The comparison

        rtc->start_secs > rtc->range_max

has a signed left-hand side and an unsigned right-hand side.
So the comparison might become true for negative start_secs which is
interpreted as a (possibly very large) positive value.

As a negative value can never be bigger than an unsigned value
the correct representation of the (mathematical) comparison

        rtc->start_secs > rtc->range_max

in C is:

        rtc->start_secs >= 0 && rtc->start_secs > rtc->range_max

Use that to fix the offset calculation currently used in the
rtc-mt6397 driver.

Fixes: 989515647e783 ("rtc: Add one offset seconds to expand RTC range")
Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
---
 drivers/rtc/class.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index e31fa0ad127e9545afac745a621d4ccbcd5fca28..a0afdeaac270f026734be8785e8ed7e1272b2348 100644
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -327,7 +327,7 @@ static void rtc_device_get_offset(struct rtc_device *rtc)
 	 *
 	 * Otherwise the offset seconds should be 0.
 	 */
-	if (rtc->start_secs > rtc->range_max ||
+	if ((rtc->start_secs >= 0 && rtc->start_secs > rtc->range_max) ||
 	    rtc->start_secs + range_secs - 1 < rtc->range_min)
 		rtc->offset_secs = rtc->start_secs - rtc->range_min;
 	else if (rtc->start_secs > rtc->range_min)

-- 
2.25.1

Hello,

On Mon, Apr 28, 2025 at 12:06:48PM +0200, Alexandre Mergnat wrote:
> The comparison
> 
>         rtc->start_secs > rtc->range_max
> 
> has a signed left-hand side and an unsigned right-hand side.
> So the comparison might become true for negative start_secs which is
> interpreted as a (possibly very large) positive value.
> 
> As a negative value can never be bigger than an unsigned value
> the correct representation of the (mathematical) comparison
> 
>         rtc->start_secs > rtc->range_max
> 
> in C is:
> 
>         rtc->start_secs >= 0 && rtc->start_secs > rtc->range_max
> 
> Use that to fix the offset calculation currently used in the
> rtc-mt6397 driver.
> 
> Fixes: 989515647e783 ("rtc: Add one offset seconds to expand RTC range")
> Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>

Reviewed-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>

Thanks
Uwe