[PATCH] app/test-pmd: fix L4 checksum with padding data

[Deng, KaiwenX]

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit at amd.com>
> Sent: Friday, November 17, 2023 8:50 AM
> To: Stephen Hemminger <stephen at networkplumber.org>
> Cc: Deng, KaiwenX <kaiwenx.deng at intel.com>; dev at dpdk.org;
> stable at dpdk.org; Yang, Qiming <qiming.yang at intel.com>; Zhou, YidingX
> <yidingx.zhou at intel.com>; Singh, Aman Deep <aman.deep.singh at intel.com>;
> Zhang, Yuying <yuying.zhang at intel.com>; Matz, Olivier
> <olivier.matz at 6wind.com>; De Lara Guarch, Pablo
> <pablo.de.lara.guarch at intel.com>
> Subject: Re: [PATCH] app/test-pmd: fix L4 checksum with padding data
> 
> On 11/16/2023 10:58 PM, Stephen Hemminger wrote:
> > On Thu, 2 Nov 2023 19:20:07 +0000
> > Ferruh Yigit <ferruh.yigit at amd.com> wrote:
> >
> >> On 8/4/2023 9:28 AM, Kaiwen Deng wrote:
> >>> IEEE 802 packets may have a minimum size limit. The data fields
> >>> should be padded when necessary. In some cases, the padding data is
> >>> not zero. Testpmd does not trim these IP packets to the true length
> >>> of the frame, so errors will occur when calculating TCP or UDP
> >>> checksum.
> >>>
> >>
> >> Hi Kaiwen,
> >>
> >> I am trying to understand the problem, what is the testcase that has
> >> checksum error?
> >>
> >> Are the received mbuf data_len & pkt_len wrong? Instead of trying to
> >> fix the mbuf during forwarding, can we fix where packet generated?
> >
> > The root cause is that get_udptcp_cksum_mbuf is using m->pkt_len which
> > maybe larger than the actual data. The real issue is there and in
> > rte_ip.h checksum code. The correct fix would be to use l3_len instead.
> >
> 
> I see, you are right.
> 
> In 'rte_ipv4_udptcp_cksum_mbuf()',
> as payload length "mbuf->pkt_len - l4_off" is used, which includes padding
> and if padding is not zero it will end up producing wrong checksum.
> 
> 
> I agree using 'l3_len' instead is correct fix.
> 
> But this requires ABI/API change,
> plus do we have any reason to keep the padding, discarding it as this patch
> does is also simpler alternative.
> 
> 
> Other alternative can be to zero the padding bytes. I guess standard doesn't
> enforce them to be zero, but we can do this to remove its impact on checksum
> calculation.
I'm not sure if this is ok, it feels like it would reduce performance. 
I can try this alternative if needed.
> 
> 
> @Kaiwen, did you able to test this with HW offload, what is the behavior of
> the HW, does is remove padding bytes?
> 
I tested the HW offload case and the same tcp/udp checksum error occurs when padding is not 0, 
But if change pkt_len to the true length of the frame, the checksum is correct.
> 
> > It also looks like test-pmd is not validating the IP header.
> > Both parse_ipv4() and parse_ipv6() should check if packet was truncated.
> > Same for both UDP and TCP lengths.
> >
>