NFS: Handle a zero-length auth flavor list

Some releases of Linux rpc.mountd (nfs-utils 1.1.4 and later) return an
empty auth flavor list if no sec= was specified for the export.  This is
notably broken server behavior.

The new auth flavor list checking added in a recent commit rejects this
case.  The OpenSolaris client does too.

The broken mountd implementation is already widely deployed.  To avoid
a behavioral regression, the kernel's mount client skips flavor checking
(ie reverts to the pre-2.6.32 behavior) if mountd returns an empty
flavor list.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/super.c

@@ -1337,6 +1337,16 @@ static int nfs_walk_authlist(struct nfs_parsed_mount_data *args,
 {
 	unsigned int i, j, server_authlist_len = *(request->auth_flav_len);
 
+	/*
+	 * Certain releases of Linux's mountd return an empty
+	 * flavor list.  To prevent behavioral regression with
+	 * these servers (ie. rejecting mounts that used to
+	 * succeed), revert to pre-2.6.32 behavior (no checking)
+	 * if the returned flavor list is empty.
+	 */
+	if (server_authlist_len == 0)
+		return 0;
+
 	/*
 	 * We avoid sophisticated negotiating here, as there are
 	 * plenty of cases where we can get it wrong, providing