gtp: add helper function to build GTP packets from an IPv6 packet

Add routine to attach an IPv6 route for the encapsulated packet, deal
with Path MTU and push GTP header.

This helper function will be used to deal with IPv4-in-IPv6-GTP.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

drivers/net/gtp.c

@@ -1047,6 +1047,63 @@ static int gtp_build_skb_outer_ip4(struct sk_buff *skb, struct net_device *dev,
 	return -EBADMSG;
 }
 
+static int gtp_build_skb_outer_ip6(struct net *net, struct sk_buff *skb,
+				   struct net_device *dev,
+				   struct gtp_pktinfo *pktinfo,
+				   struct pdp_ctx *pctx, __u8 tos)
+{
+	struct dst_entry *dst;
+	struct rt6_info *rt;
+	struct flowi6 fl6;
+	int mtu;
+
+	rt = ip6_route_output_gtp(net, &fl6, pctx->sk, &pctx->peer.addr6,
+				  &inet6_sk(pctx->sk)->saddr);
+	if (IS_ERR(rt)) {
+		netdev_dbg(dev, "no route to SSGN %pI6\n",
+			   &pctx->peer.addr6);
+		dev->stats.tx_carrier_errors++;
+		goto err;
+	}
+	dst = &rt->dst;
+
+	if (rt->dst.dev == dev) {
+		netdev_dbg(dev, "circular route to SSGN %pI6\n",
+			   &pctx->peer.addr6);
+		dev->stats.collisions++;
+		goto err_rt;
+	}
+
+	mtu = dst_mtu(&rt->dst) - dev->hard_header_len -
+		sizeof(struct ipv6hdr) - sizeof(struct udphdr);
+	switch (pctx->gtp_version) {
+	case GTP_V0:
+		mtu -= sizeof(struct gtp0_header);
+		break;
+	case GTP_V1:
+		mtu -= sizeof(struct gtp1_header);
+		break;
+	}
+
+	skb_dst_update_pmtu_no_confirm(skb, mtu);
+
+	if ((!skb_is_gso(skb) && skb->len > mtu) ||
+	    (skb_is_gso(skb) && !skb_gso_validate_network_len(skb, mtu))) {
+		netdev_dbg(dev, "packet too big, fragmentation needed\n");
+		icmpv6_ndo_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
+		goto err_rt;
+	}
+
+	gtp_set_pktinfo_ipv6(pktinfo, pctx->sk, tos, pctx, rt, &fl6, dev);
+	gtp_push_header(skb, pktinfo);
+
+	return 0;
+err_rt:
+	dst_release(dst);
+err:
+	return -EBADMSG;
+}
+
 static int gtp_build_skb_ip4(struct sk_buff *skb, struct net_device *dev,
 			     struct gtp_pktinfo *pktinfo)
 {
@@ -1087,13 +1144,10 @@ static int gtp_build_skb_ip6(struct sk_buff *skb, struct net_device *dev,
 {
 	struct gtp_dev *gtp = netdev_priv(dev);
 	struct net *net = gtp->net;
-	struct dst_entry *dst;
 	struct pdp_ctx *pctx;
 	struct ipv6hdr *ip6h;
-	struct rt6_info *rt;
-	struct flowi6 fl6;
 	__u8 tos;
-	int mtu;
+	int ret;
 
 	/* Read the IP destination address and resolve the PDP context.
 	 * Prepend PDP header with TEI/TID from PDP ctx.
@@ -1111,55 +1165,16 @@ static int gtp_build_skb_ip6(struct sk_buff *skb, struct net_device *dev,
 	}
 	netdev_dbg(dev, "found PDP context %p\n", pctx);
 
-	rt = ip6_route_output_gtp(net, &fl6, pctx->sk, &pctx->peer.addr6,
-				  &inet6_sk(pctx->sk)->saddr);
-	if (IS_ERR(rt)) {
-		netdev_dbg(dev, "no route to SSGN %pI6\n",
-			   &pctx->peer.addr6);
-		dev->stats.tx_carrier_errors++;
-		goto err;
-	}
-	dst = &rt->dst;
-
-	if (rt->dst.dev == dev) {
-		netdev_dbg(dev, "circular route to SSGN %pI6\n",
-			   &pctx->peer.addr6);
-		dev->stats.collisions++;
-		goto err_rt;
-	}
-
-	mtu = dst_mtu(&rt->dst) - dev->hard_header_len -
-		sizeof(struct ipv6hdr) - sizeof(struct udphdr);
-	switch (pctx->gtp_version) {
-	case GTP_V0:
-		mtu -= sizeof(struct gtp0_header);
-		break;
-	case GTP_V1:
-		mtu -= sizeof(struct gtp1_header);
-		break;
-	}
-
-	skb_dst_update_pmtu_no_confirm(skb, mtu);
-
-	if ((!skb_is_gso(skb) && skb->len > mtu) ||
-	    (skb_is_gso(skb) && !skb_gso_validate_network_len(skb, mtu))) {
-		netdev_dbg(dev, "packet too big, fragmentation needed\n");
-		icmpv6_ndo_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
-		goto err_rt;
-	}
-
 	tos = ipv6_get_dsfield(ip6h);
-	gtp_set_pktinfo_ipv6(pktinfo, pctx->sk, tos, pctx, rt, &fl6, dev);
-	gtp_push_header(skb, pktinfo);
+
+	ret = gtp_build_skb_outer_ip6(net, skb, dev, pktinfo, pctx, tos);
+	if (ret < 0)
+		return ret;
 
 	netdev_dbg(dev, "gtp -> IP src: %pI6 dst: %pI6\n",
 		   &ip6h->saddr, &ip6h->daddr);
 
 	return 0;
-err_rt:
-	dst_release(dst);
-err:
-	return -EBADMSG;
 }
 
 static netdev_tx_t gtp_dev_xmit(struct sk_buff *skb, struct net_device *dev)