netfilter: ip6tables: Split ip6t_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: b9e69e1273 ("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_ipv6/ip6_tables.h

@@ -29,6 +29,9 @@ int ip6t_register_table(struct net *net, const struct xt_table *table,
 			const struct nf_hook_ops *ops, struct xt_table **res);
 void ip6t_unregister_table(struct net *net, struct xt_table *table,
 			   const struct nf_hook_ops *ops);
+void ip6t_unregister_table_pre_exit(struct net *net, struct xt_table *table,
+				    const struct nf_hook_ops *ops);
+void ip6t_unregister_table_exit(struct net *net, struct xt_table *table);
 extern unsigned int ip6t_do_table(struct sk_buff *skb,
 				  const struct nf_hook_state *state,
 				  struct xt_table *table);

net/ipv6/netfilter/ip6_tables.c

@@ -1807,11 +1807,22 @@ out_free:
 	return ret;
 }
 
+void ip6t_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 ip6t_unregister_table_exit(struct net *net, struct xt_table *table)
+{
+	__ip6t_unregister_table(net, table);
+}
+
 void ip6t_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));
+		ip6t_unregister_table_pre_exit(net, table, ops);
 	__ip6t_unregister_table(net, table);
 }
 
@@ -1969,6 +1980,8 @@ static void __exit ip6_tables_fini(void)
 
 EXPORT_SYMBOL(ip6t_register_table);
 EXPORT_SYMBOL(ip6t_unregister_table);
+EXPORT_SYMBOL(ip6t_unregister_table_pre_exit);
+EXPORT_SYMBOL(ip6t_unregister_table_exit);
 EXPORT_SYMBOL(ip6t_do_table);
 
 module_init(ip6_tables_init);