hrtimer: Ensure POSIX compliance (relative CLOCK_REALTIME hrtimers)

The POSIX specification defines that relative CLOCK_REALTIME timers are not
affected by clock modifications. Those timers have to use CLOCK_MONOTONIC
to ensure POSIX compliance.

The introduction of the additional HRTIMER_MODE_PINNED mode broke this
requirement for pinned timers.

There is no user space visible impact because user space timers are not
using pinned mode, but for consistency reasons this needs to be fixed.

Check whether the mode has the HRTIMER_MODE_REL bit set instead of
comparing with HRTIMER_MODE_ABS.

Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: keescook@chromium.org
Fixes: 597d0275736d ("timers: Framework for identifying pinned timers")
Link: http://lkml.kernel.org/r/20171221104205.7269-7-anna-maria@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/time/hrtimer.c

@@ -1095,7 +1095,12 @@ static void __hrtimer_init(struct hrtimer *timer, clockid_t clock_id,
 
 	cpu_base = raw_cpu_ptr(&hrtimer_bases);
 
-	if (clock_id == CLOCK_REALTIME && mode != HRTIMER_MODE_ABS)
+	/*
+	 * POSIX magic: Relative CLOCK_REALTIME timers are not affected by
+	 * clock modifications, so they needs to become CLOCK_MONOTONIC to
+	 * ensure POSIX compliance.
+	 */
+	if (clock_id == CLOCK_REALTIME && mode & HRTIMER_MODE_REL)
 		clock_id = CLOCK_MONOTONIC;
 
 	base = hrtimer_clockid_to_base(clock_id);