From: Greg Kurz groug@kaod.org
commit e4a9ccdd1c03b3dc58214874399d24331ea0a3ab upstream.
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: stable@vger.kernel.org # v5.10+ Signed-off-by: Greg Kurz groug@kaod.org Reviewed-by: Max Reitz mreitz@redhat.com Signed-off-by: Miklos Szeredi mszeredi@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/fuse/dir.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
--- a/fs/fuse/dir.c +++ b/fs/fuse/dir.c @@ -352,6 +352,17 @@ static struct vfsmount *fuse_dentry_auto
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);