select: add a timespec_add_safe() function

For the select() rework, it's important to be able to add timespec
structures in an overflow-safe manner.

This patch adds a timespec_add_safe() function for this which is similar in
operation to ktime_add_safe(), but works on a struct timespec.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>

kernel/time.c

@@ -669,3 +669,21 @@ EXPORT_SYMBOL(get_jiffies_64);
 #endif
 
 EXPORT_SYMBOL(jiffies);
+
+/*
+ * Add two timespec values and do a safety check for overflow.
+ * It's assumed that both values are valid (>= 0)
+ */
+struct timespec timespec_add_safe(const struct timespec lhs,
+				  const struct timespec rhs)
+{
+	struct timespec res;
+
+	set_normalized_timespec(&res, lhs.tv_sec + rhs.tv_sec,
+				lhs.tv_nsec + rhs.tv_nsec);
+
+	if (res.tv_sec < lhs.tv_sec || res.tv_sec < rhs.tv_sec)
+		res.tv_sec = TIME_T_MAX;
+
+	return res;
+}