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

[Mathis Marion]

On 26/06/2024 3:45 AM, Alexander Aring wrote:
> Hi,
>
> On Tue, Jun 25, 2024 at 5:39 PM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
> > From: Mathis Marion <Mathis.Marion@xxxxxxxxxx>
> > Date: Mon, 24 Jun 2024 16:15:33 +0200
> > > From: Mathis Marion <mathis.marion@xxxxxxxxxx>
> > >
> > > 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

My point is that if a particular routing header support is disabled
through system configuration, it should be treated as any unrecognized
header. From my perspective, doing otherwise causes a regression every
time a new routing header is supported.