[PATCH] Add MSG_WAITFORONE flag to recvmmsg

[Brandon L Black <blblack@gmail.com>]

From: Brandon L Black <blblack@gmail.com>

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>

---
diff --git a/include/linux/socket.h b/include/linux/socket.h
index 7b3aae2..354cc56 100644
--- a/include/linux/socket.h
+++ b/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
 
diff --git a/net/socket.c b/net/socket.c
index 769c386..33304d1 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2133,7 +2133,10 @@ int __sys_recvmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
 
 		if (err)
 			break;
-		++datagrams;
+
+		/* MSG_WAITFORONE turns on MSG_DONTWAIT after one packet */
+		if (!datagrams++ && (flags & MSG_WAITFORONE))
+			flags |= MSG_DONTWAIT;
 
 		if (timeout) {
 			ktime_get_ts(timeout);