RDS: Make sure cmsgs aren't used in improper ways

It hasn't cropped up in the field, but this code ensures it is
impossible to issue operations that pass an rdma cookie (DEST, MAP)
in the same sendmsg call that's actually initiating rdma or atomic
ops.

Disallowing this perverse-but-technically-allowed usage makes silent
RDMA heuristics slightly easier.

Signed-off-by: Andy Grover <andy.grover@oracle.com>

net/rds/send.c

diff --git a/net/rds/send.c b/net/rds/send.c
index 38567f3..69ab1040 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -846,6 +846,7 @@
 {
 	struct cmsghdr *cmsg;
 	int size = 0;
+	int cmsg_groups = 0;
 	int retval;
 
 	for (cmsg = CMSG_FIRSTHDR(msg); cmsg; cmsg = CMSG_NXTHDR(msg, cmsg)) {
@@ -857,19 +858,23 @@
 
 		switch (cmsg->cmsg_type) {
 		case RDS_CMSG_RDMA_ARGS:
+			cmsg_groups |= 1;
 			retval = rds_rdma_extra_size(CMSG_DATA(cmsg));
 			if (retval < 0)
 				return retval;
 			size += retval;
+
 			break;
 
 		case RDS_CMSG_RDMA_DEST:
 		case RDS_CMSG_RDMA_MAP:
+			cmsg_groups |= 2;
 			/* these are valid but do no add any size */
 			break;
 
 		case RDS_CMSG_ATOMIC_CSWP:
 		case RDS_CMSG_ATOMIC_FADD:
+			cmsg_groups |= 1;
 			size += sizeof(struct scatterlist);
 			break;
 
@@ -881,6 +886,10 @@
 
 	size += ceil(data_len, PAGE_SIZE) * sizeof(struct scatterlist);
 
+	/* Ensure (DEST, MAP) are never used with (ARGS, ATOMIC) */
+	if (cmsg_groups == 3)
+		return -EINVAL;
+
 	return size;
 }