bridge: prevent bridging wrong device

The bridge code assumes ethernet addressing, so be more strict in
the what is allowed. This showed up when GRE had a bug and was not
using correct address format.

Add some more comments for increased clarity.
Signed-off-by: NStephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_if.c

@@ -377,12 +377,16 @@ int br_add_if(struct net_bridge *br, struct net_device *dev)
 	struct net_bridge_port *p;
 	int err = 0;
 
-	if (dev->flags & IFF_LOOPBACK || dev->type != ARPHRD_ETHER)
+	/* Don't allow bridging non-ethernet like devices */
+	if ((dev->flags & IFF_LOOPBACK) ||
+	    dev->type != ARPHRD_ETHER || dev->addr_len != ETH_ALEN)
 		return -EINVAL;
 
+	/* No bridging of bridges */
 	if (dev->netdev_ops->ndo_start_xmit == br_dev_xmit)
 		return -ELOOP;
 
+	/* Device is already being bridged */
 	if (dev->br_port != NULL)
 		return -EBUSY;