On Sun, Mar 28, 2021 at 6:56 PM Amir Goldstein [off-list ref] wrote:

Add a high level hook fsnotify_path_create() which is called from
syscall context where mount context is available, so that FAN_CREATE
event can be added to a mount mark mask.

This high level hook is called in addition to fsnotify_create(),
fsnotify_mkdir() and fsnotify_link() hooks in vfs helpers where the mount
context is not available.

In the context where fsnotify_path_create() will be called, a dentry flag
flag is set on the new dentry the suppress the FS_CREATE event in the vfs
level hooks.

This functionality was requested by Christian Brauner to replace
recursive inotify watches for detecting when some path was created under
an idmapped mount without having to monitor FAN_CREATE events in the
entire filesystem.

In combination with more changes to allow unprivileged fanotify listener
to watch an idmapped mount, this functionality would be usable also by
nested container managers.

Link: https://lore.kernel.org/linux-fsdevel/20210318143140.jxycfn3fpqntq34z@wittgenstein/ (local)
Cc: Christian Brauner <redacted>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Jan,

After trying several different approaches, I finally realized that
making FAN_CREATE available for mount marks is not that hard and it could
be very useful IMO.

Adding support for other "inode events" with mount mark, such as
FAN_ATTRIB, FAN_DELETE, FAN_MOVE may also be possible, but adding support
for FAN_CREATE was really easy due to the fact that all call sites are
already surrounded by filename_creat()/done_path_create() calls.

FWIW, adding support for FAN_DELETE and FAN_MOVE_SELF was not
so hard. The move event at least will also be needed for the use case
where watching when a negative path is instantiated.

https://github.com/amir73il/linux/commits/fsnotify_path_hooks

Thanks,
Amir.