net: Fix userspace RTM_NEWLINK notifications.

I received some bug reports about userspace programs having problems
because after RTM_NEWLINK was received they could not immediate access
files under /proc/sys/net/ because they had not been registered yet.

The original problem was trivially fixed by moving the userspace
notification from rtnetlink_event() to the end of
register_netdevice().

When testing that change I discovered I was still getting RTM_NEWLINK
events before I could access proc and I was also getting RTM_NEWLINK
events after I was seeing RTM_DELLINK.  Things practically guaranteed
to confuse userspace.

After a little more investigation these extra notifications proved to
be from the new notifiers NETDEV_POST_INIT and NETDEV_UNREGISTER_BATCH
hitting the default case in rtnetlink_event, and triggering
unnecessary RTM_NEWLINK messages.

rtnetlink_event now explicitly handles NETDEV_UNREGISTER_BATCH and
NETDEV_POST_INIT to avoid sending the incorrect userspace
notifications.
Signed-off-by: NEric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -5035,6 +5035,11 @@ int register_netdevice(struct net_device *dev)
 		rollback_registered(dev);
 		dev->reg_state = NETREG_UNREGISTERED;
 	}
+	/*
+	 *	Prevent userspace races by waiting until the network
+	 *	device is fully setup before sending notifications.
+	 */
+	rtmsg_ifinfo(RTM_NEWLINK, dev, ~0U);
 
 out:
 	return ret;
@@ -5597,6 +5602,12 @@ int dev_change_net_namespace(struct net_device *dev, struct net *net, const char
 	/* Notify protocols, that a new device appeared. */
 	call_netdevice_notifiers(NETDEV_REGISTER, dev);
 
+	/*
+	 *	Prevent userspace races by waiting until the network
+	 *	device is fully setup before sending notifications.
+	 */
+	rtmsg_ifinfo(RTM_NEWLINK, dev, ~0U);
+
 	synchronize_net();
 	err = 0;
 out:

net/core/rtnetlink.c

@@ -1364,15 +1364,15 @@ static int rtnetlink_event(struct notifier_block *this, unsigned long event, voi
 	case NETDEV_UNREGISTER:
 		rtmsg_ifinfo(RTM_DELLINK, dev, ~0U);
 		break;
-	case NETDEV_REGISTER:
-		rtmsg_ifinfo(RTM_NEWLINK, dev, ~0U);
-		break;
 	case NETDEV_UP:
 	case NETDEV_DOWN:
 		rtmsg_ifinfo(RTM_NEWLINK, dev, IFF_UP|IFF_RUNNING);
 		break;
+	case NETDEV_POST_INIT:
+	case NETDEV_REGISTER:
 	case NETDEV_CHANGE:
 	case NETDEV_GOING_DOWN:
+	case NETDEV_UNREGISTER_BATCH:
 		break;
 	default:
 		rtmsg_ifinfo(RTM_NEWLINK, dev, 0);