task using it has been
> reaped and the associated pid numbers are freed.
> 
> The active reference counts of the user- and pid namespace are
> decremented once the task is reaped.
> 
> Based on the namespace trees and the active reference count, a new
> listns() system call that allows userspace to iterate through namespaces
> in the system. This provides a programmatic interface to discover and
> inspect namespaces, enhancing existing namespace apis.
> 
> Currently, there is no direct way for userspace to enumerate namespaces
> in the system. Applications must resort to scanning /proc/<pid>/ns/
> across all processes, which is:
> 
> 1. Inefficient - requires iterating over all processes
> 2. Incomplete - misses inactive namespaces that aren't attached to any
>    running process but are kept alive by file descriptors, bind mounts,
>    or parent namespace references
> 3. Permission-heavy - requires access to /proc for many processes
> 4. No ordering or ownership.
> 5. No filtering per namespace type: Must always iterate and check all
>    namespaces.
> 
> The list goes on. The listns() system call solves these problems by
> providing direct kernel-level enumeration of namespaces. It is similar
> to listmount() but obviously tailored to namespaces.
> 
> /*
>  * @req: Pointer to struct ns_id_req specifying search parameters
>  * @ns_ids: User buffer to receive namespace IDs
>  * @nr_ns_ids: Size of ns_ids buffer (maximum number of IDs to return)
>  * @flags: Reserved for future use (must be 0)
>  */
> ssize_t listns(const struct ns_id_req *req, u64 *ns_ids,
>                size_t nr_ns_ids, unsigned int flags);
> 
> Returns:
> - On success: Number of namespace IDs written to ns_ids
> - On error: Negative error code
> 
> /*
>  * @size: Structure size
>  * @ns_id: Starting point for iteration; use 0 for first call, then
>  *         use the last returned ID for subsequent calls to paginate
>  * @ns_type: Bitmask of namespace types to include (from enum ns_type):
>  *           0: Return all namespace types
>  *           MNT_NS: Mount namespaces
>  *           NET_NS: Network namespaces
>  *           USER_NS: User namespaces
>  *           etc. Can be OR'd together
>  * @user_ns_id: Filter results to namespaces owned by this user namespace:
>  *              0: Return all namespaces (subject to permission checks)
>  *              LISTNS_CURRENT_USER: Namespaces owned by caller's user namespace
>  *              Other value: Namespaces owned by the specified user namespace ID
>  */
> struct ns_id_req {
>         __u32 size;         /* sizeof(struct ns_id_req) */
>         __u32 spare;        /* Reserved, must be 0 */
>         __u64 ns_id;        /* Last seen namespace ID (for pagination) */
>         __u32 ns_type;      /* Filter by namespace type(s) */
>         __u32 spare2;       /* Reserved, must be 0 */
>         __u64 user_ns_id;   /* Filter by owning user namespace */
> };
> 
> Example 1: List all namespaces
> 
> void list_all_namespaces(void)
> {
> 	struct ns_id_req req = {
> 		.size = sizeof(req),
> 		.ns_id = 0,      /* Start from beginning */
> 		.ns_type = 0,    /* All types */
> 		.user_ns_id = 0, /* All user namespaces */
> 	};
> 	uint64_t ids[100];
> 	ssize_t ret;
> 
> 	printf("All namespaces in the system:\n");
> 	do {
> 		ret = listns(&req, ids, 100, 0);
> 		if (ret < 0) {
> 			perror("listns");
> 			break;
> 		}
> 
> 		for (ssize_t i = 0; i < ret; i++)
> 			printf("  Namespace ID: %llu\n", (unsigned long long)ids[i]);
> 
> 		/* Continue from last seen ID */
> 		if (ret > 0)
> 			req.ns_id = ids[ret - 1];
> 	} while (ret == 100); /* Buffer was full, more may exist */
> }
> 
> Example 2 : List network namespaces only
> 
> void list_network_namespaces(void)
> {
> 	struct ns_id_req req = {
> 		.size = sizeof(req),
> 		.ns_id = 0,
> 		.ns_type = NET_NS, /* Only network namespaces */
