mpls: Convert mpls_dump_routes() to RCU.

mpls_dump_routes() sets fib_dump_filter.rtnl_held to true and
calls __dev_get_by_index() in mpls_valid_fib_dump_req().

This is the only RTNL dependant in mpls_dump_routes().

Also, synchronize_rcu() in resize_platform_label_table()
guarantees that net->mpls.platform_label is alive under RCU.

Let's convert mpls_dump_routes() to RCU and use dev_get_by_index_rcu().

Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Reviewed-by: Guillaume Nault <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/mpls/af_mpls.c

···
 
 		if (i == RTA_OIF) {
 			ifindex = nla_get_u32(tb[i]);
-			filter->dev = __dev_get_by_index(net, ifindex);
+			filter->dev = dev_get_by_index_rcu(net, ifindex);
 			if (!filter->dev)
 				return -ENODEV;
 			filter->filter_set = 1;
···
 	struct net *net = sock_net(skb->sk);
 	struct mpls_route __rcu **platform_label;
 	struct fib_dump_filter filter = {
-		.rtnl_held = true,
+		.rtnl_held = false,
 	};
 	unsigned int flags = NLM_F_MULTI;
 	size_t platform_labels;
 	unsigned int index;
+	int err;
 
-	ASSERT_RTNL();
+	rcu_read_lock();
 
 	if (cb->strict_check) {
-		int err;
-
 		err = mpls_valid_fib_dump_req(net, nlh, &filter, cb);
 		if (err < 0)
-			return err;
+			goto err;
 
 		/* for MPLS, there is only 1 table with fixed type and flags.
 		 * If either are set in the filter then return nothing.
···
 		if ((filter.table_id && filter.table_id != RT_TABLE_MAIN) ||
 		    (filter.rt_type && filter.rt_type != RTN_UNICAST) ||
 		     filter.flags)
-			return skb->len;
+			goto unlock;
 	}
 
 	index = cb->args[0];
 	if (index < MPLS_LABEL_FIRST_UNRESERVED)
 		index = MPLS_LABEL_FIRST_UNRESERVED;
 
-	platform_label = rtnl_dereference(net->mpls.platform_label);
+	platform_label = rcu_dereference(net->mpls.platform_label);
 	platform_labels = net->mpls.platform_labels;
 
 	if (filter.filter_set)
···
 	for (; index < platform_labels; index++) {
 		struct mpls_route *rt;
 
-		rt = rtnl_dereference(platform_label[index]);
+		rt = rcu_dereference(platform_label[index]);
 		if (!rt)
 			continue;
 
···
 	}
 	cb->args[0] = index;
 
+unlock:
+	rcu_read_unlock();
 	return skb->len;
+
+err:
+	rcu_read_unlock();
+	return err;
 }
 
 static inline size_t lfib_nlmsg_size(struct mpls_route *rt)
···
 static const struct rtnl_msg_handler mpls_rtnl_msg_handlers[] __initdata_or_module = {
 	{THIS_MODULE, PF_MPLS, RTM_NEWROUTE, mpls_rtm_newroute, NULL, 0},
 	{THIS_MODULE, PF_MPLS, RTM_DELROUTE, mpls_rtm_delroute, NULL, 0},
-	{THIS_MODULE, PF_MPLS, RTM_GETROUTE, mpls_getroute, mpls_dump_routes, 0},
+	{THIS_MODULE, PF_MPLS, RTM_GETROUTE, mpls_getroute, mpls_dump_routes,
+	 RTNL_FLAG_DUMP_UNLOCKED},
 	{THIS_MODULE, PF_MPLS, RTM_GETNETCONF,
 	 mpls_netconf_get_devconf, mpls_netconf_dump_devconf,
 	 RTNL_FLAG_DUMP_UNLOCKED},