[PATCH 0/7] fsmount: add FSMOUNT_NAMESPACE

[Christian Brauner <brauner@kernel.org>]

Add FSMOUNT_NAMESPACE flag to fsmount() that creates a new mount
namespace with the newly created filesystem attached to a copy of the
real rootfs. This returns a namespace file descriptor instead of an
O_PATH mount fd, similar to how OPEN_TREE_NAMESPACE works for
open_tree().

This allows creating a new filesystem and immediately placing it in a
new mount namespace in a single operation, which is useful for container
runtimes and other namespace-based isolation mechanisms.

This accompanies OPEN_TREE_NAMESPACE and avoids a needless detour via
OPEN_TREE_NAMESPACE to get the same effect. Will be especially useful
when you mount an actual filesystem to be used as the container rootfs.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Christian Brauner (7):
      mount: start iterating from start of rbtree
      mount: simplify __do_loopback()
      mount: add FSMOUNT_NAMESPACE
      tools: update mount.h header
      selftests/statmount: add statmount_alloc() helper
      selftests: add FSMOUNT_NAMESPACE tests
      selftests/open_tree_ns: fix compilation

 fs/namespace.c                                     |   84 +-
 include/uapi/linux/mount.h                         |    1 +
 tools/include/uapi/linux/mount.h                   |   14 +-
 .../selftests/filesystems/fsmount_ns/.gitignore    |    1 +
 .../selftests/filesystems/fsmount_ns/Makefile      |   10 +
 .../filesystems/fsmount_ns/fsmount_ns_test.c       | 1138 ++++++++++++++++++++
 .../selftests/filesystems/open_tree_ns/Makefile    |    2 +-
 .../filesystems/open_tree_ns/open_tree_ns_test.c   |   33 +-
 .../selftests/filesystems/statmount/statmount.h    |   27 +
 9 files changed, 1242 insertions(+), 68 deletions(-)
---
base-commit: 1bce1a664ac25d37a327c433a01bc347f0a81bd6
change-id: 20260121-work-fsmount-namespace-4242e3df359e