ipv6: Add GSO support on forwarding path

commit 0aa68271 upstream.

Currently we disallow GSO packets on the IPv6 forward path.
This patch fixes this.

Note that I discovered that our existing GSO MTU checks (e.g.,
IPv4 forwarding) are buggy in that they skip the check altogether,
when they really should be checking gso_size + header instead.

I have also been lazy here in that I haven't bothered to segment
the GSO packet by hand before generating an ICMP message.  Someone
should add that to be 100% correct.
Reported-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Apollon Oikonomopoulos <apoikos@gmail.com>
Signed-off-by: Faidon Liambotis <paravoid@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

net/ipv6/ip6_output.c

@@ -510,7 +510,7 @@ int ip6_forward(struct sk_buff *skb)
 		}
 	}
 
-	if (skb->len > dst_mtu(dst)) {
+	if (skb->len > dst_mtu(dst) && !skb_is_gso(skb)) {
 		/* Again, force OUTPUT device used as source address */
 		skb->dev = dst->dev;
 		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, dst_mtu(dst), skb->dev);