Merge patch series "Support foreign mount namespace with statmount/listmount"

Josef Bacik <josef@toxicpanda.com> says:

Currently the only way to iterate over mount entries in mount namespaces that
aren't your own is to trawl through /proc in order to find /proc/$PID/mountinfo
for the mount namespace that you want.  This is hugely inefficient, so extend
both statmount() and listmount() to allow specifying a mount namespace id in
order to get to mounts in other mount namespaces.

There are a few components to this

1. Having a global index of the mount namespace based on the ->seq value in the
   mount namespace.  This gives us a unique identifier that isn't re-used.
2. Support looking up mount namespaces based on that unique identifier, and
   validating the user has permission to access the given mount namespace.
3. Provide a new ioctl() on nsfs in order to extract the unique identifier we
   can use for statmount() and listmount().

The code is relatively straightforward, and there is a selftest provided to
validate everything works properly.

This is based on vfs.all as of last week, so must be applied onto a tree that
has Christians error handling rework in this area.  If you wish you can pull the
tree directly here

https://github.com/josefbacik/linux/tree/listmount.combined

Christian and I collaborated on this series, which is why there's patches from
both of us in this series.

Christian Brauner (4):
  fs: relax permissions for listmount()
  fs: relax permissions for statmount()
  fs: Allow listmount() in foreign mount namespace
  fs: Allow statmount() in foreign mount namespace

Josef Bacik (4):
  fs: keep an index of current mount namespaces
  fs: export the mount ns id via statmount
  fs: add an ioctl to get the mnt ns id from nsfs
  selftests: add a test for the foreign mnt ns extensions

fs/mount.h                                    |   2 +
 fs/namespace.c                                | 240 ++++++++++--
 fs/nsfs.c                                     |  14 +
 include/uapi/linux/mount.h                    |   6 +-
 include/uapi/linux/nsfs.h                     |   2 +
 .../selftests/filesystems/statmount/Makefile  |   2 +-
 .../filesystems/statmount/statmount.h         |  46 +++
 .../filesystems/statmount/statmount_test.c    |  53 +--
 .../filesystems/statmount/statmount_test_ns.c | 360 ++++++++++++++++++
 9 files changed, 659 insertions(+), 66 deletions(-)
 create mode 100644 tools/testing/selftests/filesystems/statmount/statmount.h
 create mode 100644 tools/testing/selftests/filesystems/statmount/statmount_test_ns.c

Link: https://lore.kernel.org/r/cover.1719243756.git.josef@toxicpanda.com
Signed-off-by: Christian Brauner <brauner@kernel.org>