Re: [PATCH v11 09/15] iio: afe: rescale: reduce risk of integer overflow

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: Wed, 22 Dec 2021 20:56:12 +0200	[thread overview]
Message-ID: <CAHp75VdrLTNLWZRgWkLXD23RAF28zh29XybywAPyMtb=GNxXbw@mail.gmail.com> (raw)
In-Reply-To: <YcNwt5RFMNFUimD/@shaak>

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.

-- 
With Best Regards,
Andy Shevchenko