From: George Prekas <prekageo@amazon.com>
To: Ferruh Yigit <ferruh.yigit@intel.com>,
Wenzhuo Lu <wenzhuo.lu@intel.com>,
Beilei Xing <beilei.xing@intel.com>,
Bernard Iremonger <bernard.iremonger@intel.com>
Cc: <dev@dpdk.org>, Stephen Hemminger <stephen@networkplumber.org>,
"George Prekas (prekageo)" <prekageo@amazon.com>
Subject: Re: [dpdk-dev] [PATCH v2] app/testpmd: fix IP checksum calculation
Date: Wed, 6 Jan 2021 23:39:39 -0600 [thread overview]
Message-ID: <6539f363-97b1-82b2-1b09-036aa75c9dc9@amazon.com> (raw)
In-Reply-To: <8c1fd6ad-302d-f45e-f48a-e81fd5f8ba85@intel.com>
On 1/6/2021 12:02 PM, Ferruh Yigit wrote:
> On 12/5/2020 5:42 AM, George Prekas wrote:
>> Strict-aliasing rules are violated by cast to uint16_t* in flowgen.c
>> and the calculated IP checksum is wrong on GCC 9 and GCC 10.
>>
>> Signed-off-by: George Prekas <prekageo@amazon.com>
>> ---
>> v2:
>> * Instead of a compiler barrier, use a compiler flag.
>> ---
>> app/test-pmd/meson.build | 1 +
>> 1 file changed, 1 insertion(+)
>>
>> diff --git a/app/test-pmd/meson.build b/app/test-pmd/meson.build
>> index 7e9c7bdd6..5d24e807f 100644
>> --- a/app/test-pmd/meson.build
>> +++ b/app/test-pmd/meson.build
>> @@ -4,6 +4,7 @@
>> # override default name to drop the hyphen
>> name = 'testpmd'
>> cflags += '-Wno-deprecated-declarations'
>> +cflags += '-fno-strict-aliasing'
>> sources = files('5tswap.c',
>> 'cmdline.c',
>> 'cmdline_flow.c',
>>
>
> Hi George,
>
> I am trying to understand this, the relevant code is as below:
> ip_hdr->hdr_checksum = ip_sum((unaligned_uint16_t *)ip_hdr, sizeof(*ip_hdr));
>
> You are suspicious of strict aliasing rule violation, with more details:
> The concern is the "struct rte_ipv4_hdr *ip_hdr;" aliased to "const
> unaligned_uint16_t *hdr", and compiler can optimize out the calculations using
> data pointed by 'hdr' pointer, since the 'hdr' pointer is not used to alter the
> data and compiler may think data is not changed at all.
>
> 1) But the pointer "hdr" is assigned in the loop, from another pointer whose
> content is changing, why this is not helping to figure out that the data 'hdr'
> pointing is changed.
>
> 2) I tried to debug this, but I am not able to reproduce the issue, 'ip_sum()'
> called each time and checksum calculated correctly. Using gcc 10.2.1-9. Can you
> able to confirm the case with debug, or from the assembly/object file?
>
>
> And if the issue is strict aliasing rule violation as you said, compiler flag is
> an option but not sure how much it reduces the compiler optimization benefit, I
> guess other options also not so good, memcpy brings too much work on runtime and
> union requires bigger change and makes code complex.
> I wonder if making 'ip_sum()' a non inline function can help, can you please
> give a try since you can reproduce it?
Hi Ferruh,
Thanks for looking into it.
I am copy-pasting at the end of this email a minimal reproduction. It calculates a checksum and prints it. The correct value is f8d9. If you compile it with -O0 or -O3 -fno-strict-aliasing, you will get the correct value. If you compile it with gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0 and -O3, you will get f8e8. You can also try it on https://godbolt.org/ and see how different versions behave.
My understanding is that the code violates the C standard (https://stackoverflow.com/a/99010).
--- cut here ---
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct rte_ipv4_hdr {
uint8_t version_ihl;
uint8_t type_of_service;
uint16_t total_length;
uint16_t packet_id;
uint16_t fragment_offset;
uint8_t time_to_live;
uint8_t next_proto_id;
uint16_t hdr_checksum;
uint32_t src_addr;
uint32_t dst_addr;
};
static inline uint16_t ip_sum(const uint16_t *hdr, int hdr_len)
{
uint32_t sum = 0;
while (hdr_len > 1)
{
sum += *hdr++;
if (sum & 0x80000000)
sum = (sum & 0xFFFF) + (sum >> 16);
hdr_len -= 2;
}
while (sum >> 16)
sum = (sum & 0xFFFF) + (sum >> 16);
return ~sum;
}
static void pkt_burst_flow_gen(void)
{
struct rte_ipv4_hdr *ip_hdr = (struct rte_ipv4_hdr *) malloc(4096);
memset(ip_hdr, 0, sizeof(*ip_hdr));
ip_hdr->version_ihl = 1;
ip_hdr->type_of_service = 2;
ip_hdr->fragment_offset = 3;
ip_hdr->time_to_live = 4;
ip_hdr->next_proto_id = 5;
ip_hdr->packet_id = 6;
ip_hdr->src_addr = 7;
ip_hdr->dst_addr = 8;
ip_hdr->total_length = 9;
ip_hdr->hdr_checksum = ip_sum((uint16_t *)ip_hdr, sizeof(*ip_hdr));
printf("%x\n", ip_hdr->hdr_checksum);
}
int main(void)
{
pkt_burst_flow_gen();
return 0;
}
next prev parent reply other threads:[~2021-01-07 5:39 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-03 13:59 [dpdk-dev] [PATCH] " George Prekas
2020-12-03 16:08 ` Stephen Hemminger
2020-12-03 16:35 ` George Prekas
2020-12-03 18:33 ` Stephen Hemminger
2020-12-04 8:59 ` Ferruh Yigit
2020-12-05 5:47 ` George Prekas
2020-12-05 5:42 ` [dpdk-dev] [PATCH v2] " George Prekas
2021-01-05 16:26 ` George Prekas
2021-01-06 18:02 ` Ferruh Yigit
2021-01-07 5:25 ` Stephen Hemminger
2021-01-07 5:39 ` George Prekas [this message]
2021-01-07 11:32 ` Ferruh Yigit
2021-01-07 13:06 ` Ferruh Yigit
2021-01-07 14:20 ` George Prekas
2021-01-07 15:22 ` Ferruh Yigit
2021-01-07 20:45 ` George Prekas
2021-01-07 15:50 ` Stephen Hemminger
2021-01-07 15:59 ` Ferruh Yigit
2021-01-07 16:29 ` Stephen Hemminger
2021-01-07 20:42 ` [dpdk-dev] [PATCH v3] " George Prekas
2021-01-18 15:20 ` Ferruh Yigit
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=6539f363-97b1-82b2-1b09-036aa75c9dc9@amazon.com \
--to=prekageo@amazon.com \
--cc=beilei.xing@intel.com \
--cc=bernard.iremonger@intel.com \
--cc=dev@dpdk.org \
--cc=ferruh.yigit@intel.com \
--cc=stephen@networkplumber.org \
--cc=wenzhuo.lu@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).