We don't set the SB_BORN flag on submounts. This is wrong as these superblocks are then considered as partially constructed or dying in the rest of the code and can break some assumptions.

One such case is when you have a virtiofs filesystem with submounts and you try to mount it again : virtio_fs_get_tree() tries to obtain a superblock with sget_fc(). The logic in sget_fc() is to loop until it has either found an existing matching superblock with SB_BORN set or to create a brand new one. It is assumed that a superblock without SB_BORN is transient and the loop is restarted. Forgetting to set SB_BORN on submounts hence causes sget_fc() to retry forever.

Setting SB_BORN requires special care, i.e. a write barrier for super_cache_count() which can check SB_BORN without taking any lock. We should call vfs_get_tree() to deal with that but this requires to have a proper ->get_tree() implementation for submounts, which is a bigger piece of work. Go for a simple bug fix in the meatime.

Fixes: bf109c64040f ("fuse: implement crossmounts") Cc: mreitz@redhat.com Cc: stable@vger.kernel.org # v5.10+ Signed-off-by: Greg Kurz groug@kaod.org --- fs/fuse/dir.c | 11 +++++++++++ 1 file changed, 11 insertions(+)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c index 3fd1b71e546b..3fa8604c21d5 100644 --- a/fs/fuse/dir.c +++ b/fs/fuse/dir.c @@ -352,6 +352,17 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)

sb->s_flags |= SB_ACTIVE; fsc->root = dget(sb->s_root); + + /* + * FIXME: setting SB_BORN requires a write barrier for + * super_cache_count(). We should actually come + * up with a proper ->get_tree() implementation + * for submounts and call vfs_get_tree() to take + * care of the write barrier. + */ + smp_wmb(); + sb->s_flags |= SB_BORN; + /* We are done configuring the superblock, so unlock it */ up_write(&sb->s_umount);