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

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Date:   Wed, 22 Dec 2021 13:38:47 -0500
From:   Liam Beguin <liambeguin@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Peter Rosin <peda@...ntia.se>, Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-iio <linux-iio@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v11 09/15] iio: afe: rescale: reduce risk of integer
 overflow

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@...il.com> wrote:
> >
> > From: Liam Beguin <lvb@...hos.com>
> >
> > Reduce the risk of integer overflow by doing the scale calculation on
> > a 64-bit integer. Since the rescaling is only performed on *val, reuse
> > the IIO_VAL_FRACTIONAL_LOG2 case.
> 
> ...
> 
> > -               tmp = 1 << *val2;
> 
> At some point this should be BIT()

I'm not against changing this, but (to me at least) 1 << *val2 seems
more explicit as we're not working with bitfields. No?

> 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;

Cheers,
Liam

> > +               if (scale_type == IIO_VAL_FRACTIONAL)
> > +                       tmp = *val2;
> > +               else
> > +                       tmp = 1 << *val2;
> 
> 
> -- 
> With Best Regards,
> Andy Shevchenko