net: dev: Add NETDEV_PRE_CHANGEADDR

The NETDEV_CHANGEADDR notification is emitted after a device address
changes. Extending this message to allow vetoing is certainly possible,
but several other notification types have instead adopted a simple
two-stage approach: first a "pre" notification is sent to make sure all
interested parties are OK with a change that's about to be done. Then
the change is done, and afterwards a "post" notification is sent.

This dual approach is easier to use: when the change is vetoed, nothing
has changed yet, and it's therefore unnecessary to roll anything back.
Therefore adopt it for NETDEV_CHANGEADDR as well.

To that end, add NETDEV_PRE_CHANGEADDR and an info structure to go along
with it.
Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2450,7 +2450,8 @@ enum netdev_cmd {
 	NETDEV_REGISTER,
 	NETDEV_UNREGISTER,
 	NETDEV_CHANGEMTU,	/* notify after mtu change happened */
-	NETDEV_CHANGEADDR,
+	NETDEV_CHANGEADDR,	/* notify after the address change */
+	NETDEV_PRE_CHANGEADDR,	/* notify before the address change */
 	NETDEV_GOING_DOWN,
 	NETDEV_CHANGENAME,
 	NETDEV_FEAT_CHANGE,
@@ -2512,6 +2513,11 @@ struct netdev_notifier_changelowerstate_info {
 	void *lower_state_info; /* is lower dev state */
 };
 
+struct netdev_notifier_pre_changeaddr_info {
+	struct netdev_notifier_info info; /* must be first */
+	const unsigned char *dev_addr;
+};
+
 static inline void netdev_notifier_info_init(struct netdev_notifier_info *info,
 					     struct net_device *dev)
 {

net/core/dev.c

@@ -1589,6 +1589,7 @@ const char *netdev_cmd_to_name(enum netdev_cmd cmd)
 	N(UDP_TUNNEL_DROP_INFO) N(CHANGE_TX_QUEUE_LEN)
 	N(CVLAN_FILTER_PUSH_INFO) N(CVLAN_FILTER_DROP_INFO)
 	N(SVLAN_FILTER_PUSH_INFO) N(SVLAN_FILTER_DROP_INFO)
+	N(PRE_CHANGEADDR)
 	}
 #undef N
 	return "UNKNOWN_NETDEV_EVENT";