From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Liam Beguin <liambeguin@gmail.com>
Cc: Peter Rosin <peda@axentia.se>,
Jonathan Cameron <jic23@kernel.org>,
Lars-Peter Clausen <lars@metafoo.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-iio <linux-iio@vger.kernel.org>,
devicetree <devicetree@vger.kernel.org>,
Rob Herring <robh+dt@kernel.org>
Subject: Re: [PATCH v11 09/15] iio: afe: rescale: reduce risk of integer overflow
Date: Sat, 8 Jan 2022 19:55:21 +0200 [thread overview]
Message-ID: <CAHp75Ve5-W7vRGBMaz8jh5DvOCyTovdKaRR_5iB7S7epiWQmfA@mail.gmail.com> (raw)
In-Reply-To: <Ydm9K3Zx3jPPv70B@shaak>
On Sat, Jan 8, 2022 at 6:34 PM Liam Beguin <liambeguin@gmail.com> wrote:
> On Wed, Dec 22, 2021 at 11:32:24PM +0200, Andy Shevchenko wrote:
> > On Wed, Dec 22, 2021 at 9:59 PM Liam Beguin <liambeguin@gmail.com> wrote:
> > > On Wed, Dec 22, 2021 at 08:56:12PM +0200, Andy Shevchenko wrote:
> > > > On Wed, Dec 22, 2021 at 8:38 PM Liam Beguin <liambeguin@gmail.com> wrote:
> > > > > On Wed, Dec 22, 2021 at 02:29:04PM +0200, Andy Shevchenko wrote:
> > > > > > On Wed, Dec 22, 2021 at 5:47 AM Liam Beguin <liambeguin@gmail.com> wrote:
...
> > > > > > > - tmp = 1 << *val2;
> > > > > >
> > > > > > At some point this should be BIT()
> > > >
> > > > Forgot to add, If it's 64-bit, then BIT_ULL().
> > > >
> > > > > I'm not against changing this, but (to me at least) 1 << *val2 seems
> > > > > more explicit as we're not working with bitfields. No?
> > > >
> > > > You may add a comment. You may use int_pow(), but it will be suboptimal.
> > > >
> > > > > > Rule of thumb (in accordance with C standard), always use unsigned
> > > > > > value as left operand of the _left_ shift.
> > > > >
> > > > > Right, that makes sense! In practice though, since we'll most likely
> > > > > never use higher bits of *val2 with IIO_VAL_FRACTIONAL_LOG2, would it be
> > > > > enough to simply typecast?
> > > > >
> > > > > tmp = 1 << (unsigned int)*val2;
> > > >
> > > > No, it's about the _left_ operand.
> > > > I haven't checked if tmp is 64-bit, then even that would be still wrong.
> > >
> > > Okay so your recommendation is to not use a left shift?
> >
> > No, I recommend not to use int type as a _leftside_ operand.
> > BIT() / BIT_ULL() does a left shift anyway.
>
> Oh, got it. Sorry for misreading your message.
> would something like this be good enough?
>
> s64 tmp;
> u64 tmp2;
> tmp2 = 1 << *val2;
This still has a UB according to the C standard. That's why
BIT()/BIT_ULL() is preferable to use since they don't have such
issues. You may open code it, of course (since I remember you wished
to show that this is not a bit, but a number).
> tmp = tmp2;
> How can I validate this?
By understanding the C standard? I dunno, actually. GCC will generate
correct code, it's just a special warning you may get when supplying a
parameter (Linux kernel doesn't use that one even on W=2 IIRC).
-Wshift-overflow=2
> > > I can look into that but given how unlikely it is to fall into those bad
> > > cases, I'd rather keep things as they are. Would that be okay?
> >
> > > Also, I don't think using BIT() or BIT_ULL() would address this as they
> > > both do the same shift, with no extra checks.
> >
> > They do slightly different versions of it. They use an unsigned int type.
> >
> > Open coded or not, it's up to you. Just convert to unsigned int.
--
With Best Regards,
Andy Shevchenko
next prev parent reply other threads:[~2022-01-08 17:56 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-22 3:46 [PATCH v11 00/15] iio: afe: add temperature rescaling support Liam Beguin
2021-12-22 3:46 ` [PATCH v11 01/15] iio: inkern: apply consumer scale on IIO_VAL_INT cases Liam Beguin
2021-12-22 3:46 ` [PATCH v11 02/15] iio: inkern: apply consumer scale when no channel scale is available Liam Beguin
2021-12-22 3:46 ` [PATCH v11 03/15] iio: inkern: make a best effort on offset calculation Liam Beguin
2021-12-22 3:46 ` [PATCH v11 04/15] iio: afe: rescale: expose scale processing function Liam Beguin
2021-12-22 10:21 ` Andy Shevchenko
2021-12-22 18:20 ` Liam Beguin
2021-12-22 18:52 ` Andy Shevchenko
2021-12-22 19:42 ` Liam Beguin
2021-12-22 19:50 ` Andy Shevchenko
2021-12-22 20:04 ` Liam Beguin
2021-12-22 3:46 ` [PATCH v11 05/15] iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support Liam Beguin
2021-12-22 3:46 ` [PATCH v11 06/15] iio: afe: rescale: add offset support Liam Beguin
2021-12-22 3:46 ` [PATCH v11 07/15] iio: afe: rescale: use s64 for temporary scale calculations Liam Beguin
2021-12-22 12:25 ` Andy Shevchenko
2021-12-22 18:21 ` Liam Beguin
2021-12-22 3:46 ` [PATCH v11 08/15] iio: afe: rescale: fix accuracy for small fractional scales Liam Beguin
2021-12-22 12:27 ` Andy Shevchenko
2021-12-22 3:46 ` [PATCH v11 09/15] iio: afe: rescale: reduce risk of integer overflow Liam Beguin
2021-12-22 12:29 ` Andy Shevchenko
2021-12-22 18:38 ` Liam Beguin
2021-12-22 18:56 ` Andy Shevchenko
2021-12-22 19:58 ` Liam Beguin
2021-12-22 21:32 ` Andy Shevchenko
2022-01-08 16:34 ` Liam Beguin
2022-01-08 17:55 ` Andy Shevchenko [this message]
2021-12-22 3:46 ` [PATCH v11 10/15] iio: afe: rescale: make use of units.h Liam Beguin
2021-12-22 12:33 ` Andy Shevchenko
2021-12-22 18:50 ` Liam Beguin
2021-12-22 18:59 ` Andy Shevchenko
2021-12-22 3:46 ` [PATCH v11 11/15] iio: test: add basic tests for the iio-rescale driver Liam Beguin
2021-12-22 12:38 ` Andy Shevchenko
2021-12-22 19:13 ` Liam Beguin
2021-12-22 3:46 ` [PATCH v11 12/15] iio: afe: rescale: add RTD temperature sensor support Liam Beguin
2021-12-22 3:46 ` [PATCH v11 13/15] iio: afe: rescale: add temperature transducers Liam Beguin
2021-12-22 12:40 ` Andy Shevchenko
2021-12-22 3:46 ` [PATCH v11 14/15] dt-bindings: iio: afe: add bindings for temperature-sense-rtd Liam Beguin
2021-12-22 3:46 ` [PATCH v11 15/15] dt-bindings: iio: afe: add bindings for temperature transducers Liam Beguin
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