gtp: add helper function to build GTP packets from an IPv4 packet

Add routine to attach an IPv4 route for the encapsulated packet, deal
with Path MTU and push GTP header.

This helper function will be used to deal with IPv6-in-IPv4-GTP.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: ummakynes

drivers/net/gtp.c

@@ -983,33 +983,16 @@ static void gtp_set_pktinfo_ipv6(struct gtp_pktinfo *pktinfo,
 	pktinfo->dev	= dev;
 }
 
-static int gtp_build_skb_ip4(struct sk_buff *skb, struct net_device *dev,
-			     struct gtp_pktinfo *pktinfo)
+static int gtp_build_skb_outer_ip4(struct sk_buff *skb, struct net_device *dev,
+				   struct gtp_pktinfo *pktinfo,
+				   struct pdp_ctx *pctx, __u8 tos,
+				   __be16 frag_off)
 {
-	struct gtp_dev *gtp = netdev_priv(dev);
-	struct pdp_ctx *pctx;
 	struct rtable *rt;
 	struct flowi4 fl4;
-	struct iphdr *iph;
 	__be16 df;
 	int mtu;
 
-	/* Read the IP destination address and resolve the PDP context.
-	 * Prepend PDP header with TEI/TID from PDP ctx.
-	 */
-	iph = ip_hdr(skb);
-	if (gtp->role == GTP_ROLE_SGSN)
-		pctx = ipv4_pdp_find(gtp, iph->saddr);
-	else
-		pctx = ipv4_pdp_find(gtp, iph->daddr);
-
-	if (!pctx) {
-		netdev_dbg(dev, "no PDP ctx found for %pI4, skip\n",
-			   &iph->daddr);
-		return -ENOENT;
-	}
-	netdev_dbg(dev, "found PDP context %p\n", pctx);
-
 	rt = ip4_route_output_gtp(&fl4, pctx->sk, pctx->peer.addr.s_addr,
 				  inet_sk(pctx->sk)->inet_saddr);
 	if (IS_ERR(rt)) {
@@ -1027,7 +1010,7 @@ static int gtp_build_skb_ip4(struct sk_buff *skb, struct net_device *dev,
 	}
 
 	/* This is similar to tnl_update_pmtu(). */
-	df = iph->frag_off;
+	df = frag_off;
 	if (df) {
 		mtu = dst_mtu(&rt->dst) - dev->hard_header_len -
 			sizeof(struct iphdr) - sizeof(struct udphdr);
@@ -1045,7 +1028,7 @@ static int gtp_build_skb_ip4(struct sk_buff *skb, struct net_device *dev,
 
 	skb_dst_update_pmtu_no_confirm(skb, mtu);
 
-	if (iph->frag_off & htons(IP_DF) &&
+	if (frag_off & htons(IP_DF) &&
 	    ((!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");
@@ -1054,19 +1037,51 @@ static int gtp_build_skb_ip4(struct sk_buff *skb, struct net_device *dev,
 		goto err_rt;
 	}
 
-	gtp_set_pktinfo_ipv4(pktinfo, pctx->sk, iph->tos, pctx, rt, &fl4, dev);
+	gtp_set_pktinfo_ipv4(pktinfo, pctx->sk, tos, pctx, rt, &fl4, dev);
 	gtp_push_header(skb, pktinfo);
 
-	netdev_dbg(dev, "gtp -> IP src: %pI4 dst: %pI4\n",
-		   &iph->saddr, &iph->daddr);
-
 	return 0;
 err_rt:
 	ip_rt_put(rt);
 err:
 	return -EBADMSG;
 }
 
+static int gtp_build_skb_ip4(struct sk_buff *skb, struct net_device *dev,
+			     struct gtp_pktinfo *pktinfo)
+{
+	struct gtp_dev *gtp = netdev_priv(dev);
+	struct pdp_ctx *pctx;
+	struct iphdr *iph;
+	int ret;
+
+	/* Read the IP destination address and resolve the PDP context.
+	 * Prepend PDP header with TEI/TID from PDP ctx.
+	 */
+	iph = ip_hdr(skb);
+	if (gtp->role == GTP_ROLE_SGSN)
+		pctx = ipv4_pdp_find(gtp, iph->saddr);
+	else
+		pctx = ipv4_pdp_find(gtp, iph->daddr);
+
+	if (!pctx) {
+		netdev_dbg(dev, "no PDP ctx found for %pI4, skip\n",
+			   &iph->daddr);
+		return -ENOENT;
+	}
+	netdev_dbg(dev, "found PDP context %p\n", pctx);
+
+	ret = gtp_build_skb_outer_ip4(skb, dev, pktinfo, pctx,
+				      iph->tos, iph->frag_off);
+	if (ret < 0)
+		return ret;
+
+	netdev_dbg(dev, "gtp -> IP src: %pI4 dst: %pI4\n",
+		   &iph->saddr, &iph->daddr);
+
+	return 0;
+}
+
 static int gtp_build_skb_ip6(struct sk_buff *skb, struct net_device *dev,
 			     struct gtp_pktinfo *pktinfo)
 {