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

[Willem de Bruijn]

> > 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.