> 		.user_ns_id = 0,
> 	};
> 	uint64_t ids[100];
> 	ssize_t ret;
> 
> 	ret = listns(&req, ids, 100, 0);
> 	if (ret < 0) {
> 		perror("listns");
> 		return;
> 	}
> 
> 	printf("Network namespaces: %zd found\n", ret);
> 	for (ssize_t i = 0; i < ret; i++)
> 		printf("  netns ID: %llu\n", (unsigned long long)ids[i]);
> }
> 
> Example 3 : List namespaces owned by current user namespace
> 
> void list_owned_namespaces(void)
> {
> 	struct ns_id_req req = {
> 		.size = sizeof(req),
> 		.ns_id = 0,
> 		.ns_type = 0,                      /* All types */
> 		.user_ns_id = LISTNS_CURRENT_USER, /* Current userns */
> 	};
> 	uint64_t ids[100];
> 	ssize_t ret;
> 
> 	ret = listns(&req, ids, 100, 0);
> 	if (ret < 0) {
> 		perror("listns");
> 		return;
> 	}
> 
> 	printf("Namespaces owned by my user namespace: %zd\n", ret);
> 	for (ssize_t i = 0; i < ret; i++)
> 		printf("  ns ID: %llu\n", (unsigned long long)ids[i]);
> }
> 
> Example 4 : List multiple namespace types
> 
> void list_network_and_mount_namespaces(void)
> {
> 	struct ns_id_req req = {
> 		.size = sizeof(req),
> 		.ns_id = 0,
> 		.ns_type = NET_NS | MNT_NS, /* Network and mount */
> 		.user_ns_id = 0,
> 	};
> 	uint64_t ids[100];
> 	ssize_t ret;
> 
> 	ret = listns(&req, ids, 100, 0);
> 	printf("Network and mount namespaces: %zd found\n", ret);
> }
> 
> Example 5 : Pagination through large namespace sets
> 
> void list_all_with_pagination(void)
> {
> 	struct ns_id_req req = {
> 		.size = sizeof(req),
> 		.ns_id = 0,
> 		.ns_type = 0,
> 		.user_ns_id = 0,
> 	};
> 	uint64_t ids[50];
> 	size_t total = 0;
> 	ssize_t ret;
> 
> 	printf("Enumerating all namespaces with pagination:\n");
> 
> 	while (1) {
> 		ret = listns(&req, ids, 50, 0);
> 		if (ret < 0) {
> 			perror("listns");
> 			break;
> 		}
> 		if (ret == 0)
> 			break; /* No more namespaces */
> 
> 		total += ret;
> 		printf("  Batch: %zd namespaces\n", ret);
> 
> 		/* Last ID in this batch becomes start of next batch */
> 		req.ns_id = ids[ret - 1];
> 
> 		if (ret < 50)
> 			break; /* Partial batch = end of results */
> 	}
> 
> 	printf("Total: %zu namespaces\n", total);
> }
> 
> listns() respects namespace isolation and capabilities:
> 
> (1) Global listing (user_ns_id = 0):
>     - Requires CAP_SYS_ADMIN in the namespace's owning user namespace
>     - OR the namespace must be in the caller's namespace context (e.g.,
>       a namespace the caller is currently using)
>     - User namespaces additionally allow listing if the caller has
>       CAP_SYS_ADMIN in that user namespace itself
> (2) Owner-filtered listing (user_ns_id != 0):
>     - Requires CAP_SYS_ADMIN in the specified owner user namespace
>     - OR the namespace must be in the caller's namespace context
>     - This allows unprivileged processes to enumerate namespaces they own
> (3) Visibility:
>     - Only "active" namespaces are listed
>     - A namespace is active if it has a non-zero __ns_ref_active count
>     - This includes namespaces used by running processes, held by open
>       file descriptors, or kept active by bind mounts
>     - Inactive namespaces (kept alive only by internal kernel
>       references) are not visible via listns()
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
> Changes in v3:
> - Expanded test-suite.
> - Moved active reference count tracking for task-attached namespaces to
>   dedicated helpers.
> - Fixed active reference count leaks when creating a new process fails.
> - Allow to be rescheduled when walking a a long namespace list.
> - Grab reference count when accessing a namespace when walking the list.
> - Link to v2: https://patch.msgid.link/20251022-work-namespace-nstree-listns-v2-0-71a588572371@kernel.org
> 
> Changes in v2:
> - Fully implement the active reference count.
> - Fix various minor issues.
> - Expand the testsuite to test complex resurrection scenarios due to SIOCGSKNS.
