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.
rescale_voltage_divider_props() seems to also use gcd() with signed
integers.
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;
> >
