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

[Ferruh Yigit]

On 11/14/2023 2:19 AM, Deng, KaiwenX wrote:
> 
> 
>> -----Original Message-----
>> From: Ferruh Yigit <ferruh.yigit at amd.com>
>> Sent: Friday, November 3, 2023 12:03 PM
>> To: Deng, KaiwenX <kaiwenx.deng at intel.com>; dev at dpdk.org
>> Cc: 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 11/3/2023 2:49 AM, Deng, KaiwenX wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Ferruh Yigit <ferruh.yigit at amd.com>
>>>> Sent: Friday, November 3, 2023 3:20 AM
>>>> To: Deng, KaiwenX <kaiwenx.deng at intel.com>; dev at dpdk.org
>>>> Cc: 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 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?
>>>>
>>> Hi Ferruh,
>>>
>>> In effect, the packet is padded by the switch.
>>> IEEE 802 packets may have a minimum size limit. The data fields should
>>> be padded by switch when necessary. In some switches, the padding data is
>> not zero.
>>>
>>> Csumonly doesn't trim these packets to the true length of the frame.
>>> In csumonly, the received mbuf data_len is the true length of the packet plus
>> the padding data len.
>>> Therefore, padding data is included in the checksum calculation.
>>> When the padding data is not zero, the checksum is wrong.
>>>
> Hi,
> Sorry for late reply.
> The minimum frame length specified by IEEE 802.3 is 64 bytes. In practice, 
> there are many packets less than 64 bytes that are padding through the switch. 
> 
> We found this issue because some customers found that their packets could not 
> calculate checksum correctly, they would send some packets less than 64 bytes, 
> but our app didn't strip the padding data for such packets.
>

OK, so switch in between is padding packets to make them compatible with
standard.

>From DPDK application perspective received packet is 64 bytes, right?
Problem happens because where verifies the checksum gets different
checksum that expected, but this is because packet is modified in
between by the networking setup.
I am not sure about trying to fix this in the testpmd.

Why not send packets that are >= 64 bytes from sender side, or configure
switch to not add padding or maybe use different switch?


>>
>> Thanks for clarification.
>>
>> Even some non-zero padding added, it will calculate the csum successfully, but
>> I assume in this case csum becomes different than expected csum and test
>> fails?
>>
>> In this case why not fix the generated packets, and make them compatible to
>> minimum size requirement? What is generating packets?
>>
>>
>>>>> This commit fixes this issue by triming IP packets to the true
>>>>> length of the frame in testpmd.
>>>>>
>>>>> Fixes: 03d17e4d0179 ("app/testpmd: do not change IP addrs in
>>>>> checksum
>>>>> engine")
>>>>> Cc: stable at dpdk.org
>>>>>
>>>>> Signed-off-by: Kaiwen Deng <kaiwenx.deng at intel.com>
>>>>> ---
>>>>>  app/test-pmd/csumonly.c | 32
>> ++++++++++++++++++++++++++++++++
>>>>>  1 file changed, 32 insertions(+)
>>>>>
>>>>> diff --git a/app/test-pmd/csumonly.c b/app/test-pmd/csumonly.c index
>>>>> 7af635e3f7..58b72b714a 100644
>>>>> --- a/app/test-pmd/csumonly.c
>>>>> +++ b/app/test-pmd/csumonly.c
>>>>> @@ -853,12 +853,14 @@ pkt_burst_checksum_forward(struct
>>>> fwd_stream *fs)
>>>>>  	uint16_t nb_rx;
>>>>>  	uint16_t nb_prep;
>>>>>  	uint16_t i;
>>>>> +	uint16_t pad_len;
>>>>>  	uint64_t rx_ol_flags, tx_ol_flags;
>>>>>  	uint64_t tx_offloads;
>>>>>  	uint32_t rx_bad_ip_csum;
>>>>>  	uint32_t rx_bad_l4_csum;
>>>>>  	uint32_t rx_bad_outer_l4_csum;
>>>>>  	uint32_t rx_bad_outer_ip_csum;
>>>>> +	uint32_t l3_off;
>>>>>  	struct testpmd_offload_info info;
>>>>>
>>>>>  	/* receive a burst of packet */
>>>>> @@ -980,6 +982,36 @@ pkt_burst_checksum_forward(struct
>> fwd_stream
>>>> *fs)
>>>>>  			l3_hdr = (char *)l3_hdr + info.outer_l3_len +
>>>> info.l2_len;
>>>>>  		}
>>>>>
>>>>> +		if (info.is_tunnel) {
>>>>> +			l3_off = info.outer_l2_len +
>>>>> +					info.outer_l3_len +
>>>>> +					info.l2_len;
>>>>> +		} else {
>>>>> +			l3_off = info.l2_len;
>>>>> +		}
>>>>> +		switch (info.ethertype) {
>>>>> +		case _htons(RTE_ETHER_TYPE_IPV4):
>>>>> +			pad_len = rte_pktmbuf_data_len(m) -
>>>>> +					(l3_off +
>>>>> +					rte_be_to_cpu_16(
>>>>> +					((struct rte_ipv4_hdr *)l3_hdr)-
>>>>> total_length));
>>>>> +			break;
>>>>> +		case _htons(RTE_ETHER_TYPE_IPV6):
>>>>> +			pad_len = rte_pktmbuf_data_len(m) -
>>>>> +					(l3_off +
>>>>> +					rte_be_to_cpu_16(
>>>>> +					((struct rte_ipv6_hdr *)l3_hdr)-
>>>>> payload_len));
>>>>> +			break;
>>>>> +		default:
>>>>> +			pad_len = 0;
>>>>> +			break;
>>>>> +		}
>>>>> +
>>>>> +		if (pad_len) {
>>>>> +			rte_pktmbuf_data_len(m) =
>>>> rte_pktmbuf_data_len(m) - pad_len;
>>>>> +			rte_pktmbuf_pkt_len(m) = rte_pktmbuf_data_len(m);
>>>>> +		}
>>>>> +
>>>>>  		/* step 2: depending on user command line configuration,
>>>>>  		 * recompute checksum either in software or flag the
>>>>>  		 * mbuf to offload the calculation to the NIC. If TSO
>>>
>