[PATCH 2/4] fuse: Fix infinite loop in sget_fc()

[Greg Kurz]

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 should go away. 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@xxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx # v5.10+
Signed-off-by: Greg Kurz <groug@xxxxxxxx>
---
 fs/fuse/dir.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 01559061cbfb..3b0482738741 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -346,6 +346,16 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 		goto out_put_sb;
 	}
 
+	/*
+	 * 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;
+
 	sb->s_flags |= SB_ACTIVE;
 	fsc->root = dget(sb->s_root);
 	/* We are done configuring the superblock, so unlock it */
-- 
2.31.1