[RFC] Exempt multicast address from five-second neighbor lifetime

[Jeff Dike]

Commit 58956317c8de 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 <jd...@akamai.com>

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index 81ee17594c32..22ced1381ede 100644
--- a/include/net/neighbour.h
+++ b/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;
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 8e39e28b0a8d..9500d28a43b0 100644
--- a/net/core/neighbour.c
+++ b/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);
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index 687971d83b4e..110d6d408edc 100644
--- a/net/ipv4/arp.c
+++ b/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.
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 27f29b957ee7..b42c9314cc4e 100644
--- a/net/ipv6/ndisc.c
+++ b/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);