rtnetlink: allow to register ops without ops->setup set

So far, it is assumed that ops->setup is filled up. But there might be
case that ops might make sense even without ->setup. In that case,
forbid to newlink and dellink.

This allows to register simple rtnl link ops containing only ->kind.
That allows consistent way of passing device kind (either device-kind or
slave-kind) to userspace.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -7091,7 +7091,7 @@ static void __net_exit default_device_exit_batch(struct list_head *net_list)
 	rtnl_lock_unregistering(net_list);
 	list_for_each_entry(net, net_list, exit_list) {
 		for_each_netdev_reverse(net, dev) {
-			if (dev->rtnl_link_ops)
+			if (dev->rtnl_link_ops && dev->rtnl_link_ops->dellink)
 				dev->rtnl_link_ops->dellink(dev, &dev_kill_list);
 			else
 				unregister_netdevice_queue(dev, &dev_kill_list);

net/core/rtnetlink.c

@@ -299,7 +299,12 @@ int __rtnl_link_register(struct rtnl_link_ops *ops)
 	if (rtnl_link_ops_get(ops->kind))
 		return -EEXIST;
 
-	if (!ops->dellink)
+	/* The check for setup is here because if ops
+	 * does not have that filled up, it is not possible
+	 * to use the ops for creating device. So do not
+	 * fill up dellink as well. That disables rtnl_dellink.
+	 */
+	if (ops->setup && !ops->dellink)
 		ops->dellink = unregister_netdevice_queue;
 
 	list_add_tail(&ops->list, &link_ops);
@@ -1777,7 +1782,7 @@ static int rtnl_dellink(struct sk_buff *skb, struct nlmsghdr *nlh)
 		return -ENODEV;
 
 	ops = dev->rtnl_link_ops;
-	if (!ops)
+	if (!ops || !ops->dellink)
 		return -EOPNOTSUPP;
 
 	ops->dellink(dev, &list_kill);
@@ -2038,6 +2043,9 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh)
 			return -EOPNOTSUPP;
 		}
 
+		if (!ops->setup)
+			return -EOPNOTSUPP;
+
 		if (!ifname[0])
 			snprintf(ifname, IFNAMSIZ, "%s%%d", ops->kind);