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

On Mon, Sep 21, 2020 at 6:22 PM Marek Zavodsky [off-list ref] 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.