arp: postpone addr_type calculation to as late as possible

The addr_type retrieval can be costly, so it's worth trying to avoid its
calculation as much as possible. This patch makes it calculated only
for gratuitous ARP packets. This is especially important since later we
may want to move is_garp calculation outside of arp_accept block, at
which point the costly operation will be executed for all setups.

The patch is the result of a discussion in net-dev:
http://marc.info/?l=linux-netdev&m=149506354216994Suggested-by: NJulian Anastasov <ja@ssi.bg>
Signed-off-by: NIhar Hrachyshka <ihrachys@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/arp.c

@@ -641,12 +641,12 @@ void arp_xmit(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(arp_xmit);
 
-static bool arp_is_garp(struct net_device *dev, int addr_type,
-			__be16 ar_op,
+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 && addr_type == RTN_UNICAST;
+	bool is_garp = tip == sip;
 
 	/* Gratuitous ARP _replies_ also require target hwaddr to be
 	 * the same as source.
@@ -659,6 +659,11 @@ static bool arp_is_garp(struct net_device *dev, int addr_type,
 			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;
 }
 
@@ -859,18 +864,23 @@ 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);
+		addr_type = -1;
 
 		/* Unsolicited ARP is not accepted by default.
 		   It is possible, that this option should be enabled for some
 		   devices (strip is candidate)
 		 */
-		is_garp = arp_is_garp(dev, addr_type, arp->ar_op,
+		is_garp = arp_is_garp(net, dev, &addr_type, arp->ar_op,
 				      sip, tip, sha, tha);
 
 		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);
 	}