On 2021-07-28 02:07, Liam Beguin wrote:
> On Fri Jul 23, 2021 at 5:17 PM EDT, Peter Rosin wrote:
>> On 2021-07-21 05:06, Liam Beguin wrote:
>>> From: Liam Beguin <lvb@xxxxxxxxxx>
>>>
>>> Reduce the risk of integer overflow by doing the scale calculation with
>>> 64bit integers and looking for a Greatest Common Divider for both parts
>>> of the fractional value when required.
>>>
>>> Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx>
>>> ---
>>> drivers/iio/afe/iio-rescale.c | 15 ++++++++++++---
>>> 1 file changed, 12 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
>>> index 6f6a711ae3ae..35fa3b4e53e0 100644
>>> --- a/drivers/iio/afe/iio-rescale.c
>>> +++ b/drivers/iio/afe/iio-rescale.c
>>> @@ -21,12 +21,21 @@
>>> int rescale_process_scale(struct rescale *rescale, int scale_type,
>>> int *val, int *val2)
>>> {
>>> - unsigned long long tmp;
>>> + s64 tmp, tmp2;
>>> + u32 factor;
>>>
>>> switch (scale_type) {
>>> case IIO_VAL_FRACTIONAL:
>>> - *val *= rescale->numerator;
>>> - *val2 *= rescale->denominator;
>>> + if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) ||
>>> + check_mul_overflow(*val2, rescale->denominator, (s32 *)&tmp2)) {
>>> + tmp = (s64)*val * rescale->numerator;
>>> + tmp2 = (s64)*val2 * rescale->denominator;
>>> + factor = gcd(tmp, tmp2);
>
> Hi Peter,
>
>>
>> Hi!
>>
>> Reiterating that gcd() only works for unsigned operands, so this is
>> broken for
>> negative values.
>
> Apologies, I didn't mean to make it seem like I ignored your comments. I
> should've added a note. After you pointed out that gcd() only works for
> unsigned elements, I added test cases for negative values, and all tests
> passed. I'll look into it more.
Maybe I've misread the code and gcd is in fact working for negative
numbers? However, I imagine it might be arch specific, so testing on
a single arch feels insufficient and deeper analysis is required.
However, looking at lib/math/gcd.c it certainly still looks like
negative values will work very poorly, and there is no macro magic
in include/linux/gcd.h to handle it by wrapping the core C routine.
> rescale_voltage_divider_props() seems to also use gcd() with signed
> integers.
The type of the operands may be s32, but if you look at how those values
are populated, and with what they are populated, I think you will find that
only positive scale factors are sensible for a voltage divider. Using
resistors with so high resistance that s32 is not enough is simply not
supported.
Cheers,
Peter
> Thanks,
> Liam
>
>>
>> Cheers,
>> Peter
>>
>>> + tmp = div_s64(tmp, factor);
>>> + tmp2 = div_s64(tmp2, factor);
>>> + }
>>> + *val = tmp;
>>> + *val2 = tmp2;
>>> return scale_type;
>>> case IIO_VAL_INT:
>>> *val *= rescale->numerator;
>>>
>
