Re: [PATCH] coccinelle: semantic patch to check for inappropriate do_div() calls

Wen Yang
2020-01-04T13:51:08Z

On 2020/1/4 4:55 下午, Julia Lawall wrote:
> On Sat, 4 Jan 2020, Wen Yang wrote:
>
>>
>>
>> On 2020/1/4 3:00 下午, Julia Lawall wrote:
>>> On Sat, 4 Jan 2020, Wen Yang wrote:
>>>
>>>> do_div() does a 64-by-32 division.
>>>> When the divisor is unsigned long, u64, or s64,
>>>> do_div() truncates it to 32 bits, this means it
>>>> can test non-zero and be truncated to zero for division.
>>>> This semantic patch is inspired by Mateusz Guzik's patch:
>>>> commit b0ab99e7736a ("sched: Fix possible divide by zero in avg_atom()
>>>> calculation")
>>>>
>>>> Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
>>>> Cc: Julia Lawall <Julia.Lawall@lip6.fr>
>>>> Cc: Gilles Muller <Gilles.Muller@lip6.fr>
>>>> Cc: Nicolas Palix <nicolas.palix@imag.fr>
>>>> Cc: Michal Marek <michal.lkml@markovi.net>
>>>> Cc: Matthias Maennich <maennich@google.com>
>>>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>>> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
>>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>>> Cc: cocci@systeme.lip6.fr
>>>> Cc: linux-kernel@vger.kernel.org
>>>> ---
>>>> scripts/coccinelle/misc/do_div.cocci | 66
>>>> ++++++++++++++++++++++++++++++++++++
>>>> 1 file changed, 66 insertions(+)
>>>> create mode 100644 scripts/coccinelle/misc/do_div.cocci
>>>>
>>>> diff --git a/scripts/coccinelle/misc/do_div.cocci
>>>> b/scripts/coccinelle/misc/do_div.cocci
>>>> new file mode 100644
>>>> index 0000000..f1b72d1
>>>> --- /dev/null
>>>> +++ b/scripts/coccinelle/misc/do_div.cocci
>>>> @@ -0,0 +1,66 @@
>>>> +// SPDX-License-Identifier: GPL-2.0-only
>>>> +/// do_div() does a 64-by-32 division.
>>>> +/// When the divisor is unsigned long, u64, or s64,
>>>> +/// do_div() truncates it to 32 bits, this means it
>>>> +/// can test non-zero and be truncated to zero for division.
>>>> +///
>>>> +//# This makes an effort to find those inappropriate do_div () calls.
>>>> +//
>>>> +// Confidence: Moderate
>>>> +// Copyright: (C) 2020 Wen Yang, Alibaba.
>>>> +// Comments:
>>>> +// Options: --no-includes --include-headers
>>>> +
>>>> +virtual context
>>>> +virtual org
>>>> +virtual report
>>>> +
>>>> +@depends on context@
>>>> +expression f;
>>>> +long l;
>>>> +unsigned long ul;
>>>> +u64 ul64;
>>>> +s64 sl64;
>>>> +
>>>> +@@
>>>> +(
>>>> +* do_div(f, l);
>>>> +|
>>>> +* do_div(f, ul);
>>>> +|
>>>> +* do_div(f, ul64);
>>>> +|
>>>> +* do_div(f, sl64);
>>>> +)
>>>> +
>>>> +@r depends on (org || report)@
>>>> +expression f;
>>>> +long l;
>>>> +unsigned long ul;
>>>> +position p;
>>>> +u64 ul64;
>>>> +s64 sl64;
>>>> +@@
>>>> +(
>>>> +do_div@p(f, l);
>>>> +|
>>>> +do_div@p(f, ul);
>>>> +|
>>>> +do_div@p(f, ul64);
>>>> +|
>>>> +do_div@p(f, sl64);
>>>> +)
>>>> +
>>>> +@script:python depends on org@
>>>> +p << r.p;
>>>> +@@
>>>> +
>>>> +msg="WARNING: WARNING: do_div() does a 64-by-32 division, which may
>>>> truncation the divisor to 32-bit"
>>>> +coccilib.org.print_todo(p[0], msg)
>>>> +
>>>> +@script:python depends on report@
>>>> +p << r.p;
>>>> +@@
>>>> +
>>>> +msg="WARNING: WARNING: do_div() does a 64-by-32 division, which may
>>>> truncation the divisor to 32-bit"
>>>> +coccilib.report.print_report(p[0], msg)
>>>
>>> A few small issues: You have WARNING: twice in each case, and truncation
>>> should be truncate.
>>>
>>
>> Thanks for your comments, we will fix it soon.
>>
>>> Is there any generic strategy for fixing these issues?
>>>
>>
>> We have done some experiments, such as:
>> https://lkml.org/lkml/2020/1/2/1354
>
> Thanks. Actually, I would appreciate knowing about such experiments when
> the semantic patch is submitted, since eg in this case I am not really an
> expert in this issue.
>
>>
>> - avg = rec->time;
>> - do_div(avg, rec->counter);
>> + avg = div64_ul(rec->time, rec->counter);
>>
>> --> Function replacement was performed here,
>> and simple code cleanup was also performed.
>>
>>
>> - do_div(stddev, rec->counter * (rec->counter - 1) * 1000);
>> + stddev = div64_ul(stddev,
>> + rec->counter * (rec->counter - 1) * 1000);
>>
>> --> Only the function replacement is performed here (because the variable
>> ‘stddev’ corresponds to a more complicated equation, cleaning it will reduce
>> readability).
>
> Would it be reasonable to extend the warning to say "consider using
> div64_ul instead"? Or do you think it is obvious to everyone?
>

Thank you for your comments.
We plan to modify it as follows:
msg="WARNING: do_div() does a 64-by-32 division, please consider using
div64_ul, div64_long, div64_u64 or div64_s64 instead."

>> In addition, there are some codes that do not need to be modified:
>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/net/can/dev.c#n263
>
> Would it be worth having a special case for constants and checking whether
> the value is obviously safe and no warning is needed?
>
Thanks.
This is very valuable in reducing false positives, and we'll try to
implement it.

--
Best Wishes,
Wen

>> So we just print a warning.
>> As for how to fix it, we need to analyze the code carefully.
>>
>> --
>> Best Wishes,
>> Wen
>>
>>
>>
>