net: use loopback dev when generating blackhole route

Existing ipv4/6_blackhole_route() code generates a blackhole route
with dst->dev pointing to the passed in dst->dev.
It is not necessary to hold reference to the passed in dst->dev
because the packets going through this route are dropped anyway.
A loopback interface is good enough so that we don't need to worry about
releasing this dst->dev when this dev is going down.

Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -2504,7 +2504,7 @@ struct dst_entry *ipv4_blackhole_route(struct net *net, struct dst_entry *dst_or
 		new->input = dst_discard;
 		new->output = dst_discard_out;
 
-		new->dev = ort->dst.dev;
+		new->dev = net->loopback_dev;
 		if (new->dev)
 			dev_hold(new->dev);
 

net/ipv6/route.c

@@ -1245,9 +1245,12 @@ EXPORT_SYMBOL_GPL(ip6_route_output_flags);
 struct dst_entry *ip6_blackhole_route(struct net *net, struct dst_entry *dst_orig)
 {
 	struct rt6_info *rt, *ort = (struct rt6_info *) dst_orig;
+	struct net_device *loopback_dev = net->loopback_dev;
 	struct dst_entry *new = NULL;
 
-	rt = dst_alloc(&ip6_dst_blackhole_ops, ort->dst.dev, 1, DST_OBSOLETE_NONE, 0);
+
+	rt = dst_alloc(&ip6_dst_blackhole_ops, loopback_dev, 1,
+		       DST_OBSOLETE_NONE, 0);
 	if (rt) {
 		rt6_info_init(rt);
 
@@ -1257,10 +1260,8 @@ struct dst_entry *ip6_blackhole_route(struct net *net, struct dst_entry *dst_ori
 		new->output = dst_discard_out;
 
 		dst_copy_metrics(new, &ort->dst);
-		rt->rt6i_idev = ort->rt6i_idev;
-		if (rt->rt6i_idev)
-			in6_dev_hold(rt->rt6i_idev);
 
+		rt->rt6i_idev = in6_dev_get(loopback_dev);
 		rt->rt6i_gateway = ort->rt6i_gateway;
 		rt->rt6i_flags = ort->rt6i_flags & ~RTF_PCPU;
 		rt->rt6i_metric = 0;