arp: allow to invalidate specific ARP entries

IPv4 over firewire needs to be able to remove ARP entries
from the ARP cache that belong to nodes that are removed, because
IPv4 over firewire uses ARP packets for private information
about nodes.

This information becomes invalid as soon as node drops
off the bus and when it reconnects, its only possible
to start talking to it after it responded to an ARP packet.
But ARP cache prevents such packets from being sent.
Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/arp.h

@@ -25,5 +25,6 @@ extern struct sk_buff *arp_create(int type, int ptype, __be32 dest_ip,
 				  const unsigned char *src_hw,
 				  const unsigned char *target_hw);
 extern void arp_xmit(struct sk_buff *skb);
+int arp_invalidate(struct net_device *dev, __be32 ip);
 
 #endif	/* _ARP_H */

net/ipv4/arp.c

@@ -1143,6 +1143,23 @@ static int arp_req_get(struct arpreq *r, struct net_device *dev)
 	return err;
 }
 
+int arp_invalidate(struct net_device *dev, __be32 ip)
+{
+	struct neighbour *neigh = neigh_lookup(&arp_tbl, &ip, dev);
+	int err = -ENXIO;
+
+	if (neigh) {
+		if (neigh->nud_state & ~NUD_NOARP)
+			err = neigh_update(neigh, NULL, NUD_FAILED,
+					   NEIGH_UPDATE_F_OVERRIDE|
+					   NEIGH_UPDATE_F_ADMIN);
+		neigh_release(neigh);
+	}
+
+	return err;
+}
+EXPORT_SYMBOL(arp_invalidate);
+
 static int arp_req_delete_public(struct net *net, struct arpreq *r,
 		struct net_device *dev)
 {
@@ -1163,7 +1180,6 @@ static int arp_req_delete(struct net *net, struct arpreq *r,
 {
 	int err;
 	__be32 ip;
-	struct neighbour *neigh;
 
 	if (r->arp_flags & ATF_PUBL)
 		return arp_req_delete_public(net, r, dev);
@@ -1181,16 +1197,7 @@ static int arp_req_delete(struct net *net, struct arpreq *r,
 		if (!dev)
 			return -EINVAL;
 	}
-	err = -ENXIO;
-	neigh = neigh_lookup(&arp_tbl, &ip, dev);
-	if (neigh) {
-		if (neigh->nud_state & ~NUD_NOARP)
-			err = neigh_update(neigh, NULL, NUD_FAILED,
-					   NEIGH_UPDATE_F_OVERRIDE|
-					   NEIGH_UPDATE_F_ADMIN);
-		neigh_release(neigh);
-	}
-	return err;
+	return arp_invalidate(dev, ip);
 }
 
 /*