SUNRPC: Add helpers for decoding list discriminators symbolically

Use these helpers in a few spots to demonstrate their use.

The remaining open-coded discriminator checks in rpcrdma will be
addressed in subsequent patches.
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/xdr.h

@@ -474,6 +474,32 @@ xdr_stream_encode_uint32_array(struct xdr_stream *xdr,
 	return ret;
 }
 
+/**
+ * xdr_item_is_absent - symbolically handle XDR discriminators
+ * @p: pointer to undecoded discriminator
+ *
+ * Return values:
+ *   %true if the following XDR item is absent
+ *   %false if the following XDR item is present
+ */
+static inline bool xdr_item_is_absent(const __be32 *p)
+{
+	return *p == xdr_zero;
+}
+
+/**
+ * xdr_item_is_present - symbolically handle XDR discriminators
+ * @p: pointer to undecoded discriminator
+ *
+ * Return values:
+ *   %true if the following XDR item is present
+ *   %false if the following XDR item is absent
+ */
+static inline bool xdr_item_is_present(const __be32 *p)
+{
+	return *p != xdr_zero;
+}
+
 /**
  * xdr_stream_decode_u32 - Decode a 32-bit integer
  * @xdr: pointer to xdr_stream

net/sunrpc/xprtrdma/rpc_rdma.c

@@ -1133,11 +1133,11 @@ rpcrdma_is_bcall(struct rpcrdma_xprt *r_xprt, struct rpcrdma_rep *rep)
 	p = xdr_inline_decode(xdr, 0);
 
 	/* Chunk lists */
-	if (*p++ != xdr_zero)
+	if (xdr_item_is_present(p++))
 		return false;
-	if (*p++ != xdr_zero)
+	if (xdr_item_is_present(p++))
 		return false;
-	if (*p++ != xdr_zero)
+	if (xdr_item_is_present(p++))
 		return false;
 
 	/* RPC header */
@@ -1215,7 +1215,7 @@ static int decode_read_list(struct xdr_stream *xdr)
 	p = xdr_inline_decode(xdr, sizeof(*p));
 	if (unlikely(!p))
 		return -EIO;
-	if (unlikely(*p != xdr_zero))
+	if (unlikely(xdr_item_is_present(p)))
 		return -EIO;
 	return 0;
 }
@@ -1234,7 +1234,7 @@ static int decode_write_list(struct xdr_stream *xdr, u32 *length)
 		p = xdr_inline_decode(xdr, sizeof(*p));
 		if (unlikely(!p))
 			return -EIO;
-		if (*p == xdr_zero)
+		if (xdr_item_is_absent(p))
 			break;
 		if (!first)
 			return -EIO;
@@ -1256,7 +1256,7 @@ static int decode_reply_chunk(struct xdr_stream *xdr, u32 *length)
 		return -EIO;
 
 	*length = 0;
-	if (*p != xdr_zero)
+	if (xdr_item_is_present(p))
 		if (decode_write_chunk(xdr, length))
 			return -EIO;
 	return 0;

net/sunrpc/xprtrdma/svc_rdma_recvfrom.c

@@ -419,7 +419,7 @@ static bool xdr_check_read_list(struct svc_rdma_recv_ctxt *rctxt)
 
 	len = 0;
 	first = true;
-	while (*p != xdr_zero) {
+	while (xdr_item_is_present(p)) {
 		p = xdr_inline_decode(&rctxt->rc_stream,
 				      rpcrdma_readseg_maxsz * sizeof(*p));
 		if (!p)
@@ -500,7 +500,7 @@ static bool xdr_check_write_list(struct svc_rdma_recv_ctxt *rctxt)
 	if (!p)
 		return false;
 	rctxt->rc_write_list = p;
-	while (*p != xdr_zero) {
+	while (xdr_item_is_present(p)) {
 		if (!xdr_check_write_chunk(rctxt, MAX_BYTES_WRITE_CHUNK))
 			return false;
 		++chcount;
@@ -532,12 +532,11 @@ static bool xdr_check_reply_chunk(struct svc_rdma_recv_ctxt *rctxt)
 	p = xdr_inline_decode(&rctxt->rc_stream, sizeof(*p));
 	if (!p)
 		return false;
-	rctxt->rc_reply_chunk = p;
-	if (*p != xdr_zero) {
+	rctxt->rc_reply_chunk = NULL;
+	if (xdr_item_is_present(p)) {
 		if (!xdr_check_write_chunk(rctxt, MAX_BYTES_SPECIAL_CHUNK))
 			return false;
-	} else {
-		rctxt->rc_reply_chunk = NULL;
+		rctxt->rc_reply_chunk = p;
 	}
 	return true;
 }
@@ -568,7 +567,7 @@ static void svc_rdma_get_inv_rkey(struct svcxprt_rdma *rdma,
 	p += rpcrdma_fixed_maxsz;
 
 	/* Read list */
-	while (*p++ != xdr_zero) {
+	while (xdr_item_is_present(p++)) {
 		p++;	/* position */
 		if (inv_rkey == xdr_zero)
 			inv_rkey = *p;
@@ -578,7 +577,7 @@ static void svc_rdma_get_inv_rkey(struct svcxprt_rdma *rdma,
 	}
 
 	/* Write list */
-	while (*p++ != xdr_zero) {
+	while (xdr_item_is_present(p++)) {
 		segcount = be32_to_cpup(p++);
 		for (i = 0; i < segcount; i++) {
 			if (inv_rkey == xdr_zero)
@@ -590,7 +589,7 @@ static void svc_rdma_get_inv_rkey(struct svcxprt_rdma *rdma,
 	}
 
 	/* Reply chunk */
-	if (*p++ != xdr_zero) {
+	if (xdr_item_is_present(p++)) {
 		segcount = be32_to_cpup(p++);
 		for (i = 0; i < segcount; i++) {
 			if (inv_rkey == xdr_zero)