linux-kernel - Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4

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Date:   Mon, 10 May 2021 09:19:43 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Dongseok Yi <dseok.yi@...sung.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6
 to 4

> > That generates TCP packets with different MSS within the same stream.
> >
> > My suggestion remains to just not change MSS at all. But this has to
> > be a new flag to avoid changing established behavior.
>
> I don't understand why the mss size should be kept in GSO step. Will
> there be any issue with different mss?

This issue has come up before and that has been the feedback from
TCP experts at one point.

> In general, upgrading mss make sense when 6 to 4. The new flag would be
> set by user to not change mss. What happened if user does not set the
> flag? I still think we should fix the issue with a general approach. Or
> can we remove the skb_increase_gso_size line?

Admins that insert such BPF packets should be aware of these issues.
And likely be using clamping. This is a known issue.

We arrived that the flag approach in bpf_skb_net_shrink. Extending
that  to bpf_skb_change_proto would be consistent.