linux-kernel - Re: [PATCH] cpufreq: intel_pstate: fix possible integer overflow

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Message-ID: <5265AE27.90600@gmail.com>
Date:	Mon, 21 Oct 2013 15:43:51 -0700
From:	Dirk Brandewie <dirk.brandewie@...il.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	"Geyslan G. Bem" <geyslan@...il.com>, kernel-br@...glegroups.com,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"open list:CPU FREQUENCY DRI..." <cpufreq@...r.kernel.org>,
	"open list:CPU FREQUENCY DRI..." <linux-pm@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: intel_pstate: fix possible integer overflow

On 10/21/2013 03:47 PM, Rafael J. Wysocki wrote:
> On Monday, October 21, 2013 08:56:22 AM Dirk Brandewie wrote:
>> On 10/19/2013 08:31 PM, Geyslan G. Bem wrote:
>>> The expression 'pstate << 8' is evaluated using 32-bit arithmetic while
>>> 'val' expects an expression of type u64.
>>>
>>> Signed-off-by: Geyslan G. Bem <geyslan@...il.com>
>> Acked-by: Dirk Brandewie <dirk.j.brandewie@...el.com>
>
> Actually, isn't (pstate << 8) guaranteed not to overflow?
>

Yes, I was assuming this was caught by a static checking tool. I
didn't see a downside to giving the compilier complete information.

>>> ---
>>>    drivers/cpufreq/intel_pstate.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
>>> index badf620..43446b5 100644
>>> --- a/drivers/cpufreq/intel_pstate.c
>>> +++ b/drivers/cpufreq/intel_pstate.c
>>> @@ -395,7 +395,7 @@ static void intel_pstate_set_pstate(struct cpudata *cpu, int pstate)
>>>    	trace_cpu_frequency(pstate * 100000, cpu->cpu);
>>>
>>>    	cpu->pstate.current_pstate = pstate;
>>> -	val = pstate << 8;
>>> +	val = (u64)pstate << 8;
>>>    	if (limits.no_turbo)
>>>    		val |= (u64)1 << 32;
>>>
>>>
>>

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