[PATCH 0/8] mount: a few gnarly fixes

[Christian Brauner <brauner@kernel.org>]

Hey,

A short while ago I received a bug report about problematic behavior in
the mount code and that let me to review and model a few corner cases
including reference counting. The result is the following set of fixes:

- Ensure that copies of unbindable mounts remain unbindable
- Ensure that MOVE_MOUNT_SET_GROUP doesn't operate on unbindable mounts
  so we don't create an undefined state where a mount is both unbindable
  and a slave mount.
- Ensure that we don't silently unmount busy mounts. This is nasty
  because it's an old bug from v4.13 where calculation of whether a
  synchronous (non-MNT_DETACH) umount would succeed drifted apart from
  the actual codepath that did the unmount.
- Ensure that we don't silently unmount busy mounts. Same class,
  different mechanism. There's a race between the reference count check
  in propagate_mount_busy() (summing up per-cpu counters) and mntget().

Fixes and test for all. No test for the mntget() race because that needs
aritifical delays.

And fwiw, I think I another vomitorious series in the pipes.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
Christian Brauner (8):
      mount: keep a copied mount unbindable
      selftests/filesystems: check that a copied mount namespace keeps unbindable
      mount: refuse MOVE_MOUNT_SET_GROUP on an unbindable mount
      selftests/move_mount_set_group: check that an unbindable target is refused
      fs: don't silently unmount busy mounts
      selftests/filesystems: check that a busy propagated copy blocks a synchronous umount
      fs: don't let a migrating task hide its reference from do_umount()
      docs: update the unmount propagation rule

 Documentation/filesystems/sharedsubtree.rst        |  20 +-
 fs/mount.h                                         |   3 +-
 fs/namespace.c                                     |  51 +++--
 fs/pnode.c                                         | 108 ++++++++--
 tools/testing/selftests/Makefile                   |   2 +
 .../filesystems/mntns_unbindable/Makefile          |   6 +
 .../mntns_unbindable/mntns_unbindable_test.c       | 227 +++++++++++++++++++++
 .../filesystems/umount_propagation/Makefile        |   6 +
 .../umount_propagation/umount_propagation_test.c   | 226 ++++++++++++++++++++
 .../move_mount_set_group_test.c                    |  74 ++++++-
 10 files changed, 673 insertions(+), 50 deletions(-)
---
base-commit: 2d2a2d7aa98741b58f54cacc99b52024e4d865f9
change-id: 20260923-work-mount-fixes-c9f19fb484eb