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

[Morten Brørup]

> From: Ferruh Yigit [mailto:ferruh.yigit at amd.com]
> Sent: Friday, 17 November 2023 10.30
> 
> On 11/17/2023 3:28 AM, Stephen Hemminger wrote:
> > On Fri, 17 Nov 2023 00:50:16 +0000
> > Ferruh Yigit <ferruh.yigit at amd.com> wrote:
> >
> >>>> 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.
> >
> >
> > Possibly an API version to change the args would work to fix.
> >
> 
> rte_ipv4_udptcp_cksum_mbuf() and rte_ipv6_udptcp_cksum_mbuf() are
> inline
> functions, unfortunately we can't version them.
> 
> But those functions already gets IP header as parameter, can't we use
> IP
> header to get the payload size? If so this can be fixed without
> updating
> API.

If rte_ipv4_udptcp_cksum_mbuf() - or any other function in the DPDK Network Headers library - includes Ethernet padding (which may be non-zero) when calculating the TCP/UDP checksum of an IPv4 packet, it is a bug, and must be fixed there.

Our test cases should use random padding to catch bugs like this.

And I just realized that Ethernet padding may be added to any IP packet, so don't assume that this bug only applies to small packets.