Subject: Re: [PATCH net-next v2 2/5] mpls: Remove incorrect PHP comment

From: Robert Shearman <[email-masked]>
Date: Mon, 23 Mar 2015 11:32:25 +0000

On 22/03/15 19:12, Eric W. Biederman wrote:
> Robert Shearman <[email-masked]> writes:
>
>> Popping the last label on the stack does not necessarily imply
>> performing penultimate hop popping. There is no reason why this
>> couldn't be the last hop in the network, so remove the comment.
>
> So this change I will disagree with.
>
> What the code implements is Penultimate hop popping.  Even if you send
> the packets over loopback that is what the code is doing.

No, RFC3031 s3.16 (https://tools.ietf.org/html/rfc3031#page-18) talks in 
terms of LSRs (label switch routers), not passes through the forwarding 
code.

> This is relevant because I think the code may actually be wrong in the
> local reception case.  By preforming penultimate hop popping and
> receving the code on loopback I think this code allows bypassing
> iptables rules that apply to incoming ip packets.  Certainly there is a
> loss of information as to which hardware interface the packet came in on
> that it may be desirable to correct.

Indeed, but network operators may well want to apply different rules to 
traffic coming in as IP versus traffic coming in as MPLS.

This may well merit a comment of its own, but this isn't directly 
relevant to the comment I'm removing.

Thanks,
Rob

>
> Eric
>
>
>> Cc: "Eric W. Biederman" <[email-masked]>
>> Signed-off-by: Robert Shearman <[email-masked]>
>> ---
>>   net/mpls/af_mpls.c | 1 -
>>   1 file changed, 1 deletion(-)
>>
>> diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
>> index 0d6763a..bf3459a 100644
>> --- a/net/mpls/af_mpls.c
>> +++ b/net/mpls/af_mpls.c
>> @@ -199,7 +199,6 @@ static int mpls_forward(struct sk_buff *skb, struct net_device *dev,
>>   	skb->protocol = htons(ETH_P_MPLS_UC);
>>
>>   	if (unlikely(!new_header_size && dec.bos)) {
>> -		/* Penultimate hop popping */
>>   		if (!mpls_egress(rt, skb, dec))
>>   			goto drop;
>>   	} else {