From: "Liam Beguin" <liambeguin@gmail.com>
To: "Peter Rosin" <peda@axentia.se>, <jic23@kernel.org>,
<lars@metafoo.de>, <pmeerw@pmeerw.net>
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
Date: Fri, 09 Jul 2021 15:22:48 -0400 [thread overview]
Message-ID: <CCOURFCFL6YC.1SGV7KHPWGIEI@shaak> (raw)
In-Reply-To: <13409f37-ecd4-5afb-e1ca-59f1f1f805b8@axentia.se>
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>
> >
> > 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@xiphos.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;
> >
next prev parent reply other threads:[~2021-07-09 19:22 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-06 16:09 [PATCH v4 00/10] iio: afe: add temperature rescaling support Liam Beguin
2021-07-06 16:09 ` [PATCH v4 01/10] iio: inkern: apply consumer scale on IIO_VAL_INT cases Liam Beguin
2021-07-06 16:09 ` [PATCH v4 02/10] iio: inkern: apply consumer scale when no channel scale is available Liam Beguin
2021-07-06 16:09 ` [PATCH v4 03/10] iio: inkern: make a best effort on offset calculation Liam Beguin
2021-07-06 16:09 ` [PATCH v4 04/10] iio: afe: rescale: reduce risk of integer overflow Liam Beguin
2021-07-09 16:24 ` Peter Rosin
2021-07-09 19:22 ` Liam Beguin [this message]
2021-07-06 16:09 ` [PATCH v4 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support Liam Beguin
2021-07-09 16:29 ` Peter Rosin
2021-07-09 19:30 ` Liam Beguin
2021-07-10 8:14 ` Peter Rosin
2021-07-10 17:45 ` Liam Beguin
2021-07-06 16:09 ` [PATCH v4 06/10] iio: afe: rescale: add offset support Liam Beguin
2021-07-06 16:09 ` [PATCH v4 07/10] iio: afe: rescale: add RTD temperature sensor support Liam Beguin
2021-07-06 16:09 ` [PATCH v4 08/10] iio: afe: rescale: add temperature transducers Liam Beguin
2021-07-06 16:09 ` [PATCH v4 09/10] dt-bindings: iio: afe: add bindings for temperature-sense-rtd Liam Beguin
2021-07-12 16:08 ` Rob Herring
2021-07-06 16:09 ` [PATCH v4 10/10] dt-bindings: iio: afe: add bindings for temperature transducers Liam Beguin
2021-07-12 16:11 ` Rob Herring
2021-07-13 4:20 ` Liam Beguin
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