On Fri, 2021-03-26 at 14:30 -0400, Willem de Bruijn wrote:
> On Thu, Mar 25, 2021 at 1:24 PM Paolo Abeni <pab...@redhat.com> wrote:
> > When UDP packets generated locally by a socket with UDP_SEGMENT
> > traverse the following path:
> >
> > UDP tunnel(xmit) -> veth (segmentation) -> veth (gro) ->
> > UDP tunnel (rx) -> UDP socket (no UDP_GRO)
> >
> > they are segmented as part of the rx socket receive operation, and
> > present a CHECKSUM_NONE after segmentation.
>
> would be good to capture how this happens, as it was not immediately obvious.
The CHECKSUM_PARTIAL is propagated up to the UDP tunnel processing,
where we have:
__iptunnel_pull_header() -> skb_pull_rcsum() ->
skb_postpull_rcsum() -> __skb_postpull_rcsum() and the latter do the
conversion.
> > Additionally the segmented packets UDP CB still refers to the original
> > GSO packet len. Overall that causes unexpected/wrong csum validation
> > errors later in the UDP receive path.
> >
> > We could possibly address the issue with some additional checks and
> > csum mangling in the UDP tunnel code. Since the issue affects only
> > this UDP receive slow path, let's set a suitable csum status there.
> >
> > v1 -> v2:
> > - restrict the csum update to the packets strictly needing them
> > - hopefully clarify the commit message and code comments
> >
> > Signed-off-by: Paolo Abeni <pab...@redhat.com>
> > + if (skb->ip_summed == CHECKSUM_NONE && !skb->csum_valid)
> > + skb->csum_valid = 1;
>
> Not entirely obvious is that UDP packets arriving on a device with rx
> checksum offload off, i.e., with CHECKSUM_NONE, are not matched by
> this test.
>
> I assume that such packets are not coalesced by the GRO layer in the
> first place. But I can't immediately spot the reason for it..
Packets with CHECKSUM_NONE are actually aggregated by the GRO engine.
Their checksum is validated by:
udp4_gro_receive -> skb_gro_checksum_validate_zero_check()
-> __skb_gro_checksum_validate ->
__skb_gro_checksum_validate_complete()
and skb->ip_summed is changed to CHECKSUM_UNNECESSARY by:
__skb_gro_checksum_validate -> skb_gro_incr_csum_unnecessary
-> __skb_incr_checksum_unnecessary()
and finally to CHECKSUM_PARTIAL by:
udp4_gro_complete() -> udp_gro_complete() -> udp_gro_complete_segment()
Do you prefer I resubmit with some more comments, either in the commit
message or in the code?
Thanks
Paolo
side note: perf probe here is fooled by skb->ip_summed being a bitfield
and does not dump the real value. I had to look at skb-
>__pkt_type_offset[0] instead.
