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

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Message-id: <568BC9F4.6020409@samsung.com>
Date:	Tue, 05 Jan 2016 14:49:40 +0100
From:	Andrzej Hajda <a.hajda@...sung.com>
To:	"Geyslan G. Bem" <geyslan@...il.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

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

Regards
Andrzej

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