[PATCH net-next v2 1/2] ipv6: check mc_forwarding before calling ip6mr_get_route()

[Wenbo Sun]

When CONFIG_IPV6_MROUTE is enabled, rt6_fill_node() calls
ip6mr_get_route() whenever an input interface is specified and the
destination address is multicast.

However, when multicast routing is not enabled in the network namespace
(i.e. mc_forwarding is 0), there is typically no matching (S,G)
multicast routing cache entry, and skb->dev is NULL in the query skb
allocated by inet6_rtm_getroute(). As a result, ip6mr_get_route()
returns -ENODEV, which causes rt6_fill_node() to jump to
nla_put_failure and report -EMSGSIZE ("RTNETLINK answers: Message too
long") to userspace.

In contrast, the IPv4 counterpart rt_fill_info() checks
IPV4_DEVCONF_ALL_RO(net, MC_FORWARDING) before calling ipmr_get_route(),
so queries with mc_forwarding disabled fall back to emitting RTA_IIF and
normal route attributes.

This discrepancy was identified with the assistance of an LLM-based code
review comparing IPv4 and IPv6 multicast route get handling, confirmed on
a UML kernel where "ip -6 route get <mcast_addr> from <src_addr> iif
<dev>" returned -EMSGSIZE on an unpatched kernel, and verified after the
change that multicast route get with iif succeeds while unicast and
non-iif multicast queries continue to work as expected.

Check atomic_read(&net->ipv6.devconf_all->mc_forwarding) before calling
ip6mr_get_route() in rt6_fill_node() so that queries with multicast
forwarding disabled report RTA_IIF and normal route attributes, matching
IPv4 behavior.

Built with W=1 on UML, x86_64 allmodconfig, and x86_64 allyesconfig with
zero new warnings or errors.

Signed-off-by: Wenbo Sun <sunwenbo@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Hangbin Liu <liuhangbin@xxxxxxxxxx>
---
 net/ipv6/route.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 2eb884558c24..6dd4a70f6490 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -5862,7 +5862,8 @@ static int rt6_fill_node(struct net *net, struct sk_buff *skb,
 #endif
 	if (iif) {
 #ifdef CONFIG_IPV6_MROUTE
-		if (ipv6_addr_is_multicast(&rt6_dst->addr)) {
+		if (ipv6_addr_is_multicast(&rt6_dst->addr) &&
+		    atomic_read(&net->ipv6.devconf_all->mc_forwarding)) {
 			int err = ip6mr_get_route(net, skb, rtm, portid);
 
 			if (err == 0)
-- 
2.33.0