linux-kernel - Re: [PATCH v4 04/10] iio: afe: rescale: reduce risk of integer overflow

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Date:   Fri, 09 Jul 2021 15:22:48 -0400
From:   "Liam Beguin" <liambeguin@...il.com>
To:     "Peter Rosin" <peda@...ntia.se>, <jic23@...nel.org>,
        <lars@...afoo.de>, <pmeerw@...erw.net>
Cc:     <linux-kernel@...r.kernel.org>, <linux-iio@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <robh+dt@...nel.org>
Subject: Re: [PATCH v4 04/10] iio: afe: rescale: reduce risk of integer
 overflow

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@...hos.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.
> > 
> > Signed-off-by: Liam Beguin <lvb@...hos.com>
> > ---
> >  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;
> >