ipv4: fib: Allow for consistent FIB dumping

The next patch will enable listeners of the FIB notification chain to
request a dump of the FIB tables. However, since RTNL isn't taken during
the dump, it's possible for the FIB tables to change mid-dump, which
will result in inconsistency between the listener's table and the
kernel's.

Allow listeners to know about changes that occurred mid-dump, by adding
a change sequence counter to each net namespace. The counter is
incremented just before a notification is sent in the FIB chain.
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/ipv4.h

@@ -135,6 +135,9 @@ struct netns_ipv4 {
 #ifdef CONFIG_IP_ROUTE_MULTIPATH
 	int sysctl_fib_multipath_use_neigh;
 #endif
+
+	unsigned int	fib_seq;	/* protected by rtnl_mutex */
+
 	atomic_t	rt_genid;
 };
 #endif

net/ipv4/fib_frontend.c

@@ -1219,6 +1219,8 @@ static int __net_init ip_fib_net_init(struct net *net)
 	int err;
 	size_t size = sizeof(struct hlist_head) * FIB_TABLE_HASHSZ;
 
+	net->ipv4.fib_seq = 0;
+
 	/* Avoid false sharing : Use at least a full cache line */
 	size = max_t(size_t, size, L1_CACHE_BYTES);
 

net/ipv4/fib_trie.c

@@ -101,6 +101,7 @@ EXPORT_SYMBOL(unregister_fib_notifier);
 int call_fib_notifiers(struct net *net, enum fib_event_type event_type,
 		       struct fib_notifier_info *info)
 {
+	net->ipv4.fib_seq++;
 	info->net = net;
 	return atomic_notifier_call_chain(&fib_chain, event_type, info);
 }