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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Mon, 10 May 2021 09:46:25 -0400
Message-ID: <CA+FuTSeyuUvKC==Mo7L+u3PS0BQyea+EdLLYjhGFrP7FQZsbEQ@mail.gmail.com>
Subject: Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6
 to 4
To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc:     Dongseok Yi <dseok.yi@samsung.com>,
        Yunsheng Lin <linyunsheng@huawei.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Mon, May 10, 2021 at 9:19 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> > > 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.

As for more generic approach: does downgrading to non-TSO by clearing
gso_size work for this edge case?