Re: [PATCH v6 08/13] iio: afe: rescale: reduce risk of integer overflow

["Liam Beguin" <liambeguin@gmail.com>]

On Wed Jul 28, 2021 at 3:47 AM EDT, Peter Rosin wrote:
> 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@xiphos.com>
> >>>
> >>> 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@xiphos.com>
> >>> ---
> >>>  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.

I agree that looking at lib/math/gcd.c odd things might happen with
negative values. I'll use the the absolute values to calculate the GCD
as it shouldn't affect the value of factor.

>
> > 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.

That makes sense!

Thanks,
Liam

>
> 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;
> >>>
> >