BUG (?) multicast loopback (IP6SKB_FORWARDED)

Hi guys,
I found a probably wrong behaviour while doing some tests with multicast routing on IPv6 with kernel 2.6.29. I will try to describe what's wrong in the code in the following. I will use the latest kernel sources (3.5-rc1) as reference source code (line numbers are taken there).
Let's assume a scenario with a node with two network interfaces acting as a multicast router. The router receives the message on one interface and needs to forward it on the other interface. Looking at the packet flow inside the kernel, we notice that

in ip6mr.c, line 2282, a flag is set:
IP6CB(skb)->flags |= IP6SKB_FORWARDED;

After this, a multicast packet can be looped back (see line 124 in ip6_output. c where function ip6_dev_loopback_xmit is called).
The packet is hence reinjected in the stack.

The packet is processed by function ipv6_rcv (ip6_input.c), and then by ipv6_mc_input (ip6_input.c).

In ipv6_rcv, line 82, the previously set flag is cleared memset(IP6CB(skb), 0, sizeof(struct inet6_skb_parm));

In ipv6_mc_input, , line 268, the flag is checked to determine if the packet has been already forwarded. Since the flag has been cleared, the kernel cannot determine that the packet has been looped back, and will hence try to forward it again.

Trying to forward a looped back packet determines a wrong behaviour of the multicast routing protocol (PIM): the kernel believes that a multicast message has been received from a wrong interface (line 1993 in ip6mr.c), discard the message (this explains why the packet does not loop forever) and triggers the transmission of an ASSERT message. Basically, the node ends up sending an ASSERT message because of a looped back packet.

WDYT? Is my analysis correct? Which is the best way to fix this issue?

Thanks,
Massimiliano D'Angelo

I guess your analysis is correct, try to revert commit 6b7fdc3ae18a0598a999156b62d55ea55220e00f ([IPV6]: Clean skb cb on IPv6 input) ?