udp: Use flow key information instead of rt->rt_{src,dst}

We have two cases.

Either the socket is in TCP_ESTABLISHED state and connect() filled
in the inet socket cork flow, or we looked up the route here and
used an on-stack flow.

Track which one it was, and use it to obtain src/dst addrs.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -791,6 +791,7 @@ int udp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 {
 	struct inet_sock *inet = inet_sk(sk);
 	struct udp_sock *up = udp_sk(sk);
+	struct flowi4 fl4_stack;
 	struct flowi4 *fl4;
 	int ulen = len;
 	struct ipcm_cookie ipc;
@@ -919,17 +920,18 @@ int udp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 	if (connected)
 		rt = (struct rtable *)sk_dst_check(sk, 0);
 
+	fl4 = &inet->cork.fl.u.ip4;
 	if (rt == NULL) {
-		struct flowi4 fl4;
 		struct net *net = sock_net(sk);
 
-		flowi4_init_output(&fl4, ipc.oif, sk->sk_mark, tos,
+		fl4 = &fl4_stack;
+		flowi4_init_output(fl4, ipc.oif, sk->sk_mark, tos,
 				   RT_SCOPE_UNIVERSE, sk->sk_protocol,
 				   inet_sk_flowi_flags(sk)|FLOWI_FLAG_CAN_SLEEP,
 				   faddr, saddr, dport, inet->inet_sport);
 
-		security_sk_classify_flow(sk, flowi4_to_flowi(&fl4));
-		rt = ip_route_output_flow(net, &fl4, sk);
+		security_sk_classify_flow(sk, flowi4_to_flowi(fl4));
+		rt = ip_route_output_flow(net, fl4, sk);
 		if (IS_ERR(rt)) {
 			err = PTR_ERR(rt);
 			rt = NULL;
@@ -950,9 +952,9 @@ int udp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 		goto do_confirm;
 back_from_confirm:
 
-	saddr = rt->rt_src;
+	saddr = fl4->saddr;
 	if (!ipc.addr)
-		daddr = ipc.addr = rt->rt_dst;
+		daddr = ipc.addr = fl4->daddr;
 
 	/* Lockless fast path for the non-corking case. */
 	if (!corkreq) {