ipv6: udp: set dst cache for a connected sk if current not valid

A new RTF_CACHE route can be created between ip6_sk_dst_lookup_flow()
and ip6_dst_store() calls in udpv6_sendmsg(), when datagram sending
results to ICMPV6_PKT_TOOBIG error:

    udp_v6_send_skb(), for example with vti6 tunnel:
        vti6_xmit(), get ICMPV6_PKT_TOOBIG error
            skb_dst_update_pmtu(), can create a RTF_CACHE clone
            icmpv6_send()
    ...
    udpv6_err()
        ip6_sk_update_pmtu()
           ip6_update_pmtu(), can create a RTF_CACHE clone
           ...
           ip6_datagram_dst_update()
                ip6_dst_store()

And after commit 33c162a9 ("ipv6: datagram: Update dst cache of
a connected datagram sk during pmtu update"), the UDPv6 error handler
can update socket's dst cache, but it can happen before the update in
the end of udpv6_sendmsg(), preventing getting the new dst cache on
the next udpv6_sendmsg() calls.

In order to fix it, save dst in a connected socket only if the current
socket's dst cache is invalid.

The previous patch prepared ip6_sk_dst_lookup_flow() to do that with
the new argument, and this patch enables it in udpv6_sendmsg().

Fixes: 33c162a9 ("ipv6: datagram: Update dst cache of a connected datagram sk during pmtu update")
Fixes: 45e4fd26 ("ipv6: Only create RTF_CACHE routes after encountering pmtu exception")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/udp.c

@@ -1308,7 +1308,7 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 
 	fl6.flowlabel = ip6_make_flowinfo(ipc6.tclass, fl6.flowlabel);
 
-	dst = ip6_sk_dst_lookup_flow(sk, &fl6, final_p, false);
+	dst = ip6_sk_dst_lookup_flow(sk, &fl6, final_p, connected);
 	if (IS_ERR(dst)) {
 		err = PTR_ERR(dst);
 		dst = NULL;
@@ -1333,7 +1333,7 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		err = PTR_ERR(skb);
 		if (!IS_ERR_OR_NULL(skb))
 			err = udp_v6_send_skb(skb, &fl6);
-		goto release_dst;
+		goto out;
 	}
 
 	lock_sock(sk);
@@ -1367,23 +1367,6 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		err = np->recverr ? net_xmit_errno(err) : 0;
 	release_sock(sk);
 
-release_dst:
-	if (dst) {
-		if (connected) {
-			ip6_dst_store(sk, dst,
-				      ipv6_addr_equal(&fl6.daddr, &sk->sk_v6_daddr) ?
-				      &sk->sk_v6_daddr : NULL,
-#ifdef CONFIG_IPV6_SUBTREES
-				      ipv6_addr_equal(&fl6.saddr, &np->saddr) ?
-				      &np->saddr :
-#endif
-				      NULL);
-		} else {
-			dst_release(dst);
-		}
-		dst = NULL;
-	}
-
 out:
 	dst_release(dst);
 	fl6_sock_release(flowlabel);