netfilter: explicit module dependency between br_netfilter and physdev

You can use physdev to match the physical interface enslaved to the
bridge device. This information is stored in skb->nf_bridge and it is
set up by br_netfilter. So, this is only available when iptables is
used from the bridge netfilter path.

Since 34666d4 ("netfilter: bridge: move br_netfilter out of the core"),
the br_netfilter code is modular. To reduce the impact of this change,
we can autoload the br_netfilter if the physdev match is used since
we assume that the users need br_netfilter in place.

Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/net/netfilter/br_netfilter.h

···
+#ifndef _BR_NETFILTER_H_
+#define _BR_NETFILTER_H_
+
+void br_netfilter_enable(void);
+
+#endif /* _BR_NETFILTER_H_ */

net/bridge/br_netfilter.c

···
 	return NF_ACCEPT;
 }
 
+void br_netfilter_enable(void)
+{
+}
+EXPORT_SYMBOL_GPL(br_netfilter_enable);
+
 /* For br_nf_post_routing, we need (prio = NF_BR_PRI_LAST), because
  * br_dev_queue_push_xmit is called afterwards */
 static struct nf_hook_ops br_nf_ops[] __read_mostly = {

net/netfilter/xt_physdev.c

···
 #include <linux/netfilter_bridge.h>
 #include <linux/netfilter/xt_physdev.h>
 #include <linux/netfilter/x_tables.h>
+#include <net/netfilter/br_netfilter.h>
 
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Bart De Schuymer <[e-mail]>");
···
 static int physdev_mt_check(const struct xt_mtchk_param *par)
 {
 	const struct xt_physdev_info *info = par->matchinfo;
+
+	br_netfilter_enable();
 
 	if (!(info->bitmask & XT_PHYSDEV_OP_MASK) ||
 	    info->bitmask & ~XT_PHYSDEV_OP_MASK)