R: Re: BUG (?) multicast loopback (IP6SKB_FORWARDED)

Hi Eric,
thanks for the quick reply. It seems that reverting the patch fixes the issue, 
and I have not observed any unintended behaviour so far.
Do you know what was the motivation for the patch?

Regards,
Massimiliano

----Messaggio originale----
Da: eric.dumazet@gmail.com
Data: 08/06/2012 19.23
A: "maxd@inwind.it"[off-list ref]
Cc: <redacted>
Ogg: Re: BUG (?) multicast loopback (IP6SKB_FORWARDED)

On Fri, 2012-06-08 at 18:48 +0200, maxd@inwind.it wrote:

quoted

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 

quoted

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 

quoted

multicast router. The router receives the message on one interface and 

needs to 

quoted

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.

quoted

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 

quoted

has been already forwarded. Since the flag has been cleared, the kernel 

cannot 

quoted

determine that the packet has been looped back, and will hence try to 

forward 

quoted

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 

quoted

has been received from a wrong interface (line 1993 in ip6mr.c), discard 

the 

quoted

message (this explains why the packet does not loop forever) and triggers 

the 

quoted

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?

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



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