ipv6: wire up skb->encapsulation

When pushing a new header before current one call skb_reset_inner_headers
to record the position of the inner headers in the various ipv6 tunnel
protocols.

We later need this to correctly identify the addresses needed to send
back an error in the xfrm layer.

This change is safe, because skb->protocol is always checked before
dereferencing data from the inner protocol.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NSteffen Klassert <steffen.klassert@secunet.com>

net/ipv6/ip6_gre.c

@@ -724,6 +724,11 @@ static netdev_tx_t ip6gre_xmit2(struct sk_buff *skb,
 		ipv6_push_nfrag_opts(skb, &opt.ops, &proto, NULL);
 	}
 
+	if (likely(!skb->encapsulation)) {
+		skb_reset_inner_headers(skb);
+		skb->encapsulation = 1;
+	}
+
 	skb_push(skb, gre_hlen);
 	skb_reset_network_header(skb);
 	skb_set_transport_header(skb, sizeof(*ipv6h));

net/ipv6/ip6_tunnel.c

@@ -1027,6 +1027,12 @@ static int ip6_tnl_xmit2(struct sk_buff *skb,
 		init_tel_txopt(&opt, encap_limit);
 		ipv6_push_nfrag_opts(skb, &opt.ops, &proto, NULL);
 	}
+
+	if (likely(!skb->encapsulation)) {
+		skb_reset_inner_headers(skb);
+		skb->encapsulation = 1;
+	}
+
 	skb_push(skb, sizeof(struct ipv6hdr));
 	skb_reset_network_header(skb);
 	ipv6h = ipv6_hdr(skb);

net/ipv6/sit.c

@@ -888,6 +888,11 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 		ttl = iph6->hop_limit;
 	tos = INET_ECN_encapsulate(tos, ipv6_get_dsfield(iph6));
 
+	if (likely(!skb->encapsulation)) {
+		skb_reset_inner_headers(skb);
+		skb->encapsulation = 1;
+	}
+
 	err = iptunnel_xmit(dev_net(dev), rt, skb, fl4.saddr, fl4.daddr,
 			    IPPROTO_IPV6, tos, ttl, df);
 	iptunnel_xmit_stats(err, &dev->stats, dev->tstats);