ipv4: do not cache looped multicasts

David, this looks like applicable to the stable releases.

Starting from 3.6 we cache output routes for
multicasts only when using route to 224/4. For local receivers we can set RTCF_LOCAL flag depending on the membership but in such case we use maddr and saddr which are not caching keys as before. Additionally, we can not use same place to cache routes that differ in RTCF_LOCAL flag value.

Fix it by caching only RTCF_MULTICAST entries
without RTCF_LOCAL (send-only, no loopback). As a side effect, we avoid unneeded lookup for fnhe when not caching because multicasts are not redirected and they do not learn PMTU.

Thanks to Maxime Bizon for showing the caching
problems in __mkroute_output for 3.6 kernels: different RTCF_LOCAL flag in cache can lead to wrong ip_mc_output or ip_output call and the visible problem is that traffic can not reach local receivers via loopback.

Upstream-ID: 636174219b52b5a8bc51bc23bbcba97cd30a65e3
Stable-trees: 3.6.x

index 200d287..df25142 100644
@@ -1785,6 +1785,7 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
 	if (dev_out->flags & IFF_LOOPBACK)
 		flags |= RTCF_LOCAL;
 
+	do_cache = true;
 	if (type == RTN_BROADCAST) {
 		flags |= RTCF_BROADCAST | RTCF_LOCAL;
 		fi = NULL;

@@ -1793,6 +1794,8 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
 		if (!ip_check_mc_rcu(in_dev, fl4->daddr, fl4->saddr,
 				     fl4->flowi4_proto))
 			flags &= ~RTCF_LOCAL;
+		else
+			do_cache = false;
 		/* If multicast route do not exist use
 		 * default one, but do not gateway in this case.
 		 * Yes, it is hack.

@@ -1802,8 +1805,8 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
 	}
 
 	fnhe = NULL;
-	do_cache = fi != NULL;
-	if (fi) {
+	do_cache &= fi != NULL;
+	if (do_cache) {
 		struct rtable __rcu **prth;
 		struct fib_nh *nh = &FIB_RES_NH(*res);

Please stop submitting networking -stable patches.

Thank you.