ipv6: Fix sporadic sendmsg -EINVAL when sending to multicast groups.

Thanks to excellent diagnosis by Eduard Guzovsky.

The core problem is that on a network with lots of active
multicast traffic, the neighbour cache can fill up.  If
we try to allocate a new route and thus neighbour cache
entry, the bog-standard GC attempt the neighbour layer does
in ineffective because route entries hold a reference
to the existing neighbour entries and GC can only liberate
entries with no references.

IPV4 already has a way to handle this, by doing a route cache
GC in such situations (when neigh attach returns -ENOBUFS).

So simply mimick this on the ipv6 side.

Tested-by: Eduard Guzovsky <eguzovsky@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 47 insertions, 5 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 18c486cf4987..76f06b94ab9f 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -627,6 +627,9 @@ static struct rt6_info *rt6_alloc_cow(struct rt6_info *ort, struct in6_addr *dad
         rt = ip6_rt_copy(ort);
 
         if (rt) {
+                struct neighbour *neigh;
+                int attempts = !in_softirq();
+
                 if (!(rt->rt6i_flags&RTF_GATEWAY)) {
                         if (rt->rt6i_dst.plen != 128 &&
                             ipv6_addr_equal(&rt->rt6i_dst.addr, daddr))
@@ -646,7 +649,35 @@ static struct rt6_info *rt6_alloc_cow(struct rt6_info *ort, struct in6_addr *dad
                 }
 #endif
 
-                rt->rt6i_nexthop = ndisc_get_neigh(rt->rt6i_dev, &rt->rt6i_gateway);
+        retry:
+                neigh = ndisc_get_neigh(rt->rt6i_dev, &rt->rt6i_gateway);
+                if (IS_ERR(neigh)) {
+                        struct net *net = dev_net(rt->rt6i_dev);
+                        int saved_rt_min_interval =
+                                net->ipv6.sysctl.ip6_rt_gc_min_interval;
+                        int saved_rt_elasticity =
+                                net->ipv6.sysctl.ip6_rt_gc_elasticity;
+
+                        if (attempts-- > 0) {
+                                net->ipv6.sysctl.ip6_rt_gc_elasticity = 1;
+                                net->ipv6.sysctl.ip6_rt_gc_min_interval = 0;
+
+                                ip6_dst_gc(net->ipv6.ip6_dst_ops);
+
+                                net->ipv6.sysctl.ip6_rt_gc_elasticity =
+                                        saved_rt_elasticity;
+                                net->ipv6.sysctl.ip6_rt_gc_min_interval =
+                                        saved_rt_min_interval;
+                                goto retry;
+                        }
+
+                        if (net_ratelimit())
+                                printk(KERN_WARNING
+                                       "Neighbour table overflow.\n");
+                        dst_free(&rt->u.dst);
+                        return NULL;
+                }
+                rt->rt6i_nexthop = neigh;
 
         }
 
@@ -945,8 +976,11 @@ struct dst_entry *icmp6_dst_alloc(struct net_device *dev,
         dev_hold(dev);
         if (neigh)
                 neigh_hold(neigh);
-        else
+        else {
                 neigh = ndisc_get_neigh(dev, addr);
+                if (IS_ERR(neigh))
+                        neigh = NULL;
+        }
 
         rt->rt6i_dev      = dev;
         rt->rt6i_idev     = idev;
@@ -1887,6 +1921,7 @@ struct rt6_info *addrconf_dst_alloc(struct inet6_dev *idev,
 {
         struct net *net = dev_net(idev->dev);
         struct rt6_info *rt = ip6_dst_alloc(net->ipv6.ip6_dst_ops);
+        struct neighbour *neigh;
 
         if (rt == NULL)
                 return ERR_PTR(-ENOMEM);
@@ -1909,11 +1944,18 @@ struct rt6_info *addrconf_dst_alloc(struct inet6_dev *idev,
                 rt->rt6i_flags |= RTF_ANYCAST;
         else
                 rt->rt6i_flags |= RTF_LOCAL;
-        rt->rt6i_nexthop = ndisc_get_neigh(rt->rt6i_dev, &rt->rt6i_gateway);
-        if (rt->rt6i_nexthop == NULL) {
+        neigh = ndisc_get_neigh(rt->rt6i_dev, &rt->rt6i_gateway);
+        if (IS_ERR(neigh)) {
                 dst_free(&rt->u.dst);
-                return ERR_PTR(-ENOMEM);
+
+                /* We are casting this because that is the return
+                 * value type.  But an errno encoded pointer is the
+                 * same regardless of the underlying pointer type,
+                 * and that's what we are returning.  So this is OK.
+                 */
+                return (struct rt6_info *) neigh;
         }
+        rt->rt6i_nexthop = neigh;
 
         ipv6_addr_copy(&rt->rt6i_dst.addr, addr);
         rt->rt6i_dst.plen = 128;