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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Mon, Sep 21, 2020 at 6:22 PM Marek Zavodsky <marek.zavodsky@gmail.com> wrote:
>
> Hi guys,
>
> My kernel knowledge is small, but I experienced this (similar) issue
> with packet encapsulation (not a redirect), therefore modifying the
> redirect branch would not help in my case.
>
> I'm working on a TC program to do GUE encap/decap (IP + UDP + GUE,
> outer header has extra 52B).
> There are no issues with small packets. But when I use curl to
> download big file HTTP server chunks data randomly. Some packets have
> MTU size, others are even bigger. Big packets are not an issue,
> however MTU sized packets fail on bpf_skb_adjust_room with -524
> (ENOTSUPP).

This is a related, but different, unresolved issue at the boundary of
GSO packets. Packets that are not GSO, but would exceed MTU once
encapsulated, will cause adjust room to fail:

            (!shrink && (skb->len + len_diff_abs > len_max &&
                         !skb_is_gso(skb))))
                return -ENOTSUPP;

As admin, this can be addressed by setting a lower route MTU on routes
that may be encapsulated. But that is not very obvious or transparent.