All submounts share the same virtio-fs device instance as the root
mount. If the same virtiofs filesystem is mounted again, sget_fc()
is likely to pick up any of these submounts and reuse it instead of
the root mount.

On the server side:

# mkdir ${some_dir}
# mkdir ${some_dir}/mnt1
# mount -t tmpfs none ${some_dir}/mnt1
# touch ${some_dir}/mnt1/THIS_IS_MNT1
# mkdir ${some_dir}/mnt2
# mount -t tmpfs none ${some_dir}/mnt2
# touch ${some_dir}/mnt2/THIS_IS_MNT2

On the client side:

# mkdir /mnt/virtiofs1
# mount -t virtiofs myfs /mnt/virtiofs1
# ls /mnt/virtiofs1
mnt1 mnt2
# grep virtiofs /proc/mounts
myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)

And now remount it again:

# mount -t virtiofs myfs /mnt/virtiofs2
# grep virtiofs /proc/mounts
myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
myfs /mnt/virtiofs2 virtiofs rw,seclabel,relatime 0 0
# ls /mnt/virtiofs2
THIS_IS_MNT2

Submount mnt2 was picked-up instead of the root mount.

Just skip submounts in virtio_fs_test_super().

Signed-off-by: Greg Kurz <redacted>
---
 fs/fuse/virtio_fs.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index e12e5190352c..8962cd033016 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c

@@ -1408,6 +1408,11 @@ static int virtio_fs_test_super(struct super_block *sb,
 	struct fuse_mount *fsc_fm = fsc->s_fs_info;
 	struct fuse_mount *sb_fm = get_fuse_mount_super(sb);
 
+
+	/* Skip submounts */
+	if (!list_is_first(&sb_fm->fc_entry, &sb_fm->fc->mounts))
+		return 0;
+
 	return fsc_fm->fc->iq.priv == sb_fm->fc->iq.priv;
 }
 

-- 
2.26.3

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