> - Currently each task takes an active reference on the user namespace as
>   credentials can be persisted for a very long time and completely
>   arbitrary reasons but we don't want to tie the lifetime of a user
>   namespace being visible to userspace to the existence of some
>   credentials being stashed somewhere. We want to tie it to it being
>   in-use by actual tasks or vfs objects and then go away. There might be
>   more clever ways of doing this but for now this is good enough.
> - TODO: Add detailed tests for multi-threaded namespace sharing.
> - Link to v1: https://patch.msgid.link/20251021-work-namespace-nstree-listns-v1-0-ad44261a8a5b@kernel.org
> 
> ---
> Christian Brauner (70):
>       libfs: allow to specify s_d_flags
>       nsfs: use inode_just_drop()
>       nsfs: raise DCACHE_DONTCACHE explicitly
>       pidfs: raise DCACHE_DONTCACHE explicitly
>       nsfs: raise SB_I_NODEV and SB_I_NOEXEC
>       cgroup: add cgroup namespace to tree after owner is set
>       nstree: simplify return
>       ns: initialize ns_list_node for initial namespaces
>       ns: add __ns_ref_read()
>       ns: rename to exit_nsproxy_namespaces()
>       ns: add active reference count
>       ns: use anonymous struct to group list member
>       nstree: introduce a unified tree
>       nstree: allow lookup solely based on inode
>       nstree: assign fixed ids to the initial namespaces
>       ns: maintain list of owned namespaces
>       nstree: add listns()
>       arch: hookup listns() system call
>       nsfs: update tools header
>       selftests/filesystems: remove CLONE_NEWPIDNS from setup_userns() helper
>       selftests/namespaces: first active reference count tests
>       selftests/namespaces: second active reference count tests
>       selftests/namespaces: third active reference count tests
>       selftests/namespaces: fourth active reference count tests
>       selftests/namespaces: fifth active reference count tests
>       selftests/namespaces: sixth active reference count tests
>       selftests/namespaces: seventh active reference count tests
>       selftests/namespaces: eigth active reference count tests
>       selftests/namespaces: ninth active reference count tests
>       selftests/namespaces: tenth active reference count tests
>       selftests/namespaces: eleventh active reference count tests
>       selftests/namespaces: twelth active reference count tests
>       selftests/namespaces: thirteenth active reference count tests
>       selftests/namespaces: fourteenth active reference count tests
>       selftests/namespaces: fifteenth active reference count tests
>       selftests/namespaces: add listns() wrapper
>       selftests/namespaces: first listns() test
>       selftests/namespaces: second listns() test
>       selftests/namespaces: third listns() test
>       selftests/namespaces: fourth listns() test
>       selftests/namespaces: fifth listns() test
>       selftests/namespaces: sixth listns() test
>       selftests/namespaces: seventh listns() test
>       selftests/namespaces: eigth listns() test
>       selftests/namespaces: ninth listns() test
>       selftests/namespaces: first listns() permission test
>       selftests/namespaces: second listns() permission test
>       selftests/namespaces: third listns() permission test
>       selftests/namespaces: fourth listns() permission test
>       selftests/namespaces: fifth listns() permission test
>       selftests/namespaces: sixth listns() permission test
>       selftests/namespaces: seventh listns() permission test
>       selftests/namespaces: first inactive namespace resurrection test
>       selftests/namespaces: second inactive namespace resurrection test
>       selftests/namespaces: third inactive namespace resurrection test
>       selftests/namespaces: fourth inactive namespace resurrection test
>       selftests/namespaces: fifth inactive namespace resurrection test
>       selftests/namespaces: sixth inactive namespace resurrection test
>       selftests/namespaces: seventh inactive namespace resurrection test
>       selftests/namespaces: eigth inactive namespace resurrection test
>       selftests/namespaces: ninth inactive namespace resurrection test
>       selftests/namespaces: tenth inactive namespace resurrection test
>       selftests/namespaces: eleventh inactive namespace resurrection test
>       selftests/namespaces: twelth inactive namespace resurrection test
>       selftests/namespace: first threaded active reference count test
>       selftests/namespace: second threaded active reference count test
>       selftests/namespace: third threaded active reference count test
