Re: [dpdk-dev] [PATCH v2] app/testpmd: fix IP checksum calculation

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 1/7/2021 2:20 PM, George Prekas wrote:
> On 1/7/2021 5:32 AM, Ferruh Yigit wrote:
>> On 1/7/2021 5:39 AM, George Prekas wrote:
>>> 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).
>>>
>>
>> Thanks for the sample code below, I copied to the godbolt:
>> https://godbolt.org/z/6fMK19
>>
>> In gcc 10, the checksum calculation is done during compilation (when
>> optimization is enabled) and the value is returned directly:
>> mov    $0xffed,%esi
>>
>> Since a calculation is happening I assume the compiler knows about the aliasing
>> and OK with it.
> 
> According to https://gcc.gnu.org/bugs/: "if compiling with -fno-strict-aliasing -fwrapv
> -fno-aggressive-loop-optimizations makes a difference ... then your code is probably not
> correct"
> 

Yep, I saw it while submitting the gcc ticket, and it seems it was right:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98582

>>
>> But that optimized calculation seems wrong, when it is disabled [1] the checksum
>> is correct again.
>>
>> [1] all following seems helping to disable compile time calculation
>> - disabling optimization
>> - putting a compiler barrier
>> - putting a 'printf' inside 'ip_sum()'
>> - fno-strict-aliasing
>>
>> gcc 8 & 9 is not doing this compile time calculation, hence they are not affected.
> 
> I just checked gcc 8.3 and gcc 9.3 on godbolt and I got f8e8 (which is wrong; the correct
> is f8d9).
> 

True, I missed that they generate wrong value.

>>
>> This feels like an optimization issue in gcc10, but not sure exactly on the root
>> cause, and how to disable it properly in our case.
> 
> I've tried with __attribute__ ((noinline)) and it fixes the problem. But keep in mind
> that we are dealing with broken C code. This attribute just prevents the optimization that
> reveals the problem. It does not guarantee that the problem will not reappear in a future
> compiler version.
> 
> I've also tried to use a union as suggested by Stephen Hemminger and it works correctly but
> it requires significant code changes: you have to copy paste the IP header structure inside
> a union and access it only through the union.
> 
> As a side note, here is a piece of opinion from Linus Torvalds regarding strict aliasing:
> https://lkml.org/lkml/2018/6/5/769
> 
> DPDK already uses -fno-strict-aliasing for librte_node and librte_vhost.

In the above ticket, 'may_alias' attribute is also suggested, which is working 
for the sample, can you please try with it too?
It may be better to allow non compatible aliasing only for single function, 
instead of whole binary.

typedef uint16_t alias_int16_t __attribute__((may_alias));

> 
>>
>>> --- 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;
>>> }
>>>
>>