rtnetlink: add reference counting to prevent module unload while dump is in progress

I don't see what prevents rmmod (unregister_all is called) while a dump
is active.

Even if we'd add rtnl lock/unlock pair to unregister_all (as done here),
thats not enough either as rtnl_lock is released right before the dump
process starts.

So this adds a refcount:
 * acquire rtnl mutex
 * bump refcount
 * release mutex
 * start the dump

... and make unregister_all remove the callbacks (no new dumps possible)
and then wait until refcount is 0.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -127,6 +127,7 @@ EXPORT_SYMBOL(lockdep_rtnl_is_held);
 #endif /* #ifdef CONFIG_PROVE_LOCKING */
 
 static struct rtnl_link *rtnl_msg_handlers[RTNL_FAMILY_MAX + 1];
+static refcount_t rtnl_msg_handlers_ref[RTNL_FAMILY_MAX + 1];
 
 static inline int rtm_msgindex(int msgtype)
 {
@@ -272,10 +273,18 @@ EXPORT_SYMBOL_GPL(rtnl_unregister);
  */
 void rtnl_unregister_all(int protocol)
 {
+	struct rtnl_link *handlers;
+
 	BUG_ON(protocol < 0 || protocol > RTNL_FAMILY_MAX);
 
-	kfree(rtnl_msg_handlers[protocol]);
+	rtnl_lock();
+	handlers = rtnl_msg_handlers[protocol];
 	rtnl_msg_handlers[protocol] = NULL;
+	rtnl_unlock();
+
+	while (refcount_read(&rtnl_msg_handlers_ref[protocol]) > 0)
+		schedule();
+	kfree(handlers);
 }
 EXPORT_SYMBOL_GPL(rtnl_unregister_all);
 
@@ -4173,6 +4182,8 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 		if (dumpit == NULL)
 			return -EOPNOTSUPP;
 
+		refcount_inc(&rtnl_msg_handlers_ref[family]);
+
 		if (type == RTM_GETLINK)
 			min_dump_alloc = rtnl_calcit(skb, nlh);
 
@@ -4186,6 +4197,7 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 			err = netlink_dump_start(rtnl, skb, nlh, &c);
 		}
 		rtnl_lock();
+		refcount_dec(&rtnl_msg_handlers_ref[family]);
 		return err;
 	}