TEE: various cleanups, add comments

Normalize function names in light of upcoming IPv6 support.
Reformat other lines.
Add comment note about tee_send4.

extensions/xt_TEE.c

@@ -29,7 +29,7 @@ static struct nf_conn tee_track;
 #include "compat_xtables.h"
 #include "xt_TEE.h"
 
-static const union nf_inet_addr zero_address;
+static const union nf_inet_addr tee_zero_address;
 
 /*
  * Try to route the packet according to the routing keys specified in
@@ -47,20 +47,16 @@ static const union nf_inet_addr zero_address;
  *         true  - if the packet was succesfully routed to the
  *                 destination desired
  */
-static bool tee_routing(struct sk_buff *skb,
+static bool
-                        const struct xt_tee_tginfo *info)
+tee_tg_route4(struct sk_buff *skb, const struct xt_tee_tginfo *info)
 {
 	int err;
 	struct rtable *rt;
-	struct iphdr *iph = ip_hdr(skb);
+	struct flowi fl;
-	struct flowi fl = {
+
-		.nl_u = {
+	memset(&fl, 0, sizeof(fl));
-			.ip4_u = {
+	fl.nl_u.ip4_u.daddr = info->gw.ip;
-				.daddr = info->gw.ip,
+	fl.nl_u.ip4_u.scope = RT_SCOPE_UNIVERSE;
-				.scope = RT_SCOPE_UNIVERSE,
-			}
-		}
-	};
 
 	/* Trying to route the packet using the standard routing table. */
 	err = ip_route_output_key(&init_net, &rt, &fl);
@@ -71,22 +67,14 @@ static bool tee_routing(struct sk_buff *skb,
 		return false;
 	}
 
-	/* Drop old route. */
 	dst_release(skb->dst);
-	skb->dst = NULL;
-
-	/*
-	 * Success if no oif specified or if the oif correspond to the
-	 * one desired.
-	 * [SC]: always the case, because we have no oif.
-	 */
 	skb->dst      = &rt->u.dst;
 	skb->dev      = skb->dst->dev;
 	skb->protocol = htons(ETH_P_IP);
 	return true;
 }
 
-static bool dev_hh_avail(const struct net_device *dev)
+static inline bool dev_hh_avail(const struct net_device *dev)
 {
 #if LINUX_VERSION_CODE <= KERNEL_VERSION(2, 6, 23)
 	return dev->hard_header != NULL;
@@ -102,14 +90,14 @@ static bool dev_hh_avail(const struct net_device *dev)
  * POST: the packet is sent with the link layer header pushed
  *       the packet is destroyed
  */
-static void tee_ip_direct_send(struct sk_buff *skb)
+static void tee_tg_send4(struct sk_buff *skb)
 {
 	const struct dst_entry *dst  = skb->dst;
 	const struct net_device *dev = dst->dev;
 	unsigned int hh_len = LL_RESERVED_SPACE(dev);
 
 	/* Be paranoid, rather than too clever. */
-	if (unlikely(skb_headroom(skb) < hh_len) && dev_hh_avail(dev)) {
+	if (unlikely(skb_headroom(skb) < hh_len && dev_hh_avail(dev))) {
 		struct sk_buff *skb2;
 
 		skb2 = skb_realloc_headroom(skb, LL_RESERVED_SPACE(dev));
@@ -141,7 +129,7 @@ static void tee_ip_direct_send(struct sk_buff *skb)
  * packets when we see they already have that ->nfct.
  */
 static unsigned int
-tee_tg(struct sk_buff **pskb, const struct xt_target_param *par)
+tee_tg4(struct sk_buff **pskb, const struct xt_target_param *par)
 {
 	const struct xt_tee_tginfo *info = par->targinfo;
 	struct sk_buff *skb = *pskb;
@@ -199,8 +187,24 @@ tee_tg(struct sk_buff **pskb, const struct xt_target_param *par)
 	nf_conntrack_get(skb->nfct);
 #endif
 
-	if (tee_routing(skb, info))
+	/*
-		tee_ip_direct_send(skb);
+	 * Normally, we would just use ip_local_out. Because iph->check is
+	 * already correct, we could take a shortcut and call dst_output
+	 * [forwards to ip_output] directly. ip_output however will invoke
+	 * Netfilter hooks and cause reentrancy. So we skip that too and go
+	 * directly to ip_finish_output. Since we should not do XFRM, control
+	 * passes to ip_finish_output2. That function is not exported, so it is
+	 * copied here as tee_ip_direct_send.
+	 *
+	 * We do no XFRM on the cloned packet on purpose! The choice of
+	 * iptables match options will control whether the raw packet or the
+	 * transformed version is cloned.
+	 *
+	 * Also on purpose, no fragmentation is done, to preserve the
+	 * packet as best as possible.
+	 */
+	if (tee_tg_route4(skb, info))
+		tee_tg_send4(skb);
 
 	return XT_CONTINUE;
 }
@@ -210,7 +214,8 @@ static bool tee_tg_check(const struct xt_tgchk_param *par)
 	const struct xt_tee_tginfo *info = par->targinfo;
 
 	/* 0.0.0.0 and :: not allowed */
-	return memcmp(&info->gw, &zero_address, sizeof(zero_address)) != 0;
+	return memcmp(&info->gw, &tee_zero_address,
+	       sizeof(tee_zero_address)) != 0;
 }
 
 static struct xt_target tee_tg_reg __read_mostly = {
@@ -218,7 +223,7 @@ static struct xt_target tee_tg_reg __read_mostly = {
 	.revision   = 0,
 	.family     = NFPROTO_IPV4,
 	.table      = "mangle",
-	.target     = tee_tg,
+	.target     = tee_tg4,
 	.targetsize = sizeof(struct xt_tee_tginfo),
 	.checkentry = tee_tg_check,
 	.me         = THIS_MODULE,
@@ -252,7 +257,7 @@ static void __exit tee_tg_exit(void)
 module_init(tee_tg_init);
 module_exit(tee_tg_exit);
 MODULE_AUTHOR("Sebastian Claßen <sebastian.classen@freenet.ag>");
-MODULE_AUTHOR("Jan Engelhardt <jengelh@computergmbh.de>");
+MODULE_AUTHOR("Jan Engelhardt <jengelh@medozas.de>");
 MODULE_DESCRIPTION("Xtables: Reroute packet copy");
 MODULE_LICENSE("GPL");
 MODULE_ALIAS("ipt_TEE");