Merge branch 'arp-always-override-existing-neigh-entries-with-gratuitous-ARP'

Ihar Hrachyshka says:

====================
arp: always override existing neigh entries with gratuitous ARP

This patchset is spurred by discussion started at
https://patchwork.ozlabs.org/patch/760372/ where we figured that there is no
real reason for enforcing override by gratuitous ARP packets only when
arp_accept is 1. Same should happen when it's 0 (the default value).

changelog v2: handled review comments by Julian Anastasov
- fixed a mistake in a comment;
- postponed addr_type calculation to as late as possible.
====================
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/arp.c

@@ -641,6 +641,32 @@ void arp_xmit(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(arp_xmit);
 
+static bool arp_is_garp(struct net *net, struct net_device *dev,
+			int *addr_type, __be16 ar_op,
+			__be32 sip, __be32 tip,
+			unsigned char *sha, unsigned char *tha)
+{
+	bool is_garp = tip == sip;
+
+	/* Gratuitous ARP _replies_ also require target hwaddr to be
+	 * the same as source.
+	 */
+	if (is_garp && ar_op == htons(ARPOP_REPLY))
+		is_garp =
+			/* IPv4 over IEEE 1394 doesn't provide target
+			 * hardware address field in its ARP payload.
+			 */
+			tha &&
+			!memcmp(tha, sha, dev->addr_len);
+
+	if (is_garp) {
+		*addr_type = inet_addr_type_dev_table(net, dev, sip);
+		if (*addr_type != RTN_UNICAST)
+			is_garp = false;
+	}
+	return is_garp;
+}
+
 /*
  *	Process an arp request.
  */
@@ -837,29 +863,25 @@ static int arp_process(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 	n = __neigh_lookup(&arp_tbl, &sip, dev, 0);
 
-	if (IN_DEV_ARP_ACCEPT(in_dev)) {
-		unsigned int addr_type = inet_addr_type_dev_table(net, dev, sip);
+	if (n || IN_DEV_ARP_ACCEPT(in_dev)) {
+		addr_type = -1;
+		is_garp = arp_is_garp(net, dev, &addr_type, arp->ar_op,
+				      sip, tip, sha, tha);
+	}
 
+	if (IN_DEV_ARP_ACCEPT(in_dev)) {
 		/* Unsolicited ARP is not accepted by default.
 		   It is possible, that this option should be enabled for some
 		   devices (strip is candidate)
 		 */
-		is_garp = tip == sip && addr_type == RTN_UNICAST;
-
-		/* Unsolicited ARP _replies_ also require target hwaddr to be
-		 * the same as source.
-		 */
-		if (is_garp && arp->ar_op == htons(ARPOP_REPLY))
-			is_garp =
-				/* IPv4 over IEEE 1394 doesn't provide target
-				 * hardware address field in its ARP payload.
-				 */
-				tha &&
-				!memcmp(tha, sha, dev->addr_len);
-
 		if (!n &&
-		    ((arp->ar_op == htons(ARPOP_REPLY)  &&
-				addr_type == RTN_UNICAST) || is_garp))
+		    (is_garp ||
+		     (arp->ar_op == htons(ARPOP_REPLY) &&
+		      (addr_type == RTN_UNICAST ||
+		       (addr_type < 0 &&
+			/* postpone calculation to as late as possible */
+			inet_addr_type_dev_table(net, dev, sip) ==
+				RTN_UNICAST)))))
 			n = __neigh_lookup(&arp_tbl, &sip, dev, 1);
 	}