From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BDAF6307AF8; Mon, 27 Oct 2025 14:10:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761574218; cv=none; b=VBtRNiowW+chGpAOXCuJ0QIFwK2BGFvo6847ZAYbaS/E7ZWQwA0kq/fQQt0iOUd05uOjiz1n6Ecbod/3nbf7XMbZZ2ghXQ8GuzzSQ+x1XRPbOHuGleJLlueUzhI0FJtQB+YqG1skYMLiUEDkEMPDdFKk4GDHRIfMoWbDvkwfv3U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761574218; c=relaxed/simple; bh=CjdQ/5cQ/4q2pAY7bUWuObYvjYAlf55jd7CNlNkzwQw=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=MTAxgOyBw2mvzWufdn/cm335KIJnsnubUKbxRINUkotjfhsW/3B4Ok/Wm3VuBzVjT7Diip0Ls0PCZ6rzYla1thVI/ApUXbPvJvmRzKr4mivkgBdSnGYG0UgsNEuCSNwPPmW/ds8dzDDtDXI1DkzxA2mjH09KI4+14ZoJT8RH1cc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=l15j1k7j; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="l15j1k7j" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A0780C4CEF1; Mon, 27 Oct 2025 14:10:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1761574218; bh=CjdQ/5cQ/4q2pAY7bUWuObYvjYAlf55jd7CNlNkzwQw=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=l15j1k7jCLo+s4hGnhGp7A8XUkqktLxNJMJx4fKmNpwSuLkcy5KLodor40FA6amNU 6Ew4Vu0n9RvqygmH70oDQLndGJF1wSU2ju5+qLPI2Fvb4SKxlu3iTkpmai5tb6erJK 3BuYMemNuMFrspEojp79juujN1xOUPxY1TCTobix/2gbISH9IcJIFgyJtgenWa2wlg qvoWSz4gl7gVXVIrak/QiKFh9FTb/C/hKRb2v8QGARb2d6iV9voRSVn/NNwv3lLRu0 z/w5ZzzSC1cWXXF2lcSfp5McsItx7vBeUq4DbBgCdVOIGG+cY+dFAfQssjugTg5aqf EwCwHVuWMvX7A== Message-ID: <5ab02765a247dbaebc7d1224ee20a3bc01adc330.camel@kernel.org> Subject: Re: [PATCH v3 00/70] nstree: listns() From: Jeff Layton To: Christian Brauner , linux-fsdevel@vger.kernel.org, Josef Bacik Cc: Jann Horn , Mike Yuan , Zbigniew =?UTF-8?Q?J=C4=99drzejewski-Szmek?= , Lennart Poettering , Daan De Meyer , Aleksa Sarai , Amir Goldstein , Tejun Heo , Johannes Weiner , Thomas Gleixner , Alexander Viro , Jan Kara , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, bpf@vger.kernel.org, Eric Dumazet , Jakub Kicinski , netdev@vger.kernel.org, Arnd Bergmann Date: Mon, 27 Oct 2025 10:10:15 -0400 In-Reply-To: <20251024-work-namespace-nstree-listns-v3-0-b6241981b72b@kernel.org> References: <20251024-work-namespace-nstree-listns-v3-0-b6241981b72b@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; 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text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2 (3.56.2-2.fc42) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2025-10-24 at 12:52 +0200, Christian Brauner wrote: > Hey, >=20 > 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. >=20 > Currently listns() is relying on active namespace reference counts which > are introduced alongside this series. >=20 > 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. >=20 > On current kernels a namespace is visible to userspace in the > following cases: >=20 > (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. >=20 > 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(). >=20 > 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 > So calls to open_related_ns() and open_namespace() will end up > resurrecting the corresponding namespace tree. >=20 > 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. >=20 > 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()). >=20 > 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. >=20 > 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//ns/. >=20 > 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. >=20 > The active reference counts of the user- and pid namespace are > decremented once the task is reaped. >=20 > 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. >=20 > Currently, there is no direct way for userspace to enumerate namespaces > in the system. Applications must resort to scanning /proc//ns/ > across all processes, which is: >=20 > 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. >=20 > 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. >=20 > /* > * @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); >=20 > Returns: > - On success: Number of namespace IDs written to ns_ids > - On error: