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Subject[PATCH] Add MSG_WAITFORONE flag to recvmmsg

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);

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