netfilter: don't attach a nat extension by default

nowadays the NAT extension only stores the interface index
(used to purge connections that got masqueraded when interface goes down)
and pptp nat information.

Previous patches moved nf_ct_nat_ext_add to those places that need it.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_nat.h

@@ -67,7 +67,7 @@ static inline bool nf_nat_oif_changed(unsigned int hooknum,
 {
 #if IS_ENABLED(CONFIG_NF_NAT_MASQUERADE_IPV4) || \
     IS_ENABLED(CONFIG_NF_NAT_MASQUERADE_IPV6)
-	return nat->masq_index && hooknum == NF_INET_POST_ROUTING &&
+	return nat && nat->masq_index && hooknum == NF_INET_POST_ROUTING &&
 	       CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL &&
 	       nat->masq_index != out->ifindex;
 #else

net/ipv4/netfilter/nf_nat_l3proto_ipv4.c

@@ -264,9 +264,7 @@ nf_nat_ipv4_fn(void *priv, struct sk_buff *skb,
 	if (!ct)
 		return NF_ACCEPT;
 
-	nat = nf_ct_nat_ext_add(ct);
-	if (nat == NULL)
-		return NF_ACCEPT;
+	nat = nfct_nat(ct);
 
 	switch (ctinfo) {
 	case IP_CT_RELATED:

net/ipv6/netfilter/nf_nat_l3proto_ipv6.c

@@ -273,9 +273,7 @@ nf_nat_ipv6_fn(void *priv, struct sk_buff *skb,
 	if (!ct)
 		return NF_ACCEPT;
 
-	nat = nf_ct_nat_ext_add(ct);
-	if (nat == NULL)
-		return NF_ACCEPT;
+	nat = nfct_nat(ct);
 
 	switch (ctinfo) {
 	case IP_CT_RELATED:

net/netfilter/nf_nat_core.c

@@ -408,12 +408,6 @@ nf_nat_setup_info(struct nf_conn *ct,
 		  enum nf_nat_manip_type maniptype)
 {
 	struct nf_conntrack_tuple curr_tuple, new_tuple;
-	struct nf_conn_nat *nat;
-
-	/* nat helper or nfctnetlink also setup binding */
-	nat = nf_ct_nat_ext_add(ct);
-	if (nat == NULL)
-		return NF_ACCEPT;
 
 	NF_CT_ASSERT(maniptype == NF_NAT_MANIP_SRC ||
 		     maniptype == NF_NAT_MANIP_DST);