s390/time: fix clocksource steering for negative clock offsets

The TOD clock offset injected by an STP sync check can be negative.
If the resulting total tod_steering_delta gets negative the kernel
will panic.

Change the type of tod_steering_delta to a signed type.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 75c7b6f3f6ba ("s390/time: steer clocksource on STP sync events")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
index 33082f6..867d0a0 100644 (file)
--- a/arch/s390/kernel/time.c
+++ b/arch/s390/kernel/time.c
@@ -63,7 +63,7 @@ unsigned char ptff_function_mask[16];
 static unsigned long long lpar_offset;
 static unsigned long long initial_leap_seconds;
 static unsigned long long tod_steering_end;
-static unsigned long long tod_steering_delta;
+static long long tod_steering_delta;
 
 /*
  * Get time offsets with PTFF
@@ -223,8 +223,7 @@ static cycle_t read_tod_clock(struct clocksource *cs)
                 * therefore steered in ~9h. The adjust will decrease
                 * over time, until it finally reaches 0.
                 */
-               now += ((s64) tod_steering_delta < 0) ?
-                       (adj >> 15) : -(adj >> 15);
+               now += (tod_steering_delta < 0) ? (adj >> 15) : -(adj >> 15);
        preempt_enable();
        return now;
 }
@@ -412,7 +411,7 @@ static void clock_sync_global(unsigned long long delta)
        adj = tod_steering_end - now;
        if (unlikely((s64) adj >= 0))
                /* Calculate how much of the old adjustment is left. */
-               tod_steering_delta = ((s64) tod_steering_delta < 0) ?
+               tod_steering_delta = (tod_steering_delta < 0) ?
                        -(adj >> 15) : (adj >> 15);
        tod_steering_delta += delta;
        if ((abs(tod_steering_delta) >> 48) != 0)