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

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Date:   Fri, 16 Feb 2018 16:22:50 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        linux-rtc@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

On Thu, Feb 15, 2018 at 8:59 PM, Colin King <colin.king@...onical.com> 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.
>
> Detected by CoverityScan, CID#1465292 ("Unintended sign extension")
>
> Fixes: 0e1492330cd2 ("rtc: add rtc-tx4939 driver")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>

>         spin_unlock_irq(&pdata->lock);
> -       sec = (buf[5] << 24) | (buf[4] << 16) | (buf[3] << 8) | buf[2];
> +       sec = ((unsigned long)buf[5] << 24) | (buf[4] << 16) |
> +               (buf[3] << 8) | buf[2];

Wouldn't be better to use le32_to_cpu() or get_unaligned()?

-- 
With Best Regards,
Andy Shevchenko