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

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