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From:   "Dongseok Yi" <dseok.yi@samsung.com>
To:     "'Willem de Bruijn'" <willemdebruijn.kernel@gmail.com>
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Subject: RE: [PATCH bpf] bpf: check for data_len before upgrading mss when 6
 to 4
Date:   Tue, 11 May 2021 10:11:08 +0900
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Precedence: bulk

On Mon, May 10, 2021 at 09:46:25AM -0400, Willem de Bruijn wrote:
> 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?

It can hit __skb_linearize in validate_xmit_skb and frags will be
copied to a linear part. The linear part size can exceed the MTU of
skb->dev unexpectedly.

I will make another patch with the flag approach.