Nginx: hijack recvmsg.

Since Nginx calls `recvmsg` in stream with udp, we must
hijack this function, so that the network IO can pass through
f-stack.

app/nginx-1.11.10/src/event/modules/ngx_ff_module.c

@@ -109,6 +109,7 @@ static ssize_t (*real_send)(int, const void *, size_t, int);
 static ssize_t (*real_sendto)(int, const void *, size_t, int,
     const struct sockaddr*, socklen_t);
 static ssize_t (*real_sendmsg)(int, const struct msghdr*, int);
+static ssize_t (*real_recvmsg)(int, struct msghdr *, int);
 static ssize_t (*real_writev)(int, const struct iovec *, int);
 static ssize_t (*real_readv)(int, const struct iovec *, int);
 
@@ -292,6 +293,16 @@ sendmsg(int sockfd, const struct msghdr *msg, int flags)
     return SYSCALL(sendmsg)(sockfd, msg, flags);
 }
 
+ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags)
+{
+    if(is_fstack_fd(sockfd)){
+        sockfd = restore_fstack_fd(sockfd);
+        return ff_recvmsg(sockfd, msg, flags);
+    }
+
+    return SYSCALL(recvmsg)(sockfd, msg, flags);
+}
+
 ssize_t
 recv(int sockfd, void *buf, size_t len, int flags)
 {