[PATCH] fs/namespace: notify pollers of legacy propagation changes

[Guopeng Zhang <guopeng.zhang@linux.dev>]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

Changing mount propagation through the legacy mount API changes
user-visible mountinfo contents, including the shared: and master:
optional fields.

The mount_setattr() path already touches the mount namespace after
change_mnt_propagation(), so pollers of /proc/<pid>/mountinfo are woken
when the namespace event changes.

The legacy mount --make-* path also changes propagation through
change_mnt_propagation(), and MOVE_MOUNT_SET_GROUP updates the
propagation relationship of the target mount. Both paths currently
return without touching the affected mount namespace.

As a result, userspace polling /proc/<pid>/mountinfo can miss these
propagation-only changes even though mountinfo has changed.

A simple reproducer that polls /proc/self/mountinfo while changing
propagation shows the inconsistency.

Before this change:

  legacy MS_SHARED: poll ret=0 revents=0x0
  mount_setattr MS_SHARED: poll ret=1 revents=0xa

After this change:

  legacy MS_SHARED: poll ret=1 revents=0xa
  mount_setattr MS_SHARED: poll ret=1 revents=0xa

Touch the affected mount namespace after successfully changing
propagation state in do_change_type() and do_set_group(). Take the
vfsmount lock for write around touch_mnt_namespace(), as required by
its locking rules.

Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 fs/namespace.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 9a66a806a9b8..f871c7bf3bc8 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2908,6 +2908,10 @@ static int do_change_type(const struct path *path, int ms_flags)
 	for (m = mnt; m; m = (recurse ? next_mnt(m, mnt) : NULL))
 		change_mnt_propagation(m, type);
 
+	lock_mount_hash();
+	touch_mnt_namespace(mnt->mnt_ns);
+	unlock_mount_hash();
+
 	return 0;
 }
 
@@ -3479,6 +3483,11 @@ static int do_set_group(const struct path *from_path, const struct path *to_path
 		list_add(&to->mnt_share, &from->mnt_share);
 		set_mnt_shared(to);
 	}
+
+	lock_mount_hash();
+	touch_mnt_namespace(to->mnt_ns);
+	unlock_mount_hash();
+
 	return 0;
 }
 
-- 
2.43.0