time: add missing implementation for timespec64_add_safe()

timespec64_add_safe() has been defined in time64.h for 64 bit systems.
But, 32 bit systems only have an extern function prototype defined.
Provide a definition for the above function.

The function will be necessary as part of y2038 changes.  struct
timespec is not y2038 safe.  All references to timespec will be replaced
by struct timespec64.  The function is meant to be a replacement for
timespec_add_safe().

The implementation is similar to timespec_add_safe().

Link: http://lkml.kernel.org/r/1461947989-21926-2-git-send-email-deepa.kernel@gmail.com
Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/time64.h

@@ -136,13 +136,11 @@ extern void set_normalized_timespec64(struct timespec64 *ts, time64_t sec, s64 n
 
 /*
  * timespec64_add_safe assumes both values are positive and checks for
- * overflow. It will return TIME_T_MAX if the returned value would be
- * smaller then either of the arguments.
+ * overflow. It will return TIME64_MAX in case of overflow.
  */
 extern struct timespec64 timespec64_add_safe(const struct timespec64 lhs,
 					 const struct timespec64 rhs);
 
-
 static inline struct timespec64 timespec64_add(struct timespec64 lhs,
 						struct timespec64 rhs)
 {

kernel/time/time.c

@@ -769,3 +769,28 @@ struct timespec timespec_add_safe(const struct timespec lhs,
 
 	return res;
 }
+
+#if __BITS_PER_LONG != 64
+
+/*
+ * Add two timespec64 values and do a safety check for overflow.
+ * It's assumed that both values are valid (>= 0).
+ * And, each timespec64 is in normalized form.
+ */
+struct timespec64 timespec64_add_safe(const struct timespec64 lhs,
+				const struct timespec64 rhs)
+{
+	struct timespec64 res;
+
+	set_normalized_timespec64(&res, lhs.tv_sec + rhs.tv_sec,
+			lhs.tv_nsec + rhs.tv_nsec);
+
+	if (unlikely(res.tv_sec < lhs.tv_sec || res.tv_sec < rhs.tv_sec)) {
+		res.tv_sec = TIME64_MAX;
+		res.tv_nsec = 0;
+	}
+
+	return res;
+}
+
+#endif