ipv6: give an IPv6 dev to blackhole_netdev

IPv6 addrconf notifiers wants the loopback device to
be the last device being dismantled at netns deletion.

This caused many limitations and work arounds.

Back in linux-5.3, Mahesh added a per host blackhole_netdev
that can be used whenever we need to make sure objects no longer
refer to a disappearing device.

If we attach to blackhole_netdev an ip6_ptr (allocate an idev),
then we can use this special device (which is never freed)
in place of the loopback_dev (which can be freed).

This will permit improvements in netdev_run_todo() and other parts
of the stack where had steps to make sure loopback_dev was
the last device to disappear.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -372,7 +372,7 @@ static struct inet6_dev *ipv6_add_dev(struct net_device *dev)
 
 	ASSERT_RTNL();
 
-	if (dev->mtu < IPV6_MIN_MTU)
+	if (dev->mtu < IPV6_MIN_MTU && dev != blackhole_netdev)
 		return ERR_PTR(-EINVAL);
 
 	ndev = kzalloc(sizeof(struct inet6_dev), GFP_KERNEL);
@@ -400,6 +400,7 @@ static struct inet6_dev *ipv6_add_dev(struct net_device *dev)
 	/* We refer to the device */
 	dev_hold_track(dev, &ndev->dev_tracker, GFP_KERNEL);
 
+	if (dev != blackhole_netdev) {
 		if (snmp6_alloc_dev(ndev) < 0) {
 			netdev_dbg(dev, "%s: cannot allocate memory for statistics\n",
 				   __func__);
@@ -414,7 +415,7 @@ static struct inet6_dev *ipv6_add_dev(struct net_device *dev)
 				   __func__, dev->name);
 			goto err_release;
 		}
-
+	}
 	/* One reference from device. */
 	refcount_set(&ndev->refcnt, 1);
 
@@ -445,15 +446,18 @@ static struct inet6_dev *ipv6_add_dev(struct net_device *dev)
 
 	ipv6_mc_init_dev(ndev);
 	ndev->tstamp = jiffies;
+	if (dev != blackhole_netdev) {
 		err = addrconf_sysctl_register(ndev);
 		if (err) {
 			ipv6_mc_destroy_dev(ndev);
 			snmp6_unregister_dev(ndev);
 			goto err_release;
 		}
+	}
 	/* protected by rtnl_lock */
 	rcu_assign_pointer(dev->ip6_ptr, ndev);
 
+	if (dev != blackhole_netdev) {
 		/* Join interface-local all-node multicast group */
 		ipv6_dev_mc_inc(dev, &in6addr_interfacelocal_allnodes);
 
@@ -463,7 +467,7 @@ static struct inet6_dev *ipv6_add_dev(struct net_device *dev)
 		/* Join all-router multicast group if forwarding is set */
 		if (ndev->cnf.forwarding && (dev->flags & IFF_MULTICAST))
 			ipv6_dev_mc_inc(dev, &in6addr_linklocal_allrouters);
-
+	}
 	return ndev;
 
 err_release:
@@ -7233,26 +7237,8 @@ int __init addrconf_init(void)
 		goto out_nowq;
 	}
 
-	/* The addrconf netdev notifier requires that loopback_dev
-	 * has it's ipv6 private information allocated and setup
-	 * before it can bring up and give link-local addresses
-	 * to other devices which are up.
-	 *
-	 * Unfortunately, loopback_dev is not necessarily the first
-	 * entry in the global dev_base list of net devices.  In fact,
-	 * it is likely to be the very last entry on that list.
-	 * So this causes the notifier registry below to try and
-	 * give link-local addresses to all devices besides loopback_dev
-	 * first, then loopback_dev, which cases all the non-loopback_dev
-	 * devices to fail to get a link-local address.
-	 *
-	 * So, as a temporary fix, allocate the ipv6 structure for
-	 * loopback_dev first by hand.
-	 * Longer term, all of the dependencies ipv6 has upon the loopback
-	 * device and it being up should be removed.
-	 */
 	rtnl_lock();
-	idev = ipv6_add_dev(init_net.loopback_dev);
+	idev = ipv6_add_dev(blackhole_netdev);
 	rtnl_unlock();
 	if (IS_ERR(idev)) {
 		err = PTR_ERR(idev);

net/ipv6/route.c

@@ -156,14 +156,10 @@ void rt6_uncached_list_del(struct rt6_info *rt)
 	}
 }
 
-static void rt6_uncached_list_flush_dev(struct net *net, struct net_device *dev)
+static void rt6_uncached_list_flush_dev(struct net_device *dev)
 {
-	struct net_device *loopback_dev = net->loopback_dev;
 	int cpu;
 
-	if (dev == loopback_dev)
-		return;
-
 	for_each_possible_cpu(cpu) {
 		struct uncached_list *ul = per_cpu_ptr(&rt6_uncached_list, cpu);
 		struct rt6_info *rt;
@@ -174,7 +170,7 @@ static void rt6_uncached_list_flush_dev(struct net *net, struct net_device *dev)
 			struct net_device *rt_dev = rt->dst.dev;
 
 			if (rt_idev->dev == dev) {
-				rt->rt6i_idev = in6_dev_get(loopback_dev);
+				rt->rt6i_idev = in6_dev_get(blackhole_netdev);
 				in6_dev_put(rt_idev);
 			}
 
@@ -371,13 +367,12 @@ static void ip6_dst_ifdown(struct dst_entry *dst, struct net_device *dev,
 {
 	struct rt6_info *rt = (struct rt6_info *)dst;
 	struct inet6_dev *idev = rt->rt6i_idev;
-	struct net_device *loopback_dev =
-		dev_net(dev)->loopback_dev;
 
-	if (idev && idev->dev != loopback_dev) {
-		struct inet6_dev *loopback_idev = in6_dev_get(loopback_dev);
-		if (loopback_idev) {
-			rt->rt6i_idev = loopback_idev;
+	if (idev && idev->dev != blackhole_netdev) {
+		struct inet6_dev *blackhole_idev = in6_dev_get(blackhole_netdev);
+
+		if (blackhole_idev) {
+			rt->rt6i_idev = blackhole_idev;
 			in6_dev_put(idev);
 		}
 	}
@@ -4892,7 +4887,7 @@ void rt6_sync_down_dev(struct net_device *dev, unsigned long event)
 void rt6_disable_ip(struct net_device *dev, unsigned long event)
 {
 	rt6_sync_down_dev(dev, event);
-	rt6_uncached_list_flush_dev(dev_net(dev), dev);
+	rt6_uncached_list_flush_dev(dev);
 	neigh_ifdown(&nd_tbl, dev);
 }