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

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Message-ID: <Ydm9K3Zx3jPPv70B@shaak>
Date:   Sat, 8 Jan 2022 11:34:51 -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

Hi Andy,

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@...il.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@...il.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@...il.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;
	tmp = tmp2;

	...

How can I validate this?

> > 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

Cheers,
Liam