[NETPOLL]: make arp replies through netpoll use mac address of sender

Back in 2.4 arp requests that were recevied by netpoll were processed
in netconsole_receive_skb, where they were responded to using the src
mac of the request sender.  In the 2.6 kernel arp_reply is responsible
for this function, but instead of using the src mac address of the
incomming request, the stored mac address that was registered for the
netconsole application is used.  While this is usually ok, it can lead
to failures in netpoll in some situations (specifically situations
where a network may have two gateways, as arp requests from one may be
responded to using the mac address of the other).  This patch reverts
the behavior to what we had in 2.4, in which all arp requests are sent
back using the src address of the request sender.
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Chris Lalancette <clalance@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -330,6 +330,7 @@ static void arp_reply(struct sk_buff *skb)
 	unsigned char *arp_ptr;
 	int size, type = ARPOP_REPLY, ptype = ETH_P_ARP;
 	__be32 sip, tip;
+	unsigned char *sha;
 	struct sk_buff *send_skb;
 	struct netpoll *np = NULL;
 
@@ -356,9 +357,14 @@ static void arp_reply(struct sk_buff *skb)
 	    arp->ar_op != htons(ARPOP_REQUEST))
 		return;
 
-	arp_ptr = (unsigned char *)(arp+1) + skb->dev->addr_len;
+	arp_ptr = (unsigned char *)(arp+1);
+	/* save the location of the src hw addr */
+	sha = arp_ptr;
+	arp_ptr += skb->dev->addr_len;
 	memcpy(&sip, arp_ptr, 4);
-	arp_ptr += 4 + skb->dev->addr_len;
+	arp_ptr += 4;
+	/* if we actually cared about dst hw addr, it would get copied here */
+	arp_ptr += skb->dev->addr_len;
 	memcpy(&tip, arp_ptr, 4);
 
 	/* Should we ignore arp? */
@@ -381,7 +387,7 @@ static void arp_reply(struct sk_buff *skb)
 
 	if (np->dev->hard_header &&
 	    np->dev->hard_header(send_skb, skb->dev, ptype,
-				 np->remote_mac, np->local_mac,
+				 sha, np->local_mac,
 				 send_skb->len) < 0) {
 		kfree_skb(send_skb);
 		return;
@@ -405,7 +411,7 @@ static void arp_reply(struct sk_buff *skb)
 	arp_ptr += np->dev->addr_len;
 	memcpy(arp_ptr, &tip, 4);
 	arp_ptr += 4;
-	memcpy(arp_ptr, np->remote_mac, np->dev->addr_len);
+	memcpy(arp_ptr, sha, np->dev->addr_len);
 	arp_ptr += np->dev->addr_len;
 	memcpy(arp_ptr, &sip, 4);