SUNRPC: Add XDR encoding helper for opaque_auth

RFC 5531 defines an MSG_ACCEPTED Reply message like this:

	struct accepted_reply {
		opaque_auth verf;
		union switch (accept_stat stat) {
		case SUCCESS:
		   ...

In the current server code, struct opaque_auth encoding is open-
coded. Introduce a helper that encodes an opaque_auth data item
within the context of a xdr_stream.

Done as part of hardening the server-side RPC header decoding and
encoding paths.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/xdr.h

@@ -348,6 +348,8 @@ ssize_t xdr_stream_decode_string_dup(struct xdr_stream *xdr, char **str,
 		size_t maxlen, gfp_t gfp_flags);
 ssize_t xdr_stream_decode_opaque_auth(struct xdr_stream *xdr, u32 *flavor,
 		void **body, unsigned int *body_len);
+ssize_t xdr_stream_encode_opaque_auth(struct xdr_stream *xdr, u32 flavor,
+		void *body, unsigned int body_len);
 
 /**
  * xdr_align_size - Calculate padded size of an object

net/sunrpc/xdr.c

@@ -2310,3 +2310,32 @@ ssize_t xdr_stream_decode_opaque_auth(struct xdr_stream *xdr, u32 *flavor,
 	return len + ret;
 }
 EXPORT_SYMBOL_GPL(xdr_stream_decode_opaque_auth);
+
+/**
+ * xdr_stream_encode_opaque_auth - Encode struct opaque_auth (RFC5531 S8.2)
+ * @xdr: pointer to xdr_stream
+ * @flavor: verifier flavor to encode
+ * @body: content of body to encode
+ * @body_len: length of body to encode
+ *
+ * Return values:
+ *   On success, returns length in bytes of XDR buffer consumed
+ *   %-EBADMSG on XDR buffer overflow
+ *   %-EMSGSIZE if the size of @body exceeds 400 octets
+ */
+ssize_t xdr_stream_encode_opaque_auth(struct xdr_stream *xdr, u32 flavor,
+				      void *body, unsigned int body_len)
+{
+	ssize_t ret, len;
+
+	if (unlikely(body_len > RPC_MAX_AUTH_SIZE))
+		return -EMSGSIZE;
+	len = xdr_stream_encode_u32(xdr, flavor);
+	if (unlikely(len < 0))
+		return len;
+	ret = xdr_stream_encode_opaque(xdr, body, body_len);
+	if (unlikely(ret < 0))
+		return ret;
+	return len + ret;
+}
+EXPORT_SYMBOL_GPL(xdr_stream_encode_opaque_auth);