linux-kernel - Re: [PATCH] iio: adc: Fix potential integer overflow

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Message-ID: <01297ad3-34a9-994a-f6f4-874dfeb9242b@metafoo.de>
Date:   Mon, 24 Sep 2018 19:19:34 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Jonathan Cameron <jic23@...nel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: adc: Fix potential integer overflow

On 09/24/2018 07:18 PM, Lars-Peter Clausen wrote:
> On 09/22/2018 03:42 PM, Jonathan Cameron wrote:
>> On Tue, 18 Sep 2018 07:53:14 -0500
>> "Gustavo A. R. Silva" <gustavo@...eddedor.com> wrote:
>>
>>> Cast factor to s64 in order to give the compiler complete information
>>> about the proper arithmetic to use and avoid a potential integer
>>> overflow. Notice that such variable is being used in a context
>>> that expects an expression of type s64 (64 bits, signed).
>>>
>>> Addresses-Coverity-ID: 1324146 ("Unintentional integer overflow")
>>> Fixes: e13d757279bb ("iio: adc: Add QCOM SPMI PMIC5 ADC driver")
>>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
>>> ---
>>>  drivers/iio/adc/qcom-vadc-common.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/iio/adc/qcom-vadc-common.c b/drivers/iio/adc/qcom-vadc-common.c
>>> index dcd7fb5..e360e27 100644
>>> --- a/drivers/iio/adc/qcom-vadc-common.c
>>> +++ b/drivers/iio/adc/qcom-vadc-common.c
>>> @@ -282,7 +282,7 @@ static int qcom_vadc_scale_code_voltage_factor(u16 adc_code,
>>>  	voltage = div64_s64(voltage, data->full_scale_code_volt);
>>>  	if (voltage > 0) {
>>>  		voltage *= prescale->den;
>>> -		temp = prescale->num * factor;
>>> +		temp = prescale->num * (s64)factor;
>> So factor is an unsigned int so could be 32 bits.  In reality it only
>> takes a small set of values between 1 and 1000
>>
>> Maximum numerator is 10 so a maximum of 10,000.
>>
>> Hence this is a false positive, be it one that would be very hard
>> for a static checker to identify.
> 
> I think the reason why it complains is because temp is s64. So it infers
> that the idea was that the result of the multiplication can be larger
> than 64 bit. For 32bit * 32bit -> 32bit it should not complain.

"lager than 32 bit"

> 
>>
>> So that moves it from a fix to a warning suppression change.
>> I have no problem with those, but description needs to reflect that.
> 
> Maybe just change the type of temp to u32. There is also
> mul_u64_u32_div() which could be used here to further simplify things.
>