tipc: introduce new socket option TIPC_SOCK_RECVQ_USED

When using TIPC_SOCK_RECVQ_DEPTH for getsockopt(), it returns the
number of buffers in receive socket buffer which is not so helpful
for user space applications.

This commit introduces the new option TIPC_SOCK_RECVQ_USED which
returns the current allocated bytes of the receive socket buffer.
This helps user space applications dimension its buffer usage to
avoid buffer overload issue.
Signed-off-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tipc.h

@@ -190,6 +190,7 @@ struct sockaddr_tipc {
 #define TIPC_MCAST_REPLICAST    134     /* Default: TIPC selects. No arg */
 #define TIPC_GROUP_JOIN         135     /* Takes struct tipc_group_req* */
 #define TIPC_GROUP_LEAVE        136     /* No argument */
+#define TIPC_SOCK_RECVQ_USED    137     /* Default: none (read only) */
 
 /*
  * Flag values

net/tipc/socket.c

@@ -3070,6 +3070,9 @@ static int tipc_getsockopt(struct socket *sock, int lvl, int opt,
 	case TIPC_SOCK_RECVQ_DEPTH:
 		value = skb_queue_len(&sk->sk_receive_queue);
 		break;
+	case TIPC_SOCK_RECVQ_USED:
+		value = sk_rmem_alloc_get(sk);
+		break;
 	case TIPC_GROUP_JOIN:
 		seq.type = 0;
 		if (tsk->group)