linux-kernel - Re: [PATCH v1 2/2] ipv6: always accept routing headers with 0 segments left

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Date: Tue, 25 Jun 2024 21:45:18 -0400
From: Alexander Aring <aahringo@...hat.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: mathis.marion@...abs.com, alex.aring@...il.com, davem@...emloft.net, 
	dsahern@...nel.org, edumazet@...gle.com, jerome.pouiller@...abs.com, 
	kuba@...nel.org, kylian.balan@...abs.com, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, pabeni@...hat.com, 
	Michael Richardson <mcr@...delman.ca>
Subject: Re: [PATCH v1 2/2] ipv6: always accept routing headers with 0
 segments left

Hi,

On Tue, Jun 25, 2024 at 5:39 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From: Mathis Marion <Mathis.Marion@...abs.com>
> Date: Mon, 24 Jun 2024 16:15:33 +0200
> > From: Mathis Marion <mathis.marion@...abs.com>
> >
> > Routing headers of type 3 and 4 would be rejected even if segments left
> > was 0, in the case that they were disabled through system configuration.
> >
> > RFC 8200 section 4.4 specifies:
> >
> >       If Segments Left is zero, the node must ignore the Routing header
> >       and proceed to process the next header in the packet, whose type
> >       is identified by the Next Header field in the Routing header.
>
> I think this part is only applied to an unrecognized Routing Type,
> so only applied when the network stack does not know the type.
>
>    https://www.rfc-editor.org/rfc/rfc8200.html#section-4.4
>
>    If, while processing a received packet, a node encounters a Routing
>    header with an unrecognized Routing Type value, the required behavior
>    of the node depends on the value of the Segments Left field, as
>    follows:
>
>       If Segments Left is zero, the node must ignore the Routing header
>       and proceed to process the next header in the packet, whose type
>       is identified by the Next Header field in the Routing header.
>
> That's why RPL with segment length 0 was accepted before 8610c7c6e3bd.
>
> But now the kernel recognizes RPL and it's intentionally disabled
> by default with net.ipv6.conf.$DEV.rpl_seg_enabled since introduced.
>
> And SRv6 has been rejected since 1ababeba4a21f for the same reason.

so there might be a need to have an opt-in knob to actually tell the
kernel ipv6 stack to recognize or not recognize a next header field
for users wanting to bypass certain next header fields to the user
space?

- Alex