bridge: fix seq check in br_mdb_dump()

In case of rehashing, introduce a global variable 'br_mdb_rehash_seq'
which gets increased every time when rehashing, and assign
net->dev_base_seq + br_mdb_rehash_seq to cb->seq.

In theory cb->seq could be wrapped to zero, but this is not
easy to fix, as net->dev_base_seq is not visible inside
br_mdb_rehash(). In practice, this is rare.

Cc: Herbert Xu <[e-mail]>
Cc: Stephen Hemminger <[e-mail]>
Cc: "David S. Miller" <[e-mail]>
Cc: Thomas Graf <[e-mail]>
Cc: Jesper Dangaard Brouer <[e-mail]>
Signed-off-by: Cong Wang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/bridge/br_mdb.c

···
 
 	rcu_read_lock();
 
-	/* TODO: in case of rehashing, we need to check
-	 * consistency for dumping.
-	 */
-	cb->seq = net->dev_base_seq;
+	/* In theory this could be wrapped to 0... */
+	cb->seq = net->dev_base_seq + br_mdb_rehash_seq;
 
 	for_each_netdev_rcu(net, dev) {
 		if (dev->priv_flags & IFF_EBRIDGE) {

net/bridge/br_multicast.c

···
 	rcu_dereference_protected(X, lockdep_is_held(&br->multicast_lock))
 
 static void br_multicast_start_querier(struct net_bridge *br);
+unsigned int br_mdb_rehash_seq;
 
 #if IS_ENABLED(CONFIG_IPV6)
 static inline int ipv6_is_transient_multicast(const struct in6_addr *addr)
···
 		return err;
 	}
 
+	br_mdb_rehash_seq++;
 	call_rcu_bh(&mdb->rcu, br_mdb_free);
 
 out:

net/bridge/br_private.h

···
 
 /* br_multicast.c */
 #ifdef CONFIG_BRIDGE_IGMP_SNOOPING
+extern unsigned int br_mdb_rehash_seq;
 extern int br_multicast_rcv(struct net_bridge *br,
 			    struct net_bridge_port *port,
 			    struct sk_buff *skb);