Negative error code >=20 > /* > * @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 na= mespace > * Other value: Namespaces owned by the specified user names= pace 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 */ > }; >=20 > Example 1: List all namespaces >=20 > void list_all_namespaces(void) > { > struct ns_id_req req =3D { > .size =3D sizeof(req), > .ns_id =3D 0, /* Start from beginning */ > .ns_type =3D 0, /* All types */ > .user_ns_id =3D 0, /* All user namespaces */ > }; > uint64_t ids[100]; > ssize_t ret; >=20 > printf("All namespaces in the system:\n"); > do { > ret =3D listns(&req, ids, 100, 0); > if (ret < 0) { > perror("listns"); > break; > } >=20 > for (ssize_t i =3D 0; i < ret; i++) > printf(" Namespace ID: %llu\n", (unsigned long long)ids[i]); >=20 > /* Continue from last seen ID */ > if (ret > 0) > req.ns_id =3D ids[ret - 1]; > } while (ret =3D=3D 100); /* Buffer was full, more may exist */ > } >=20 > Example 2 : List network namespaces only >=20 > void list_network_namespaces(void) > { > struct ns_id_req req =3D { > .size =3D sizeof(req), > .ns_id =3D 0, > .ns_type =3D NET_NS, /* Only network namespaces */ > .user_ns_id =3D 0, > }; > uint64_t ids[100]; > ssize_t ret; >=20 > ret =3D listns(&req, ids, 100, 0); > if (ret < 0) { > perror("listns"); > return; > } >=20 > printf("Network namespaces: %zd found\n", ret); > for (ssize_t i =3D 0; i < ret; i++) > printf(" netns ID: %llu\n", (unsigned long long)ids[i]); > } >=20 > Example 3 : List namespaces owned by current user namespace >=20 > void list_owned_namespaces(void) > { > struct ns_id_req req =3D { > .size =3D sizeof(req), > .ns_id =3D 0, > .ns_type =3D 0, /* All types */ > .user_ns_id =3D LISTNS_CURRENT_USER, /* Current userns */ > }; > uint64_t ids[100]; > ssize_t ret; >=20 > ret =3D listns(&req, ids, 100, 0); > if (ret < 0) { > perror("listns"); > return; > } >=20 > printf("Namespaces owned by my user namespace: %zd\n", ret); > for (ssize_t i =3D 0; i < ret; i++) > printf(" ns ID: %llu\n", (unsigned long long)ids[i]); > } >=20 > Example 4 : List multiple namespace types >=20 > void list_network_and_mount_namespaces(void) > { > struct ns_id_req req =3D { > .size =3D sizeof(req), > .ns_id =3D 0, > .ns_type =3D NET_NS | MNT_NS, /* Network and mount */ > .user_ns_id =3D 0, > }; > uint64_t ids[100]; > ssize_t ret; >=20 > ret =3D listns(&req, ids, 100, 0); > printf("Network and mount namespaces: %zd found\n", ret); > } >=20 > Example 5 : Pagination through large namespace sets >=20 > void list_all_with_pagination(void) > { > struct ns_id_req req =3D { > .size =3D sizeof(req), > .ns_id =3D 0, > .ns_type =3D 0, > .user_ns_id =3D 0, > }; > uint64_t ids[50]; > size_t total =3D 0; > ssize_t ret; >=20 > printf("Enumerating all namespaces with pagination:\n"); >=20 > while (1) { > ret =3D listns(&req, ids, 50, 0); > if (ret < 0) { > perror("listns"); > break; > } > if (ret =3D=3D 0) > break; /* No more namespaces */ >=20 > total +=3D ret; > printf(" Batch: %zd namespaces\n", ret); >=20 > /* Last ID in this batch becomes start of next batch */ > req.ns_id =3D ids[ret - 1]; >=20 > if (ret < 50) > break; /* Partial batch =3D end of results */ > } >=20 > printf("Total: %zu namespaces\n", total); > } >=20 > listns() respects namespace isolation and capabilities: >=20 > (1) Global listing (user_ns_id =3D 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 !=3D 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() >=20 > Signed-off-by: Christian Brauner > --- > 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-lis= tns-v2-0-71a588572371@kernel.org >=20 > Changes in v2: > - Fully implement the active reference count. > - Fix various minor issues. > - Expand the testsuite to test complex resurrection scenarios due to SIOC= GSKNS. > - 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-lis= tns-v1-0-ad44261a8a5b@kernel.org >=20 > --- > 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() he= lper > 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 >=20 > 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