linux-kernel - Re: [PATCH][next] rtc: msc313: Fix unintentional sign extension issue on left shift of a u16

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Date:   Wed, 29 Sep 2021 21:56:36 +0900
From:   Daniel Palmer <daniel@...f.com>
To:     Romain Perier <romain.perier@...il.com>
Cc:     Colin King <colin.king@...onical.com>,
        Daniel Palmer <daniel@...ngy.jp>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Nobuhiro Iwamatsu <iwamatsu@...auri.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-rtc@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] rtc: msc313: Fix unintentional sign extension issue
 on left shift of a u16

Hi Romain,

On Tue, 28 Sept 2021 at 22:55, Romain Perier <romain.perier@...il.com> wrote:
>
> Hi,
>
> Le mar. 28 sept. 2021 à 15:31, Daniel Palmer <daniel@...f.com> a écrit :
> The crazy stuff being, I ran rtctest from selftests and rtc-range (1)
> that tests a variety
> of dates including 2038 and 2106 for example. Both tests passed :) (probably
> because *this case* specifically did not happen while running the test)

I suspect it works because for reading the time because seconds is a
u32 not unsigned long like the other functions.
So if the high word of the register is read, is promoted to a wider
type and sign extended it doesn't actually matter because it gets
truncated to 32 bits so the sign extended part is gone.

Cheers,

Daniel