rtc: Fix offset calculation for .start_secs < 0

commit fe9f5f9 upstream.

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: 9895156 ("rtc: Add one offset seconds to expand RTC range")
Signed-off-by: Alexandre Mergnat <[email protected]>
Reviewed-by: Uwe Kleine-König <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexandre Belloni <[email protected]>
Signed-off-by: Uwe Kleine-König <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: amergnat

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)