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

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Date:   Thu, 15 Feb 2018 19:25:01 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        linux-rtc@...r.kernel.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: NACK: [PATCH] rtc: tx4939: avoid unintended sign extension on a 24
 bit shift

On 15/02/18 18:59, 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.
> 
> 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>
> ---
>  drivers/rtc/rtc-tx4939.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/rtc/rtc-tx4939.c b/drivers/rtc/rtc-tx4939.c
> index feededce3ded..b8a066cbcc42 100644
> --- a/drivers/rtc/rtc-tx4939.c
> +++ b/drivers/rtc/rtc-tx4939.c
> @@ -170,7 +170,8 @@ static int tx4939_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
>  	alrm->enabled = (ctl & TX4939_RTCCTL_ALME) ? 1 : 0;
>  	alrm->pending = (ctl & TX4939_RTCCTL_ALMD) ? 1 : 0;
>  	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];
>  	rtc_time_to_tm(sec, &alrm->time);
>  	return rtc_valid_tm(&alrm->time);
>  }
> 
Nack, there are two occurrences of this sign extension, I missed the
other one. I'll re-send a fix.

Colin