macvlan: Support creating macvlans from macvlans

When running in a network namespace whose only link to
the outside world is a macvlan device, not being
able to create another macvlan is a real pain.

So modify macvlan creation to allow automatically forward
a creation of a macvlan on a macvlan to become a creation
of a macvlan on the underlying network device.
Signed-off-by: NEric Biederman <ebiederm@aristanetworks.com>
Acked-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/macvlan.c

@@ -461,12 +461,13 @@ static int macvlan_newlink(struct net_device *dev,
 	if (lowerdev == NULL)
 		return -ENODEV;
 
-	/* Don't allow macvlans on top of other macvlans - its not really
-	 * wrong, but lockdep can't handle it and its not useful for anything
-	 * you couldn't do directly on top of the real device.
+	/* When creating macvlans on top of other macvlans - use
+	 * the real device as the lowerdev.
 	 */
-	if (lowerdev->rtnl_link_ops == dev->rtnl_link_ops)
-		return -ENODEV;
+	if (lowerdev->rtnl_link_ops == dev->rtnl_link_ops) {
+		struct macvlan_dev *lowervlan = netdev_priv(lowerdev);
+		lowerdev = lowervlan->lowerdev;
+	}
 
 	if (!tb[IFLA_MTU])
 		dev->mtu = lowerdev->mtu;