netdev - Re: [PATCH bpf-next] selftests/bpf: reserve flags in bpf_skb_net

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Date:   Fri, 12 Apr 2019 09:49:46 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Alan Maguire <alan.maguire@...cle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: reserve flags in bpf_skb_net_shrink

On Fri, Apr 12, 2019 at 9:45 AM Alan Maguire <alan.maguire@...cle.com> wrote:
>
> On Fri, 12 Apr 2019, Willem de Bruijn wrote:
>
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > The ENCAP flags in bpf_skb_adjust_room are ignored on decap with
> > bpf_skb_net_shrink. Reserve these bits for future use.
> >
> > Fixes: 868d523535c2d ("bpf: add bpf_skb_adjust_room encap flags")
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>
> Reviewed-by: Alan Maguire <alan.maguire@...cle.com>

Thanks Alan.

> One thought; should we add a line to the growth-only flags in
> the description of bpf_skb_adjust_room (ENCAP_L[234]) in bpf.h;
> something like:
>
> "This flag is valid for use with positive len_diff values only."

That is easily gleaned from reading the code. I'm not a fan of
documentation that duplicates concise readable code.

Plus, we'd add it and then remove again before long. We just need to
implement the decap behavior. I actually maintain TC ingress decap
code in C from that I wrote before eBPF and want to deprecate one day.
But it will take some time to do properly.