ext4: don't fail GETFSUUID when the caller provides a long buffer

If userspace provides a longer UUID buffer than is required, we
shouldn't fail the call with EINVAL -- rather, we can fill the caller's
buffer with the bytes we /can/ fill, and update the length field to
reflect what we copied.  This doesn't break the UAPI since we're
enabling a case that currently fails, and so far Ted hasn't released a
version of e2fsprogs that uses the new ext4 ioctl.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Catherine Hoang <catherine.hoang@oracle.com>
Link: https://lore.kernel.org/r/166811139478.327006.13879198441587445544.stgit@magnoliaSigned-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/ioctl.c

@@ -1160,14 +1160,16 @@ static int ext4_ioctl_getuuid(struct ext4_sb_info *sbi,
 		return 0;
 	}
 
-	if (fsuuid.fsu_len != UUID_SIZE || fsuuid.fsu_flags != 0)
+	if (fsuuid.fsu_len < UUID_SIZE || fsuuid.fsu_flags != 0)
 		return -EINVAL;
 
 	lock_buffer(sbi->s_sbh);
 	memcpy(uuid, sbi->s_es->s_uuid, UUID_SIZE);
 	unlock_buffer(sbi->s_sbh);
 
-	if (copy_to_user(&ufsuuid->fsu_uuid[0], uuid, UUID_SIZE))
+	fsuuid.fsu_len = UUID_SIZE;
+	if (copy_to_user(ufsuuid, &fsuuid, sizeof(fsuuid)) ||
+	    copy_to_user(&ufsuuid->fsu_uuid[0], uuid, UUID_SIZE))
 		return -EFAULT;
 	return 0;
 }