gre: better support for ICMP messages for gre+ipv6

ipgre_err() can call ip6_err_gen_icmpv6_unreach() for proper
support of ipv4+gre+icmp+ipv6+... frames, used for example
by traceroute/mtr.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip_tunnels.h

@@ -157,6 +157,7 @@ struct tnl_ptk_info {
 	__be16 proto;
 	__be32 key;
 	__be32 seq;
+	int hdr_len;
 };
 
 #define PACKET_RCVD	0

net/ipv4/gre_demux.c

@@ -117,6 +117,7 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 		if ((*(u8 *)options & 0xF0) != 0x40)
 			hdr_len += 4;
 	}
+	tpi->hdr_len = hdr_len;
 	return hdr_len;
 }
 EXPORT_SYMBOL(gre_parse_header);

net/ipv4/ip_gre.c

@@ -187,6 +187,12 @@ static void ipgre_err(struct sk_buff *skb, u32 info,
 	if (!t)
 		return;
 
+#if IS_ENABLED(CONFIG_IPV6)
+       if (tpi->proto == htons(ETH_P_IPV6) &&
+           !ip6_err_gen_icmpv6_unreach(skb, iph->ihl * 4 + tpi->hdr_len, type))
+               return;
+#endif
+
 	if (t->parms.iph.daddr == 0 ||
 	    ipv4_is_multicast(t->parms.iph.daddr))
 		return;