rtc: cmos: zero-init wkalrm when reading from CMOS

cmos_read_alarm() may leave certain fields of a struct rtc_wkalrm
untouched; therefore, these fields contain garbage if not properly
initialized, leading to inconsistent values when converting into
time64_t. This patch to zero initialize the struct before calling
cmos_read_alarm().

Note that this patch is not intended to produce a correct time64_t, it
is only to produce a consistent value. In the case of suspend/resume, a
correct time64_t is not necessary; a consistent value is sufficient to
correctly perform an equality test for t_current_expires and
t_saved_expires. Logic to deduce a correct time64_t is expensive and
hence should be avoided.

Signed-off-by: Victor Ding <victording@google.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200814191654.v2.1.Iaf7638a2f2a87ff68d85fcb8dec615e41340c97f@changeid

drivers/rtc/rtc-cmos.c

@@ -1006,6 +1006,7 @@ static int cmos_suspend(struct device *dev)
 			enable_irq_wake(cmos->irq);
 	}
 
+	memset(&cmos->saved_wkalrm, 0, sizeof(struct rtc_wkalrm));
 	cmos_read_alarm(dev, &cmos->saved_wkalrm);
 
 	dev_dbg(dev, "suspend%s, ctrl %02x\n",
@@ -1054,6 +1055,7 @@ static void cmos_check_wkalrm(struct device *dev)
 		return;
 	}
 
+	memset(&current_alarm, 0, sizeof(struct rtc_wkalrm));
 	cmos_read_alarm(dev, &current_alarm);
 	t_current_expires = rtc_tm_to_time64(&current_alarm.time);
 	t_saved_expires = rtc_tm_to_time64(&cmos->saved_wkalrm.time);