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

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Message-ID: <CAHp75VfssNnd9zvNu+N9xc74RO+qBPC_qhF5ed_G8p5HJ8LWvw@mail.gmail.com>
Date: Wed, 11 Dec 2024 13:51:31 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Dan Carpenter <dan.carpenter@...aro.org>
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 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?

Wondering, do we deserve to have something like
#define SEC_PER_HOUR 3600UL
somewhere in the headers, if not already exists?

-- 
With Best Regards,
Andy Shevchenko