rtc: interface: Validate alarm-time before handling rollover

In function __rtc_read_alarm() its possible for an alarm time-stamp to
be invalid even after replacing missing components with current
time-stamp. The condition 'alarm->time.tm_year < 70' will trigger this
case and will cause the call to 'rtc_tm_to_time64(&alarm->time)'
return a negative value for variable t_alm.

While handling alarm rollover this negative t_alm (assumed to seconds
offset from '1970-01-01 00:00:00') is converted back to rtc_time via
rtc_time64_to_tm() which results in this error log with seemingly
garbage values:

"rtc rtc0: invalid alarm value: -2-1--1041528741
2005511117:71582844:32"

This error was generated when the rtc driver (rtc-opal in this case)
returned an alarm time-stamp of '00-00-00 00:00:00' to indicate that
the alarm is disabled. Though I have submitted a separate fix for the
rtc-opal driver, this issue may potentially impact other
existing/future rtc drivers.

To fix this issue the patch validates the alarm time-stamp just after
filling up the missing datetime components and if rtc_valid_tm() still
reports it to be invalid then bails out of the function without
handling the rollover.

Reported-by: Steve Best <sbest@redhat.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/interface.c

@@ -227,6 +227,13 @@ int __rtc_read_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
 			missing = year;
 	}
 
+	/* Can't proceed if alarm is still invalid after replacing
+	 * missing fields.
+	 */
+	err = rtc_valid_tm(&alarm->time);
+	if (err)
+		goto done;
+
 	/* with luck, no rollover is needed */
 	t_now = rtc_tm_to_time64(&now);
 	t_alm = rtc_tm_to_time64(&alarm->time);
@@ -278,9 +285,9 @@ int __rtc_read_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
 		dev_warn(&rtc->dev, "alarm rollover not handled\n");
 	}
 
-done:
 	err = rtc_valid_tm(&alarm->time);
 
+done:
 	if (err) {
 		dev_warn(&rtc->dev, "invalid alarm value: %d-%d-%d %d:%d:%d\n",
 			alarm->time.tm_year + 1900, alarm->time.tm_mon + 1,