netfilter: xtables: allow table definitions not backed by hook_ops

The ip(6)tables nat table is currently receiving skbs from the netfilter
core, after a followup patch skbs will be coming from the netfilter nat
core instead, so the table is no longer backed by normal hook_ops.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/ip_tables.c

@@ -1782,6 +1782,8 @@ int ipt_register_table(struct net *net, const struct xt_table *table,
 
 	/* set res now, will see skbs right after nf_register_net_hooks */
 	WRITE_ONCE(*res, new_table);
+	if (!ops)
+		return 0;
 
 	ret = nf_register_net_hooks(net, ops, hweight32(table->valid_hooks));
 	if (ret != 0) {
@@ -1799,6 +1801,7 @@ int ipt_register_table(struct net *net, const struct xt_table *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(net, table);
 }

net/ipv6/netfilter/ip6_tables.c

@@ -1792,6 +1792,8 @@ int ip6t_register_table(struct net *net, const struct xt_table *table,
 
 	/* set res now, will see skbs right after nf_register_net_hooks */
 	WRITE_ONCE(*res, new_table);
+	if (!ops)
+		return 0;
 
 	ret = nf_register_net_hooks(net, ops, hweight32(table->valid_hooks));
 	if (ret != 0) {
@@ -1809,6 +1811,7 @@ int ip6t_register_table(struct net *net, const struct xt_table *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(net, table);
 }