Re: Possible bug in linux-6.2/tools/testing/selftests/powerpc/pmu/sampling_tests/mmcra_thresh_marked_sample_test.c

[kajoljain <kjain@linux.ibm.com>]

On 2/27/23 10:56, Michael Ellerman wrote:
> David Binderman <dcb314@hotmail.com> writes:
>> Hello there,
>>
>> I ran the static analyser cppcheck over the linux-6.2 source code and got this:
>>
>> linux-6.2/tools/testing/selftests/powerpc/pmu/sampling_tests/mmcra_thresh_marked_sample_test.c:68:10: style: Same expression '0x3' found multiple times in chain of '&' operators. [duplicateExpression]

Hi,
  Thanks David for reporting it.

> 
> Thanks.
> 
>> Source code is
>>
>>     FAIL_IF(EV_CODE_EXTRACT(event.attr.config, sample & 0x3) !=
>>             get_mmcra_sample_mode(get_reg_value(intr_regs, "MMCRA"), 4));
>>
>> but
>>
>> #define EV_CODE_EXTRACT(x, y)   \
>>     ((x >> ev_shift_##y) & ev_mask_##y)
>>
>>
>> Given the token pasting, I very much doubt an expression like "sample & 0x3"
>> will work correctly. Same thing on the line above 
>>
>>     FAIL_IF(EV_CODE_EXTRACT(event.attr.config, sample >> 2) !=
>>             get_mmcra_rand_samp_elig(get_reg_value(intr_regs, "MMCRA"), 4));
>>
>> "sample >> 2" doesn't look like a valid token to me.
> 
> It expands to:
> 
>  if ((((event.attr.config >> ev_shift_sample >> 2) & ev_mask_sample >> 2) != get_mmcra_rand_samp_elig(get_reg_value(intr_regs, "MMCRA"), 4))) 
> 
> Which AFAICS is valid, and does compile.
> 
> Whether it's what the author actually intended is less clear.
> 
> And the other example with & 0x3 seems obviously wrong, it expands to:
> 
>   if ((((event.attr.config >> ev_shift_sample & 0x3) & ev_mask_sample & 0x3) != get_mmcra_sample_mode(get_reg_value(intr_regs, "MMCRA"), 4)))
> 
> The shift is 24, so bitwise anding it with 0x3 gets 0 which doesn't seem
> likely to be what was intended.
> 

Hi Michael,
   Thanks for checking it. The intention is to check 3 bits of
rand_samp_elig field and 2 bits of rand_samp_mode field from the
sampling bits. Basically we first want to extract that sample field
using EV_CODE_EXTRACT macro and then fetch required value of
rand_samp_elig and rand_samp_mode, to compare it with MMCRA bits.

Right approach to do that would be:

 FAIL_IF((EV_CODE_EXTRACT(event.attr.config, sample) >> 2) !=
get_mmcra_rand_samp_elig(get_reg_value(intr_regs, "MMCRA"), 4));

 FAIL_IF((EV_CODE_EXTRACT(event.attr.config, sample) & 0x3) !=
get_mmcra_rand_samp_elig(get_reg_value(intr_regs, "MMCRA"), 4));

I will send a fix patch for same.

Thanks,
Kajol Jain

> cheers