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

[Willem de Bruijn]

On Mon, May 10, 2021 at 9:11 PM Dongseok Yi <dseok.yi@xxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxx> 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.

When does skb_needs_linearize return true here (besides lack of
scatter-gather support, which would also preclude TSO)?

> I will make another patch with the flag approach.
>