[PATCH RFC 04/12] time: Add rtc_tm_to_time() safe version(using time64_t)

["pang.xunlei" <pang.xunlei@linaro.org>]

The kernel uses 32-bit signed value(time_t) for seconds since 1970-01-01:00:00:00, thus it
will overflow at 2038-01-19 03:14:08 on 32-bit systems. We call this "2038 safety" issue.

As part of addressing 2038 saftey for in-kernel uses, this patch adds the safe rtc_tm_to_time()
using time64_t. After this patch, rtc_tm_to_time_unsafe() should be replaced by rtc_tm_to_time()
one by one.

Eventually, rtc_tm_to_time_unsafe() will be removed from the kernel when it has no users.

Signed-off-by: pang.xunlei <pang.xunlei@linaro.org>
---
 drivers/rtc/rtc-lib.c |    9 +++++++++
 include/linux/rtc.h   |    1 +
 2 files changed, 10 insertions(+)

diff --git a/drivers/rtc/rtc-lib.c b/drivers/rtc/rtc-lib.c
index 168d2a2..829eef8 100644
--- a/drivers/rtc/rtc-lib.c
+++ b/drivers/rtc/rtc-lib.c
@@ -111,6 +111,15 @@ EXPORT_SYMBOL(rtc_valid_tm);
 /*
  * Convert Gregorian date to seconds since 01-01-1970 00:00:00.
  */
+int rtc_tm_to_time(struct rtc_time *tm, time64_t *time)
+{
+        *time = mktime(tm->tm_year + 1900, tm->tm_mon + 1, tm->tm_mday,
+                        tm->tm_hour, tm->tm_min, tm->tm_sec);
+        return 0;
+}
+EXPORT_SYMBOL(rtc_tm_to_time);
+
+/* TODO: [2038 safety] unsafe for legacy, and should be replaced by rtc_tm_to_time() */
 int rtc_tm_to_time_unsafe(struct rtc_time *tm, unsigned long *time)
 {
 	*time = (unsigned long) mktime(tm->tm_year + 1900, tm->tm_mon + 1, tm->tm_mday,
diff --git a/include/linux/rtc.h b/include/linux/rtc.h
index 97bbb6f..ffd22e9 100644
--- a/include/linux/rtc.h
+++ b/include/linux/rtc.h
@@ -19,6 +19,7 @@
 extern int rtc_month_days(unsigned int month, unsigned int year);
 extern int rtc_year_days(unsigned int day, unsigned int month, unsigned int year);
 extern int rtc_valid_tm(struct rtc_time *tm);
+extern int rtc_tm_to_time(struct rtc_time *tm, time64_t *time);
 extern int rtc_tm_to_time_unsafe(struct rtc_time *tm, unsigned long *time);
 extern void rtc_time_to_tm(unsigned long time, struct rtc_time *tm);
 ktime_t rtc_tm_to_ktime(struct rtc_time tm);
-- 
1.7.9.5