netfilter: add nf_ipv6_ops hook to fix xt_addrtype with IPv6

Quoting https://bugzilla.netfilter.org/show_bug.cgi?id=812:

[ ip6tables -m addrtype ]
When I tried to use in the nat/PREROUTING it messes up the
routing cache even if the rule didn't matched at all.
[..]
If I remove the --limit-iface-in from the non-working scenario, so just
use the -m addrtype --dst-type LOCAL it works!

This happens when LOCAL type matching is requested with --limit-iface-in,
and the default ipv6 route is via the interface the packet we test
arrived on.

Because xt_addrtype uses ip6_route_output, the ipv6 routing implementation
creates an unwanted cached entry, and the packet won't make it to the
real/expected destination.

Silently ignoring --limit-iface-in makes the routing work but it breaks
rule matching (--dst-type LOCAL with limit-iface-in is supposed to only
match if the dst address is configured on the incoming interface;
without --limit-iface-in it will match if the address is reachable
via lo).

The test should call ipv6_chk_addr() instead.  However, this would add
a link-time dependency on ipv6.

There are two possible solutions:

1) Revert the commit that moved ipt_addrtype to xt_addrtype,
   and put ipv6 specific code into ip6t_addrtype.
2) add new "nf_ipv6_ops" struct to register pointers to ipv6 functions.

While the former might seem preferable, Pablo pointed out that there
are more xt modules with link-time dependeny issues regarding ipv6,
so lets go for 2).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter_ipv6.h

@@ -17,6 +17,22 @@ extern __sum16 nf_ip6_checksum(struct sk_buff *skb, unsigned int hook,
 
 extern int ipv6_netfilter_init(void);
 extern void ipv6_netfilter_fini(void);
+
+/*
+ * Hook functions for ipv6 to allow xt_* modules to be built-in even
+ * if IPv6 is a module.
+ */
+struct nf_ipv6_ops {
+	int (*chk_addr)(struct net *net, const struct in6_addr *addr,
+			const struct net_device *dev, int strict);
+};
+
+extern const struct nf_ipv6_ops __rcu *nf_ipv6_ops;
+static inline const struct nf_ipv6_ops *nf_get_ipv6_ops(void)
+{
+	return rcu_dereference(nf_ipv6_ops);
+}
+
 #else /* CONFIG_NETFILTER */
 static inline int ipv6_netfilter_init(void) { return 0; }
 static inline void ipv6_netfilter_fini(void) { return; }

include/net/addrconf.h

@@ -65,7 +65,7 @@ extern int			addrconf_set_dstaddr(struct net *net,
 
 extern int			ipv6_chk_addr(struct net *net,
 					      const struct in6_addr *addr,
-					      struct net_device *dev,
+					      const struct net_device *dev,
 					      int strict);
 
 #if defined(CONFIG_IPV6_MIP6) || defined(CONFIG_IPV6_MIP6_MODULE)