Re: BPF selftests and strict aliasing

[Yonghong Song <yonghong.song@linux.dev>]

On 1/29/24 8:15 AM, Eduard Zingerman wrote:
> On Sun, 2024-01-28 at 21:33 -0800, Yonghong Song wrote:
> [...]
>> I tried below example with the above prog/dynptr_fail.c case with gcc 11.4
>> for native x86 target and didn't trigger the warning. Maybe this requires
>> latest gcc? Or test C file is not sufficient enough to trigger the warning?
>>
>> [~/tmp1]$ cat t.c
>> struct t {
>>     char a;
>>     short b;
>>     int c;
>> };
>> void init(struct t *);
>> long foo() {
>>     struct t dummy;
>>     init(&dummy);
>>     return *(int *)&dummy;
>> }
>> [~/tmp1]$ gcc -Wall -Werror -O2 -g -Wno-compare-distinct-pointer-types -c t.c
>> [~/tmp1]$ gcc --version
>> gcc (GCC) 11.4.1 20230605 (Red Hat 11.4.1-2)
> I managed to trigger this warning for gcc 13.2.1:
>
>      $ gcc -fstrict-aliasing -Wstrict-aliasing=1 -c test-punning.c -o /dev/null
>      test-punning.c: In function ‘foo’:
>      test-punning.c:10:19: warning: dereferencing type-punned pointer might break strict-aliasing rules [-Wstrict-aliasing]
>         10 |    return *(int *)&dummy;
>            |                   ^~~~~~
>      
> Note the -Wstrict-aliasing=1 option, w/o =1 suffix it does not trigger.

Thanks for confirmation. My question is that in our selftests bpf compilation, we do
not have -fstrict-aliasing flag, so even for gcc we should not have strict-aliasing
warning, right?

Jose, did I miss anything? Or you added -fstrict-aliasing to the compilation flag
to selftests/bpf Makefile?

>
> Grepping words "strict-aliasing", "strictaliasing", "strict_aliasing"
> through clang code-base does not show any diagnostic related tests or
> detection logic. It appears to me clang does not warn about strict
> aliasing violations at all and -Wstrict-aliasing=* are just stubs at
> the moment.

I also did some search in clang source. This appears indeed the case.