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

@@ -846,6 +846,7 @@ static int rds_rm_size(struct msghdr *msg, int data_len)
 {
 	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 @@ static int rds_rm_size(struct msghdr *msg, int data_len)
 
 		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 @@ static int rds_rm_size(struct msghdr *msg, int data_len)
 
 	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;
 }