Re: [PATCH net-next v2 1/8] udp: fixup csum for GSO receive slow path

[Paolo Abeni <pabeni@redhat.com>]

On Fri, 2021-03-26 at 14:30 -0400, Willem de Bruijn wrote:
> On Thu, Mar 25, 2021 at 1:24 PM Paolo Abeni <pabeni@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 <pabeni@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.