time: Avoid undefined behaviour in ktime_add_safe()

I ran into this:

    ================================================================================
    UBSAN: Undefined behaviour in kernel/time/hrtimer.c:310:16
    signed integer overflow:
    9223372036854775807 + 50000 cannot be represented in type 'long long int'
    CPU: 2 PID: 4798 Comm: trinity-c2 Not tainted 4.8.0-rc1+ #91
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org 04/01/2014
     0000000000000000 ffff88010ce6fb88 ffffffff82344740 0000000041b58ab3
     ffffffff84f97a20 ffffffff82344694 ffff88010ce6fbb0 ffff88010ce6fb60
     000000000000c350 ffff88010ce6f968 dffffc0000000000 ffffffff857bc320
    Call Trace:
     [<ffffffff82344740>] dump_stack+0xac/0xfc
     [<ffffffff82344694>] ? _atomic_dec_and_lock+0xc4/0xc4
     [<ffffffff8242df78>] ubsan_epilogue+0xd/0x8a
     [<ffffffff8242e6b4>] handle_overflow+0x202/0x23d
     [<ffffffff8242e4b2>] ? val_to_string.constprop.6+0x11e/0x11e
     [<ffffffff8236df71>] ? timerqueue_add+0x151/0x410
     [<ffffffff81485c48>] ? hrtimer_start_range_ns+0x3b8/0x1380
     [<ffffffff81795631>] ? memset+0x31/0x40
     [<ffffffff8242e6fd>] __ubsan_handle_add_overflow+0xe/0x10
     [<ffffffff81488ac9>] hrtimer_nanosleep+0x5d9/0x790
     [<ffffffff814884f0>] ? hrtimer_init_sleeper+0x80/0x80
     [<ffffffff813a9ffb>] ? __might_sleep+0x5b/0x260
     [<ffffffff8148be10>] common_nsleep+0x20/0x30
     [<ffffffff814906c7>] SyS_clock_nanosleep+0x197/0x210
     [<ffffffff81490530>] ? SyS_clock_getres+0x150/0x150
     [<ffffffff823c7113>] ? __this_cpu_preempt_check+0x13/0x20
     [<ffffffff8162ef60>] ? __context_tracking_exit.part.3+0x30/0x1b0
     [<ffffffff81490530>] ? SyS_clock_getres+0x150/0x150
     [<ffffffff81007bd3>] do_syscall_64+0x1b3/0x4b0
     [<ffffffff845f85aa>] entry_SYSCALL64_slow_path+0x25/0x25
    ================================================================================

Add a new ktime_add_unsafe() helper which doesn't check for overflow, but
doesn't throw a UBSAN warning when it does overflow either.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

include/linux/ktime.h

@@ -63,6 +63,13 @@ static inline ktime_t ktime_set(const s64 secs, const unsigned long nsecs)
 #define ktime_add(lhs, rhs) \
 		({ (ktime_t){ .tv64 = (lhs).tv64 + (rhs).tv64 }; })
 
+/*
+ * Same as ktime_add(), but avoids undefined behaviour on overflow; however,
+ * this means that you must check the result for overflow yourself.
+ */
+#define ktime_add_unsafe(lhs, rhs) \
+		({ (ktime_t){ .tv64 = (u64) (lhs).tv64 + (rhs).tv64 }; })
+
 /*
  * Add a ktime_t variable and a scalar nanosecond value.
  * res = kt + nsval:

kernel/time/hrtimer.c

@@ -307,7 +307,7 @@ EXPORT_SYMBOL_GPL(__ktime_divns);
  */
 ktime_t ktime_add_safe(const ktime_t lhs, const ktime_t rhs)
 {
-	ktime_t res = ktime_add(lhs, rhs);
+	ktime_t res = ktime_add_unsafe(lhs, rhs);
 
 	/*
 	 * We use KTIME_SEC_MAX here, the maximum timeout which we can