[PATCH] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

[PATCH] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

[Colin King]

From: Colin Ian King <colin.king@canonical.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@canonical.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);
 }
-- 
2.15.1

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

[Colin Ian King]

On 15/02/18 18:59, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.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@canonical.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

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

[Andy Shevchenko]

On Thu, Feb 15, 2018 at 8:59 PM, Colin King <colin.king@canonical.com> wrote:
> From: Colin Ian King <colin.king@canonical.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@canonical.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

RE: [PATCH] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

[David Laight]

From: Andy Shevchenko
> Sent: 16 February 2018 14:23
...
> Wouldn't be better to use le32_to_cpu() or get_unaligned()?

Or shoot the members of the ANSI C committee who insisted on
promotion of unsigned char/short to int and sign preserving
promotion of int to unsigned long :-)

	David