netdev - [PATCH net-next] Exempt multicast addresses from five-second neighbor lifetime

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Date:   Mon, 2 Nov 2020 12:47:56 -0500
From:   Jeff Dike <jdike@...mai.com>
To:     <netdev@...r.kernel.org>
CC:     David Ahern <dsahern@...il.com>, Jeff Dike <jdike@...mai.com>
Subject: [PATCH net-next] Exempt multicast addresses from five-second neighbor lifetime

Commit 58956317c8de ("neighbor: Improve garbage collection")
guarantees neighbour table entries a five-second lifetime.  Processes
which make heavy use of multicast can fill the neighour table with
multicast addresses in five seconds.  At that point, neighbour entries
can't be GC-ed because they aren't five seconds old yet, the kernel
log starts to fill up with "neighbor table overflow!" messages, and
sends start to fail.

This patch allows multicast addresses to be thrown out before they've
lived out their five seconds.  This makes room for non-multicast
addresses and makes messages to all addresses more reliable in these
circumstances.

Signed-off-by: Jeff Dike <jdike@...mai.com>
---
 include/net/neighbour.h | 1 +
 net/core/neighbour.c    | 2 ++
 net/ipv4/arp.c          | 8 ++++++++
 net/ipv6/ndisc.c        | 7 +++++++
 4 files changed, 18 insertions(+)

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..1032ad76a3f5 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -125,6 +125,7 @@ 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 +157,7 @@ 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 +930,12 @@ static void parp_redo(struct sk_buff *skb)
 	arp_process(dev_net(skb->dev), NULL, skb);
 }
 
+#define IN_MULTICAST(a) ((((long)(a)) & 0xf0000000) == 0xe0000000)
+
+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..6aed5536fc5c 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);
-- 
2.17.1