ipv6: pass up EMSGSIZE msg for UDP socket in Ipv6

In ipv4,  when  the machine receives a ICMP_FRAG_NEEDED message,  the
connected UDP socket will get EMSGSIZE message on its next read from the
socket.
However, this is not the case for ipv6.
This fix modifies the udp err handler in Ipv6 for ICMP6_PKT_TOOBIG to
make it similar to ipv4 behavior. That is when the machine gets an
ICMP6_PKT_TOOBIG message, the connected UDP socket will get EMSGSIZE
message on its next read from the socket.
Signed-off-by: NWei Wang <weiwan@google.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/udp.c

@@ -548,6 +548,7 @@ void __udp6_lib_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 	const struct in6_addr *daddr = &hdr->daddr;
 	struct udphdr *uh = (struct udphdr *)(skb->data+offset);
 	struct sock *sk;
+	int harderr;
 	int err;
 	struct net *net = dev_net(skb->dev);
 
@@ -559,26 +560,27 @@ void __udp6_lib_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		return;
 	}
 
+	harderr = icmpv6_err_convert(type, code, &err);
+	np = inet6_sk(sk);
+
 	if (type == ICMPV6_PKT_TOOBIG) {
 		if (!ip6_sk_accept_pmtu(sk))
 			goto out;
 		ip6_sk_update_pmtu(skb, sk, info);
+		if (np->pmtudisc != IPV6_PMTUDISC_DONT)
+			harderr = 1;
 	}
 	if (type == NDISC_REDIRECT) {
 		ip6_sk_redirect(skb, sk);
 		goto out;
 	}
 
-	np = inet6_sk(sk);
-
-	if (!icmpv6_err_convert(type, code, &err) && !np->recverr)
+	if (!np->recverr) {
+		if (!harderr || sk->sk_state != TCP_ESTABLISHED)
 			goto out;
-
-	if (sk->sk_state != TCP_ESTABLISHED && !np->recverr)
-		goto out;
-
-	if (np->recverr)
+	} else {
 		ipv6_icmp_error(sk, skb, err, uh->dest, ntohl(info), (u8 *)(uh+1));
+	}
 
 	sk->sk_err = err;
 	sk->sk_error_report(sk);