net: allow setting mac address of loopback device

We are trying to mirror the local traffic from lo to eth0,
allowing setting mac address of lo to eth0 would make
the ether addresses in these packets correct, so that
we don't have to modify the ether header again.

Since usually no one cares about its mac address (all-zero),
it is safe to allow those who care to set its mac address.

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: NCong Wang <xiyou.wangcong@gmail.com>
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/loopback.c

@@ -160,6 +160,7 @@ static const struct net_device_ops loopback_ops = {
 	.ndo_init      = loopback_dev_init,
 	.ndo_start_xmit= loopback_xmit,
 	.ndo_get_stats64 = loopback_get_stats64,
+	.ndo_set_mac_address = eth_mac_addr,
 };
 
 /*
@@ -174,6 +175,7 @@ static void loopback_setup(struct net_device *dev)
 	dev->tx_queue_len	= 0;
 	dev->type		= ARPHRD_LOOPBACK;	/* 0x0001*/
 	dev->flags		= IFF_LOOPBACK;
+	dev->priv_flags		|= IFF_LIVE_ADDR_CHANGE;
 	dev->priv_flags	       &= ~IFF_XMIT_DST_RELEASE;
 	dev->hw_features	= NETIF_F_ALL_TSO | NETIF_F_UFO;
 	dev->features 		= NETIF_F_SG | NETIF_F_FRAGLIST