sunrpc: Change ret code of xdr_stream_decode_opaque_fixed

Since the opaque is fixed in size, the caller already knows how many
bytes were decoded, on success. Thus, xdr_stream_decode_opaque_fixed()
doesn't need to return that value. And, xdr_stream_decode_u32 and _u64
both return zero on success.

This patch simplifies the caller's error checking to avoid potential
integer promotion issues.

Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Sergey Bashirov <sergeybashirov@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: sbashiro

include/linux/sunrpc/xdr.h

@@ -721,7 +721,7 @@ xdr_stream_decode_u64(struct xdr_stream *xdr, __u64 *ptr)
  * @len: size of buffer pointed to by @ptr
  *
  * Return values:
- *   On success, returns size of object stored in @ptr
+ *   %0 on success
  *   %-EBADMSG on XDR buffer overflow
  */
 static inline ssize_t
@@ -732,7 +732,7 @@ xdr_stream_decode_opaque_fixed(struct xdr_stream *xdr, void *ptr, size_t len)
 	if (unlikely(!p))
 		return -EBADMSG;
 	xdr_decode_opaque_fixed(p, ptr, len);
-	return len;
+	return 0;
 }
 
 /**

tools/net/sunrpc/xdrgen/templates/C/typedef/decoder/fixed_length_opaque.j2

@@ -13,5 +13,5 @@ xdrgen_decode_{{ name }}(struct xdr_stream *xdr, {{ classifier }}{{ name }} *ptr
 {% if annotate %}
 	/* (fixed-length opaque) */
 {% endif %}
-	return xdr_stream_decode_opaque_fixed(xdr, ptr, {{ size }}) >= 0;
+	return xdr_stream_decode_opaque_fixed(xdr, ptr, {{ size }}) == 0;
 };