Re: BPF redirect API design issue for BPF-prog MTU feedback?

[Lorenz Bauer <lmb@cloudflare.com>]

On Sat, 19 Sep 2020 at 00:06, Maciej Żenczykowski <maze@google.com> wrote:
>
> > This is a good point.  As bpf_skb_adjust_room() can just be run after
> > bpf_redirect() call, then a MTU check in bpf_redirect() actually
> > doesn't make much sense.  As clever/bad BPF program can then avoid the
> > MTU check anyhow.  This basically means that we have to do the MTU
> > check (again) on kernel side anyhow to catch such clever/bad BPF
> > programs.  (And I don't like wasting cycles on doing the same check two
> > times).
>
> If you get rid of the check in bpf_redirect() you might as well get
> rid of *all* the checks for excessive mtu in all the helpers that
> adjust packet size one way or another way.  They *all* then become
> useless overhead.
>
> I don't like that.  There may be something the bpf program could do to
> react to the error condition (for example in my case, not modify
> things and just let the core stack deal with things - which will
> probably just generate packet too big icmp error).
>
> btw. right now our forwarding programs first adjust the packet size
> then call bpf_redirect() and almost immediately return what it
> returned.
>
> but this could I think easily be changed to reverse the ordering, so
> we wouldn't increase packet size before the core stack was informed we
> would be forwarding via a different interface.

We do the same, except that we also use XDP_TX when appropriate. This
complicates the matter, because there is no helper call we could
return an error from.

My preference would be to have three helpers: get MTU for a device,
redirect ctx to a device (with MTU check), resize ctx (without MTU
check) but that doesn't work with XDP_TX. Your idea of doing checks in
redirect and adjust_room is pragmatic and seems easier to implement.

-- 
Lorenz Bauer  |  Systems Engineer
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