net: Add MSG_WAITFORONE flag to recvmmsg

Add new flag MSG_WAITFORONE for the recvmmsg() syscall.
When this flag is specified for a blocking socket, recvmmsg()
will only block until at least 1 packet is available.  The
default behavior is to block until all vlen packets are
available.  This flag has no effect on non-blocking sockets
or when used in combination with MSG_DONTWAIT.

Signed-off-by: Brandon L Black <blblack@gmail.com>
Acked-by: Ulrich Drepper <drepper@redhat.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/socket.h

@@ -255,6 +255,7 @@ struct ucred {
 #define MSG_ERRQUEUE	0x2000	/* Fetch message from error queue */
 #define MSG_NOSIGNAL	0x4000	/* Do not generate SIGPIPE */
 #define MSG_MORE	0x8000	/* Sender will send more */
+#define MSG_WAITFORONE	0x10000	/* recvmmsg(): block until 1+ packets avail */
 
 #define MSG_EOF         MSG_FIN
 

net/socket.c

@@ -2135,6 +2135,10 @@ int __sys_recvmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
 			break;
 		++datagrams;
 
+		/* MSG_WAITFORONE turns on MSG_DONTWAIT after one packet */
+		if (flags & MSG_WAITFORONE)
+			flags |= MSG_DONTWAIT;
+
 		if (timeout) {
 			ktime_get_ts(timeout);
 			*timeout = timespec_sub(end_time, *timeout);