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

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Date:	Sun, 22 Mar 2015 14:12:59 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Robert Shearman <rshearma@...cade.com>
Cc:	<davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 2/5] mpls: Remove incorrect PHP comment

Robert Shearman <rshearma@...cade.com> 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.

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.

Eric

> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> Signed-off-by: Robert Shearman <rshearma@...cade.com>
> ---
>  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 {
--
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