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. I will make another patch with the flag approach.
