linux-kernel - Re: [PATCH v6] coccinelle: tests: unsigned value cannot be lesser than zero

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Date:	Tue, 5 Jan 2016 11:18:04 -0300
From:	"Geyslan G. Bem" <geyslan@...il.com>
To:	Andrzej Hajda <a.hajda@...sung.com>
Cc:	Julia Lawall <Julia.Lawall@...6.fr>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Gilles Muller <Gilles.Muller@...6.fr>,
	Nicolas Palix <nicolas.palix@...g.fr>,
	Michal Marek <mmarek@...e.com>,
	open list <linux-kernel@...r.kernel.org>,
	"moderated list:COCCINELLE/Semantic Patches (SmPL)" 
	<cocci@...teme.lip6.fr>
Subject: Re: [PATCH v6] coccinelle: tests: unsigned value cannot be lesser
 than zero

2016-01-05 10:49 GMT-03:00 Andrzej Hajda <a.hajda@...sung.com>:
> On 01/05/2016 01:59 PM, Geyslan G. Bem wrote:
>> Hello,
>>
>> 2016-01-04 4:45 GMT-03:00 Andrzej Hajda <a.hajda@...sung.com>:
>>> Unsigned expressions cannot be lesser than zero. Presence of comparisons
>>> 'unsigned (<|<=|>|>=) 0' often indicates a bug, usually wrong type of variable.
>>> The patch beside finding such comparisons tries to eliminate false positives,
>>> mainly by bypassing range checks.
>>>
>>> gcc can detect such comparisons also using -Wtype-limits switch, but it warns
>>> also in correct cases, making too much noise.
>>>
>>> Signed-off-by: Andrzej Hajda <a.hajda@...sung.com>
>>> ---
>>> v6: improved range check detection (according to Julia suggestion)
>>> v5: improved range check detection
>>> v4: added range check detection, added full check in case value holds a result
>>>     of signed function
>>> v3: added bool type
>>> v2: added --all-includes option
>>> ---
>>>  .../tests/unsigned_lesser_than_zero.cocci          | 76 ++++++++++++++++++++++
>>>  1 file changed, 76 insertions(+)
>>>  create mode 100644 scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci
>>>
>>> diff --git a/scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci b/scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci
>>> new file mode 100644
>>> index 0000000..b9c7ed8
>>> --- /dev/null
>>> +++ b/scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci
>>> @@ -0,0 +1,76 @@
>>> +/// Unsigned expressions cannot be lesser than zero. Presence of
>>> +/// comparisons 'unsigned (<|<=|>|>=) 0' often indicates a bug,
>>> +/// usually wrong type of variable.
>>> +///
>>> +/// To reduce number of false positives following tests have been added:
>>> +/// - parts of range checks are skipped, eg. "if (u < 0 || u > 15) ...",
>>> +///   developers prefer to keep such code,
>>> +/// - comparisons "<= 0" and "> 0" are performed only on results of
>>> +///   signed functions/macros,
>> Why common unsigned comparisons with <= 0 are not being detected? I
>> think that it misleads the code reading and induces further bugs.
>> Just reading "var <= 0" infers that var can receive signed value. The
>> be clear the comparison should be against zero only "var == 0" or
>> depending of the context "!var".
>>
>
> Many developers prefer to use "unsigned <= 0" comparison, as more
> descriptive
> and less fragile. See for example for the last phrase of Linus email[1].
>
> [1]: http://permalink.gmane.org/gmane.linux.kernel/2054063

I got it. Tks.

>
> Regards
> Andrzej
>



-- 
Regards,

Geyslan G. Bem
hackingbits.com
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