[netdevice] Avoid registering duplicate network devices

Reject network devices which appear to be duplicates of those already
available via a different underlying hardware device.  On a Xen PV-HVM
system, this allows us to filter out the emulated PCI NICs (which
would otherwise appear alongside the netfront NICs).

Note that we cannot use the Xen facility to "unplug" the emulated PCI
NICs, since there is no guarantee that the OS we subsequently load
will have a native netfront driver.

We permit devices with the same MAC address if they are attached to
the same underlying hardware device (e.g. VLAN devices).

Inspired-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/netdevice.h

 extern struct net_device * find_netdev_by_index ( unsigned int index );
 extern struct net_device * find_netdev_by_location ( unsigned int bus_type,
 						     unsigned int location );
+extern struct net_device *
+find_netdev_by_ll_addr ( struct ll_protocol *ll_protocol, const void *ll_addr );
 extern struct net_device * last_opened_netdev ( void );
 extern int net_tx ( struct io_buffer *iobuf, struct net_device *netdev,
 		    struct net_protocol *net_protocol, const void *ll_dest,

src/net/netdevice.c

 int register_netdev ( struct net_device *netdev ) {
 	struct ll_protocol *ll_protocol = netdev->ll_protocol;
 	struct net_driver *driver;
+	struct net_device *duplicate;
 	uint32_t seed;
 	int rc;
 
+	/* Set initial link-layer address, if not already set */
+	if ( ! netdev_has_ll_addr ( netdev ) ) {
+		ll_protocol->init_addr ( netdev->hw_addr, netdev->ll_addr );
+	}
+
+	/* Reject network devices that are already available via a
+	 * different hardware device.
+	 */
+	duplicate = find_netdev_by_ll_addr ( ll_protocol, netdev->ll_addr );
+	if ( duplicate && ( duplicate->dev != netdev->dev ) ) {
+		DBGC ( netdev, "NETDEV rejecting duplicate (phys %s) of %s "
+		       "(phys %s)\n", netdev->dev->name, duplicate->name,
+		       duplicate->dev->name );
+		rc = -EEXIST;
+		goto err_duplicate;
+	}
+
 	/* Record device index and create device name */
 	netdev->index = netdev_index++;
 	if ( netdev->name[0] == '\0' ) {
 			   netdev->index );
 	}
 
-	/* Set initial link-layer address, if not already set */
-	if ( ! netdev_has_ll_addr ( netdev ) ) {
-		ll_protocol->init_addr ( netdev->hw_addr, netdev->ll_addr );
-	}
-
 	/* Use least significant bits of the link-layer address to
 	 * improve the randomness of the (non-cryptographic) random
 	 * number generator.
 	clear_settings ( netdev_settings ( netdev ) );
 	unregister_settings ( netdev_settings ( netdev ) );
  err_register_settings:
+ err_duplicate:
 	return rc;
 }
 
 	return NULL;	
 }
 
+/**
+ * Get network device by link-layer address
+ *
+ * @v ll_protocol	Link-layer protocol
+ * @v ll_addr		Link-layer address
+ * @ret netdev		Network device, or NULL
+ */
+struct net_device * find_netdev_by_ll_addr ( struct ll_protocol *ll_protocol,
+					     const void *ll_addr ) {
+	struct net_device *netdev;
+
+	list_for_each_entry ( netdev, &net_devices, list ) {
+		if ( ( netdev->ll_protocol == ll_protocol ) &&
+		     ( memcmp ( netdev->ll_addr, ll_addr,
+				ll_protocol->ll_addr_len ) == 0 ) )
+			return netdev;
+	}
+
+	return NULL;
+}
+
 /**
  * Get most recently opened network device
  *