netfilter: iptables: Split ipt_unregister_table() into pre_exit and exit helpers.

The pre_exit will un-register the underlying hook and .exit will do the
table freeing. The netns core does an unconditional synchronize_rcu after
the pre_exit hooks insuring no packets are in flight that have picked up
the pointer before completing the un-register.

Fixes: b9e69e12 ("netfilter: xtables: don't hook tables by default")
Signed-off-by: David Wilder <dwilder@us.ibm.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter_ipv4/ip_tables.h

@@ -25,6 +25,12 @@
 int ipt_register_table(struct net *net, const struct xt_table *table,
 		       const struct ipt_replace *repl,
 		       const struct nf_hook_ops *ops, struct xt_table **res);
+
+void ipt_unregister_table_pre_exit(struct net *net, struct xt_table *table,
+		       const struct nf_hook_ops *ops);
+
+void ipt_unregister_table_exit(struct net *net, struct xt_table *table);
+
 void ipt_unregister_table(struct net *net, struct xt_table *table,
 			  const struct nf_hook_ops *ops);
 

net/ipv4/netfilter/ip_tables.c

@@ -1797,11 +1797,22 @@ int ipt_register_table(struct net *net, const struct xt_table *table,
 	return ret;
 }
 
+void ipt_unregister_table_pre_exit(struct net *net, struct xt_table *table,
+				   const struct nf_hook_ops *ops)
+{
+	nf_unregister_net_hooks(net, ops, hweight32(table->valid_hooks));
+}
+
+void ipt_unregister_table_exit(struct net *net, struct xt_table *table)
+{
+	__ipt_unregister_table(net, table);
+}
+
 void ipt_unregister_table(struct net *net, struct xt_table *table,
 			  const struct nf_hook_ops *ops)
 {
 	if (ops)
-		nf_unregister_net_hooks(net, ops, hweight32(table->valid_hooks));
+		ipt_unregister_table_pre_exit(net, table, ops);
 	__ipt_unregister_table(net, table);
 }
 
@@ -1958,6 +1969,8 @@ static void __exit ip_tables_fini(void)
 
 EXPORT_SYMBOL(ipt_register_table);
 EXPORT_SYMBOL(ipt_unregister_table);
+EXPORT_SYMBOL(ipt_unregister_table_pre_exit);
+EXPORT_SYMBOL(ipt_unregister_table_exit);
 EXPORT_SYMBOL(ipt_do_table);
 module_init(ip_tables_init);
 module_exit(ip_tables_fini);