Messages in this thread | | | Date | Tue, 27 Jul 2021 20:07:30 -0400 | Subject | Re: [PATCH v6 08/13] iio: afe: rescale: reduce risk of integer overflow | From | "Liam Beguin" <> |
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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.
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; > >
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