[PATCH v2 00/33] ns: support file handles

[Christian Brauner <brauner@kernel.org>]

For a while now we have supported file handles for pidfds. This has
proven to be very useful.

Extend the concept to cover namespaces as well. After this patchset it
is possible to encode and decode namespace file handles using the
commong name_to_handle_at() and open_by_handle_at() apis.

Namespaces file descriptors can already be derived from pidfds which
means they aren't subject to overmount protection bugs. IOW, it's
irrelevant if the caller would not have access to an appropriate
/proc/<pid>/ns/ directory as they could always just derive the namespace
based on a pidfd already.

It has the same advantage as pidfds. It's possible to reliably and for
the lifetime of the system refer to a namespace without pinning any
resources and to compare them.

Permission checking is kept simple. If the caller is located in the
namespace the file handle refers to they are able to open it otherwise
they must hold privilege over the owning namespace of the relevant
namespace.

Both the network namespace and the mount namespace already have an
associated cookie that isn't recycled and is fully exposed to userspace.
Move this into ns_common and use the same id space for all namespaces so
they can trivially and reliably be compared.

There's more coming based on the iterator infrastructure but the series
is large enough and focuses on file handles.

Extensive selftests included.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Changes in v2:
- Address various review comments.
- Use a common NS_GET_ID ioctl() instead of individual ioctls.
- Link to v1: https://lore.kernel.org/20250910-work-namespace-v1-0-4dd56e7359d8@kernel.org

---
Christian Brauner (33):
      pidfs: validate extensible ioctls
      nsfs: drop tautological ioctl() check
      nsfs: validate extensible ioctls
      block: use extensible_ioctl_valid()
      ns: move to_ns_common() to ns_common.h
      nsfs: add nsfs.h header
      ns: uniformly initialize ns_common
      cgroup: use ns_common_init()
      ipc: use ns_common_init()
      mnt: use ns_common_init()
      net: use ns_common_init()
      pid: use ns_common_init()
      time: use ns_common_init()
      user: use ns_common_init()
      uts: use ns_common_init()
      ns: remove ns_alloc_inum()
      nstree: make iterator generic
      mnt: support ns lookup
      cgroup: support ns lookup
      ipc: support ns lookup
      net: support ns lookup
      pid: support ns lookup
      time: support ns lookup
      user: support ns lookup
      uts: support ns lookup
      ns: add to_<type>_ns() to respective headers
      nsfs: add current_in_namespace()
      nsfs: support file handles
      nsfs: support exhaustive file handles
      nsfs: add missing id retrieval support
      tools: update nsfs.h uapi header
      selftests/namespaces: add identifier selftests
      selftests/namespaces: add file handle selftests

 block/blk-integrity.c                              |    8 +-
 fs/fhandle.c                                       |    6 +
 fs/internal.h                                      |    1 +
 fs/mount.h                                         |   10 +-
 fs/namespace.c                                     |  156 +--
 fs/nsfs.c                                          |  201 ++-
 fs/pidfs.c                                         |    2 +-
 include/linux/cgroup.h                             |    5 +
 include/linux/exportfs.h                           |    6 +
 include/linux/fs.h                                 |   14 +
 include/linux/ipc_namespace.h                      |    5 +
 include/linux/ns_common.h                          |   29 +
 include/linux/nsfs.h                               |   40 +
 include/linux/nsproxy.h                            |   11 -
 include/linux/nstree.h                             |   89 ++
 include/linux/pid_namespace.h                      |    5 +
 include/linux/proc_ns.h                            |   32 +-
 include/linux/time_namespace.h                     |    9 +
 include/linux/user_namespace.h                     |    5 +
 include/linux/utsname.h                            |    5 +
 include/net/net_namespace.h                        |    6 +
 include/uapi/linux/fcntl.h                         |    1 +
 include/uapi/linux/nsfs.h                          |   15 +-
 init/main.c                                        |    2 +
 ipc/msgutil.c                                      |    1 +
 ipc/namespace.c                                    |   12 +-
 ipc/shm.c                                          |    2 +
 kernel/Makefile                                    |    2 +-
 kernel/cgroup/cgroup.c                             |    2 +
 kernel/cgroup/namespace.c                          |   24 +-
 kernel/nstree.c                                    |  233 ++++
 kernel/pid_namespace.c                             |   13 +-
 kernel/time/namespace.c                            |   23 +-
 kernel/user_namespace.c                            |   17 +-
 kernel/utsname.c                                   |   28 +-
 net/core/net_namespace.c                           |   59 +-
 tools/include/uapi/linux/nsfs.h                    |   17 +-
 tools/testing/selftests/namespaces/.gitignore      |    2 +
 tools/testing/selftests/namespaces/Makefile        |    7 +
 tools/testing/selftests/namespaces/config          |    7 +
 .../selftests/namespaces/file_handle_test.c        | 1429 ++++++++++++++++++++
 tools/testing/selftests/namespaces/nsid_test.c     |  986 ++++++++++++++
 42 files changed, 3257 insertions(+), 270 deletions(-)
---
base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
change-id: 20250905-work-namespace-c68826dda0d4