linux-kernel - Re: [PATCH] rtc: tps6594: Fix integer overflow on 32bit systems

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Message-ID: <e434930a-f30d-427f-9cc6-41562a31d8dc@stanley.mountain>
Date: Wed, 11 Dec 2024 15:29:27 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: Esteban Blanc <eblanc@...libre.com>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>,
	linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] rtc: tps6594: Fix integer overflow on 32bit systems

On Wed, Dec 11, 2024 at 01:51:31PM +0200, Andy Shevchenko wrote:
> On Wed, Dec 11, 2024 at 11:32 AM Dan Carpenter <dan.carpenter@...aro.org> wrote:
> >
> > The problem is this multiply in tps6594_rtc_set_offset()
> >
> >         tmp = offset * TICKS_PER_HOUR;
> >
> > The "tmp" variable is an s64 but "offset" is a long in the
> > (-277774)-277774 range.  On 32bit systems a long can hold numbers up to
> > approximately two billion.  The number of TICKS_PER_HOUR is really large,
> > (32768 * 3600) or roughly a hundred million.  When you start multiplying
> > by a hundred million it doesn't take long to overflow the two billion
> > mark.
> >
> > Probably the safest way to fix this is to change the type of
> > TICKS_PER_HOUR to long long because it's such a large number.
> 
> ...
> 
> > -#define TICKS_PER_HOUR (32768 * 3600)
> > +#define TICKS_PER_HOUR (32768 * 3600LL)
> 
> Hmm... And why signed?

It needs to be signed for negatives.  That's deliberate.

regards,
dan carpenter