Exempt multicast address from five-second neighbor lifetime

Commit 5895631 guarantees arp table entries a five-second lifetime.  We have some apps which make heavy use of multicast, and these can cause the table to overflow by filling it with multicast addresses which can't be GC-ed until their five seconds are up.
This patch allows multicast addresses to be thrown out before they've lived out their five seconds.

Signed-off-by: Jeff Dike <jdike@akamai.com>

include/net/neighbour.h

@@ -204,6 +204,7 @@ struct neigh_table {
 	int			(*pconstructor)(struct pneigh_entry *);
 	void			(*pdestructor)(struct pneigh_entry *);
 	void			(*proxy_redo)(struct sk_buff *skb);
+	int			(*is_multicast)(const void *pkey);
 	bool			(*allow_add)(const struct net_device *dev,
 					     struct netlink_ext_ack *extack);
 	char			*id;

net/core/neighbour.c

@@ -235,6 +235,8 @@ static int neigh_forced_gc(struct neigh_table *tbl)
 
 			write_lock(&n->lock);
 			if ((n->nud_state == NUD_FAILED) ||
+			    (tbl->is_multicast &&
+			     tbl->is_multicast(n->primary_key)) ||
 			    time_after(tref, n->updated))
 				remove = true;
 			write_unlock(&n->lock);

net/ipv4/arp.c

@@ -79,6 +79,7 @@
 #include <linux/socket.h>
 #include <linux/sockios.h>
 #include <linux/errno.h>
+#define __UAPI_DEF_IN_CLASS 1
 #include <linux/in.h>
 #include <linux/mm.h>
 #include <linux/inet.h>
@@ -125,6 +126,7 @@ static int arp_constructor(struct neighbour *neigh);
 static void arp_solicit(struct neighbour *neigh, struct sk_buff *skb);
 static void arp_error_report(struct neighbour *neigh, struct sk_buff *skb);
 static void parp_redo(struct sk_buff *skb);
+static int arp_is_multicast(const void *pkey);
 
 static const struct neigh_ops arp_generic_ops = {
 	.family =		AF_INET,
@@ -156,6 +158,7 @@ struct neigh_table arp_tbl = {
 	.key_eq		= arp_key_eq,
 	.constructor	= arp_constructor,
 	.proxy_redo	= parp_redo,
+	.is_multicast   = arp_is_multicast,
 	.id		= "arp_cache",
 	.parms		= {
 		.tbl			= &arp_tbl,
@@ -928,6 +931,10 @@ static void parp_redo(struct sk_buff *skb)
 	arp_process(dev_net(skb->dev), NULL, skb);
 }
 
+static int arp_is_multicast(const void *pkey)
+{
+	return IN_MULTICAST(htonl(*((u32 *) pkey)));
+}
 
 /*
  *	Receive an arp request from the device layer.

net/ipv6/ndisc.c

@@ -81,6 +81,7 @@ static void ndisc_error_report(struct neighbour *neigh, struct sk_buff *skb);
 static int pndisc_constructor(struct pneigh_entry *n);
 static void pndisc_destructor(struct pneigh_entry *n);
 static void pndisc_redo(struct sk_buff *skb);
+static int ndisc_is_multicast(const void *pkey);
 
 static const struct neigh_ops ndisc_generic_ops = {
 	.family =		AF_INET6,
@@ -115,6 +116,7 @@ struct neigh_table nd_tbl = {
 	.pconstructor =	pndisc_constructor,
 	.pdestructor =	pndisc_destructor,
 	.proxy_redo =	pndisc_redo,
+	.is_multicast = ndisc_is_multicast,
 	.allow_add  =   ndisc_allow_add,
 	.id =		"ndisc_cache",
 	.parms = {
@@ -1706,6 +1708,11 @@ static void pndisc_redo(struct sk_buff *skb)
 	kfree_skb(skb);
 }
 
+static int ndisc_is_multicast(const void *pkey)
+{
+	return (((struct in6_addr *) pkey)->in6_u.u6_addr8[0] & 0xf0) == 0xf0;
+}
+
 static bool ndisc_suppress_frag_ndisc(struct sk_buff *skb)
 {
 	struct inet6_dev *idev = __in6_dev_get(skb->dev);