netdev - Re: [PATCH net-next RFC 0/5] ipv6: sr: introduce seg6local End.BPF action

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Date:   Tue, 3 Apr 2018 14:40:22 +0100
From:   David Lebrun <dav.lebrun@...il.com>
To:     Mathieu Xhonneux <m.xhonneux@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     netdev@...r.kernel.org, David Lebrun <dlebrun@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH net-next RFC 0/5] ipv6: sr: introduce seg6local End.BPF
 action

On 04/03/2018 12:16 PM, Mathieu Xhonneux wrote:
> 
>> In patch 2 I was a bit concerned that:
>> +       struct seg6_bpf_srh_state *srh_state = (struct seg6_bpf_srh_state *)
>> +                                              &skb->cb;
>> would not collide with other users of skb->cb, but it seems the way
>> the hook is placed such usage should always be valid.
>> Would be good to add a comment describing the situation.
> Yes, it's indeed a little hack, but this should be OK since the IPv6 layer does
> not use the cb field. Another solution would be to create a new field in
> __sk_buff but it's more cumbersome.
> I will add a comment.

Good point. The IPv6 layer *does* use the cb field through the IP6CB() 
macro. It is first filled in ipv6_rcv() for ingress packets and used, 
among others, in the input path by extension headers processing 
functions to store EH offsets.

Given that input_action_end_bpf is called in the forwarding path 	 and 
terminates with a call to dst_input(), IP6CB() will be then reset by 
ipv6_rcv(), and the use of skb->cb here indeed should not collide with 
other users.

> 
>> Looks like somewhat odd 'End.BPF' name comes from similar names in SRv6 draft.
>> Do you plan to disclose such End.BPF action in the draft as well?
> This is something I've discussed with David Lebrun (the author of the Segment
> Routing implementation). There's no plan to disclose an End.BPF action as-is
> in the draft, since eBPF is really specific to Linux, and David doesn't mind not
> having a 1:1 mapping between the actions of the draft and the implemented
> ones. Writing "End.BPF" instead of just "bpf" is important to indicate that the
> action will advance to the next segment by itself, like all other End actions.
> One could imagine adding later a T.BPF action (a transit action), whose SID
> wouldn't have to be a segment, but that could still e.g. add/edit/delete TLVs.
> 

To clarify, I don't see why we shouldn't support "experimental" features 
that are not defined in draft-6man-segment-routing-header. However, we 
could create a separate draft describing the End.BPF feature, but that's 
perhaps best left for after the ongoing draft's last call.

David