linux-kernel - Re: [PATCH][V2] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

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Date:   Fri, 16 Feb 2018 16:24:40 +0100
From:   Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>, linux-rtc@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][V2] rtc: tx4939: avoid unintended sign extension on a 24
 bit shift

On 15/02/2018 at 21:44:53 +0100, Alexandre Belloni wrote:
> On 15/02/2018 at 19:36:14 +0000, Colin King wrote:
> > From: Colin Ian King <colin.king@...onical.com>
> > 
> > The shifting of buf[5] by 24 bits to the left will be promoted to
> > a 32 bit signed int and then sign-extended to an unsigned long. If
> > the top bit of buf[5] is set then all then all the upper bits sec
> > end up as also being set because of the sign-extension. Fix this by
> > casting buf[5] to an unsigned long before the shift.
> > 
> 
> The timing of the discovery of this issue is suspicious. I believe it is
> because I just enabled COMPILE_TEST on that driver and now this gets
> compiled on a 64bit architecture.
> 
> Can I ask on which architecture this is an issue? I don't think (and a
> small test program confirms) x86 does the sign extension because both
> sec and buf are unsigned.
> 

Actually, my test program was wrong and you are right.

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com