linux-kernel - Re: [PATCH v4 2/5] rtc: Fix offset calculation for .start

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Message-ID: <yhcdgillcnlwditdmitklbhkfd2lh5on7ibvuqdxnhricjoele@c246pnf4uc42>
Date: Mon, 28 Apr 2025 19:00:18 +0200
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: Alexandre Mergnat <amergnat@...libre.com>
Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/5] rtc: Fix offset calculation for .start_secs < 0

Hello,

On Mon, Apr 28, 2025 at 12:06:48PM +0200, Alexandre Mergnat wrote:
> The comparison
> 
>         rtc->start_secs > rtc->range_max
> 
> has a signed left-hand side and an unsigned right-hand side.
> So the comparison might become true for negative start_secs which is
> interpreted as a (possibly very large) positive value.
> 
> As a negative value can never be bigger than an unsigned value
> the correct representation of the (mathematical) comparison
> 
>         rtc->start_secs > rtc->range_max
> 
> in C is:
> 
>         rtc->start_secs >= 0 && rtc->start_secs > rtc->range_max
> 
> Use that to fix the offset calculation currently used in the
> rtc-mt6397 driver.
> 
> Fixes: 989515647e783 ("rtc: Add one offset seconds to expand RTC range")
> Signed-off-by: Alexandre Mergnat <amergnat@...libre.com>

Reviewed-by: Uwe Kleine-König <u.kleine-koenig@...libre.com>

Thanks
Uwe

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