On Fri Jul 9, 2021 at 12:24 PM EDT, Peter Rosin wrote:
>
> On 2021-07-06 18:09, 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.
> >
> > Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx>
> > ---
> > drivers/iio/afe/iio-rescale.c | 12 +++++++++---
> > 1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> > index 774eb3044edd..ba3bdcc69b16 100644
> > --- a/drivers/iio/afe/iio-rescale.c
> > +++ b/drivers/iio/afe/iio-rescale.c
> > @@ -39,7 +39,8 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
> > int *val, int *val2, long mask)
> > {
> > struct rescale *rescale = iio_priv(indio_dev);
> > - unsigned long long tmp;
> > + s64 tmp, tmp2;
> > + u32 factor;
> > int ret;
> >
> > switch (mask) {
> > @@ -67,8 +68,13 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
> > }
> > switch (ret) {
> > case IIO_VAL_FRACTIONAL:
> > - *val *= rescale->numerator;
> > - *val2 *= rescale->denominator;
> > + tmp = (s64)*val * rescale->numerator;
> > + tmp2 = (s64)*val2 * rescale->denominator;
> > + factor = gcd(tmp, tmp2);
Hi Peter,
>
> Hi!
>
> gcd() isn't exactly free. I do not think it is suitable to call it for
> each
> and every value. So, if you really need it, then it should only be used
> when there is an actual overflow (or if there is a high risk if that's
> somehow easier).
Understood, digging into this a little bit, it seems like
check_mul_overflow() could be used here.
I'll give it a try and will look at implementing Jonathan's suggestion
in case we're dealing with a case where gcd() returns 1.
Thanks,
Liam
>
> Cheers,
> Peter
>
> > + do_div(tmp, factor);
> > + *val = tmp;
> > + do_div(tmp2, factor);
> > + *val2 = tmp2;
> > return ret;
> > case IIO_VAL_INT:
> > *val *= rescale->numerator;
> >
