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[198.48.202.89]) by smtp.gmail.com with ESMTPSA id n25sm2513685qkh.21.2021.07.27.17.07.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 Jul 2021 17:07:31 -0700 (PDT) Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Tue, 27 Jul 2021 20:07:30 -0400 Message-Id: Cc: , , , Subject: Re: [PATCH v6 08/13] iio: afe: rescale: reduce risk of integer overflow From: "Liam Beguin" To: "Peter Rosin" , , , References: <20210721030613.3105327-1-liambeguin@gmail.com> <20210721030613.3105327-9-liambeguin@gmail.com> <9e0e4398-873e-b5c0-0f0c-50a186ed2228@axentia.se> In-Reply-To: <9e0e4398-873e-b5c0-0f0c-50a186ed2228@axentia.se> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > >=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 when required. > >=20 > > Signed-off-by: Liam Beguin > > --- > > drivers/iio/afe/iio-rescale.c | 15 ++++++++++++--- > > 1 file changed, 12 insertions(+), 3 deletions(-) > >=20 > > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescal= e.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; > > =20 > > switch (scale_type) { > > case IIO_VAL_FRACTIONAL: > > - *val *=3D rescale->numerator; > > - *val2 *=3D rescale->denominator; > > + if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) || > > + check_mul_overflow(*val2, rescale->denominator, (s32 *)&tmp2)) { > > + tmp =3D (s64)*val * rescale->numerator; > > + tmp2 =3D (s64)*val2 * rescale->denominator; > > + factor =3D 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 =3D div_s64(tmp, factor); > > + tmp2 =3D div_s64(tmp2, factor); > > + } > > + *val =3D tmp; > > + *val2 =3D tmp2; > > return scale_type; > > case IIO_VAL_INT: > > *val *=3D rescale->numerator; > >=20