On Thu Jul 15, 2021 at 6:23 AM EDT, Peter Rosin wrote:
> On 2021-07-15 05:12, 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 774eb3044edd..4c3cfd4d5181 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 +68,16 @@ 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;
> > + if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) ||
> > + check_mul_overflow(*val2, rescale->denominator, (s32 *)&tmp2)) {
Hi Peter,
>
> The white space should be like this, methinks.
>
> if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) ||
> check_mul_overflow(*val2, rescale->denominator, (s32 *)&tmp2))
> {
>
Sorry about that... Like I said in the cover letter, I'm working on
getting kunit tests running for the iio-rescale. At the moment it still
requires copying part of the code over and sure enough I forgot to copy
some of it back. My apologies for the noise...
This is what I meant to send:
case IIO_VAL_FRACTIONAL:
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);
do_div(tmp, factor);
do_div(tmp2, factor);
}
*val = tmp;
*val2 = tmp2;
return ret;
I'll also move the opening bracket on a new line if you prefer.
> > + factor = gcd(tmp, tmp2);
>
> And I just realized, gcd() works on unsigned values which is a bit safer
> for the
> scale factor. But here, for the actual values, more care is needed.
>
I added negative test cases to take this into account. I'll update and
resend. I'm going to find a way to get the test cases ready for the next
revision.
> > + do_div(tmp, factor);
> > + do_div(tmp2, factor);
> > + }
> > + *val = tmp;
> > + *val2 = tmp2;
>
> And beside the above points, the whole mechanism seems broken. The
> returned value
> in the third argument to check_mul_overflow isn't useful if there is an
> overflow.
> Yet, the code continues to use tmp and tmp2 in case of overflow. And why
> do you
> first multiply tmp and tmp2 without checks, only to then do the same mul
> again
> but with checks? Or have I completely misunderstood how
> check_mul_overflow
> works?
>
Again, my apologies for this. It's not what I meant to send.
Hopefully the snippet above makes more sense.
Thanks for your time,
Liam
> Cheers,
> Peter
>
> > return ret;
> > case IIO_VAL_INT:
> > *val *= rescale->numerator;
> >
