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From:   Romain Perier <romain.perier@gmail.com>
Date:   Tue, 28 Sep 2021 15:54:53 +0200
Message-ID: <CABgxDoLPTcRbZZgAdJ9+=9OG+a=F59x9SQ9HvQkVvGmkDjO6-A@mail.gmail.com>
Subject: Re: [PATCH][next] rtc: msc313: Fix unintentional sign extension issue
 on left shift of a u16
To:     Daniel Palmer <daniel@0x0f.com>
Cc:     Colin King <colin.king@canonical.com>,
        Daniel Palmer <daniel@thingy.jp>,
        Alessandro Zummo <a.zummo@towertech.it>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Nobuhiro Iwamatsu <iwamatsu@nigauri.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-rtc@vger.kernel.org, kernel-janitors@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi,

Le mar. 28 sept. 2021 =C3=A0 15:31, Daniel Palmer <daniel@0x0f.com> a =C3=
=A9crit :
>
> Hi Colin,
>
> On Tue, 28 Sept 2021 at 21:39, Colin King <colin.king@canonical.com> wrot=
e:
> >Shifting the u16 value returned by readw by 16 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 the readw is set then
> >the shifted value will be sign extended and the top 32 bits of
> >the result will be set.

Good catch !

>
> Ah,.. C is fun in all the wrong places. :)
> These chips are full of 32bit registers that are split into two 16
> registers 4 bytes apart when seen from the ARM CPU so we probably have
> this same mistake in a few other places.
>
> A similar pattern is used a bit later on in the same file to read the cou=
nter:
>
> seconds =3D readw(priv->rtc_base + REG_RTC_CNT_VAL_L)
> | (readw(priv->rtc_base + REG_RTC_CNT_VAL_H) << 16);
>
> I guess it works at the moment because the top bit won't be set until 203=
8.

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 :) (probabl=
y
because *this case* specifically did not happen while running the test)

1. https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/rtc-tools.git/t=
ree/rtc-range.c

Thanks,
Regards,
Romain