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

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Message-ID: <CA+FuTSdx_a+DGG5dSFRB3wkowwNb1ZXHFed=qA3sj5y6U3VtiA@mail.gmail.com>
Date:   Mon, 21 Sep 2020 23:17:16 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Marek Zavodsky <marek.zavodsky@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Maciej Żenczykowski <maze@...gle.com>,
        Saeed Mahameed <saeed@...nel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        BPF-dev-list <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Shaun Crampton <shaun@...era.io>,
        David Miller <davem@...emloft.net>,
        Marek Majkowski <marek@...udflare.com>
Subject: Re: BPF redirect API design issue for BPF-prog MTU feedback?

On Mon, Sep 21, 2020 at 6:22 PM Marek Zavodsky <marek.zavodsky@...il.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.