>       selftests/namespace: commit_creds() active reference tests
>       selftests/namespace: add stress test
>       selftests/namespace: test listns() pagination
> 
>  arch/alpha/kernel/syscalls/syscall.tbl             |    1 +
>  arch/arm/tools/syscall.tbl                         |    1 +
>  arch/arm64/tools/syscall_32.tbl                    |    1 +
>  arch/m68k/kernel/syscalls/syscall.tbl              |    1 +
>  arch/microblaze/kernel/syscalls/syscall.tbl        |    1 +
>  arch/mips/kernel/syscalls/syscall_n32.tbl          |    1 +
>  arch/mips/kernel/syscalls/syscall_n64.tbl          |    1 +
>  arch/mips/kernel/syscalls/syscall_o32.tbl          |    1 +
>  arch/parisc/kernel/syscalls/syscall.tbl            |    1 +
>  arch/powerpc/kernel/syscalls/syscall.tbl           |    1 +
>  arch/s390/kernel/syscalls/syscall.tbl              |    1 +
>  arch/sh/kernel/syscalls/syscall.tbl                |    1 +
>  arch/sparc/kernel/syscalls/syscall.tbl             |    1 +
>  arch/x86/entry/syscalls/syscall_32.tbl             |    1 +
>  arch/x86/entry/syscalls/syscall_64.tbl             |    1 +
>  arch/xtensa/kernel/syscalls/syscall.tbl            |    1 +
>  fs/libfs.c                                         |    1 +
>  fs/namespace.c                                     |    8 +-
>  fs/nsfs.c                                          |   95 +-
>  fs/pidfs.c                                         |    1 +
>  include/linux/ns_common.h                          |  166 +-
>  include/linux/nsfs.h                               |    3 +
>  include/linux/nsproxy.h                            |    5 +-
>  include/linux/nstree.h                             |   26 +-
>  include/linux/pseudo_fs.h                          |    1 +
>  include/linux/syscalls.h                           |    4 +
>  include/linux/user_namespace.h                     |    4 +-
>  include/uapi/asm-generic/unistd.h                  |    4 +-
>  include/uapi/linux/nsfs.h                          |   58 +
>  init/version-timestamp.c                           |    5 +
>  ipc/msgutil.c                                      |    5 +
>  ipc/namespace.c                                    |    1 +
>  kernel/cgroup/cgroup.c                             |   11 +-
>  kernel/cgroup/namespace.c                          |    3 +-
>  kernel/cred.c                                      |    6 +
>  kernel/exit.c                                      |    3 +-
>  kernel/fork.c                                      |    3 +-
>  kernel/nscommon.c                                  |  227 +-
>  kernel/nsproxy.c                                   |   25 +-
>  kernel/nstree.c                                    |  540 +++-
>  kernel/pid.c                                       |   10 +
>  kernel/pid_namespace.c                             |    1 +
>  kernel/time/namespace.c                            |    6 +
>  kernel/user.c                                      |    5 +
>  kernel/user_namespace.c                            |    1 +
>  kernel/utsname.c                                   |    1 +
>  net/core/net_namespace.c                           |    3 +-
>  scripts/syscall.tbl                                |    1 +
>  tools/include/uapi/linux/nsfs.h                    |   70 +
>  tools/testing/selftests/filesystems/utils.c        |    2 +-
>  tools/testing/selftests/namespaces/.gitignore      |    7 +
>  tools/testing/selftests/namespaces/Makefile        |   20 +-
>  .../selftests/namespaces/cred_change_test.c        |  814 ++++++
>  .../selftests/namespaces/listns_pagination_bug.c   |  138 +
>  .../selftests/namespaces/listns_permissions_test.c |  759 ++++++
>  tools/testing/selftests/namespaces/listns_test.c   |  679 +++++
>  .../selftests/namespaces/ns_active_ref_test.c      | 2672 ++++++++++++++++++++
>  .../testing/selftests/namespaces/siocgskns_test.c  | 1824 +++++++++++++
>  tools/testing/selftests/namespaces/stress_test.c   |  626 +++++
>  tools/testing/selftests/namespaces/wrappers.h      |   35 +
>  60 files changed, 8835 insertions(+), 60 deletions(-)
> ---
> base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
> change-id: 20251020-work-namespace-nstree-listns-9fd71518515c

This looks pretty great overall, Christian. Nice work!

I hate the fact that we have to deal with resurrection here since it
makes things much messier, but I don't see a great alternative. I found
the nsfs filehandle format, btw, so that seems fine.

You can add this to patches 1-19, though I'd still prefer that you
split the ns_owner_tree handling out of patch #17 and into a separate
patch.

Reviewed-by: Jeff Layton <jlayton@kernel.org>