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

Wen Yang
2020-01-04T08:50:01Z

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

- 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).

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

So we just print a warning.
As for how to fix it, we need to analyze the code carefully.

--
Best Wishes,
Wen