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

[Wen Yang]

On 2020/1/4 3:00 äå, 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@xxxxxxxxxxxxxxxxx>
> > Cc: Julia Lawall <Julia.Lawall@xxxxxxx>
> > Cc: Gilles Muller <Gilles.Muller@xxxxxxx>
> > Cc: Nicolas Palix <nicolas.palix@xxxxxxx>
> > Cc: Michal Marek <michal.lkml@xxxxxxxxxxx>
> > Cc: Matthias Maennich <maennich@xxxxxxxxxx>
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: cocci@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> >   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 âstddevâ 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