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Date:   Fri, 09 Jul 2021 15:22:48 -0400
Message-Id: <CCOURFCFL6YC.1SGV7KHPWGIEI@shaak>
Cc:     <linux-kernel@vger.kernel.org>, <linux-iio@vger.kernel.org>,
        <devicetree@vger.kernel.org>, <robh+dt@kernel.org>
Subject: Re: [PATCH v4 04/10] iio: afe: rescale: reduce risk of integer
 overflow
From:   "Liam Beguin" <liambeguin@gmail.com>
To:     "Peter Rosin" <peda@axentia.se>, <jic23@kernel.org>,
        <lars@metafoo.de>, <pmeerw@pmeerw.net>
References: <20210706160942.3181474-1-liambeguin@gmail.com>
 <20210706160942.3181474-5-liambeguin@gmail.com>
 <13409f37-ecd4-5afb-e1ca-59f1f1f805b8@axentia.se>
In-Reply-To: <13409f37-ecd4-5afb-e1ca-59f1f1f805b8@axentia.se>
Precedence: bulk

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@xiphos.com>
> >=20
> > 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.
> >=20
> > Signed-off-by: Liam Beguin <lvb@xiphos.com>
> > ---
> >  drivers/iio/afe/iio-rescale.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> >=20
> > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescal=
e.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 =3D iio_priv(indio_dev);
> > -	unsigned long long tmp;
> > +	s64 tmp, tmp2;
> > +	u32 factor;
> >  	int ret;
> > =20
> >  	switch (mask) {
> > @@ -67,8 +68,13 @@ static int rescale_read_raw(struct iio_dev *indio_de=
v,
> >  		}
> >  		switch (ret) {
> >  		case IIO_VAL_FRACTIONAL:
> > -			*val *=3D rescale->numerator;
> > -			*val2 *=3D rescale->denominator;
> > +			tmp =3D (s64)*val * rescale->numerator;
> > +			tmp2 =3D (s64)*val2 * rescale->denominator;
> > +			factor =3D 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 =3D tmp;
> > +			do_div(tmp2, factor);
> > +			*val2 =3D tmp2;
> >  			return ret;
> >  		case IIO_VAL_INT:
> >  			*val *=3D rescale->numerator;
> >=20