When calculating the current delta since the last tick, we currently have no hard protections to prevent a multiplciation overflow from ocurring.
This patch introduces such a cap that limits the read delta value to the max_cycles value, which is where an overflow would occur. There was some concern that capping time like this could cause problems as we may stop expiring timers, which could go circular if the timer that triggers time accumulation were misscheduled too far in the future, which would cause time to stop. However, since the mult overflow would result in a smaller time value, we would effectively have the same problem there. Cc: Dave Jones <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Richard Cochran <[email protected]> Cc: Prarit Bhargava <[email protected]> Cc: Stephen Boyd <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Peter Zijlstra <[email protected]> Signed-off-by: John Stultz <[email protected]> --- kernel/time/timekeeping.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 0dcceba..9740fd8 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -202,6 +202,9 @@ static inline s64 timekeeping_get_ns(struct tk_read_base *tkr) /* calculate the delta since the last update_wall_time: */ delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask); + /* Cap delta value to the max_cycles values to avoid mult overflows */ + delta = min(delta, tkr->clock->max_cycles); + nsec = delta * tkr->mult + tkr->xtime_nsec; nsec >>= tkr->shift; @@ -221,6 +224,9 @@ static inline s64 timekeeping_get_ns_raw(struct timekeeper *tk) /* calculate the delta since the last update_wall_time: */ delta = clocksource_delta(cycle_now, tk->tkr.cycle_last, tk->tkr.mask); + /* Cap delta value to the max_cycles values to avoid mult overflows */ + delta = min(delta, tk->tkr.clock->max_cycles); + /* convert delta to nanoseconds. */ nsec = clocksource_cyc2ns(delta, clock->mult, clock->shift); @@ -482,6 +488,9 @@ static void timekeeping_forward_now(struct timekeeper *tk) delta = clocksource_delta(cycle_now, tk->tkr.cycle_last, tk->tkr.mask); tk->tkr.cycle_last = cycle_now; + /* Cap delta value to the max_cycles values to avoid mult overflows */ + delta = min(delta, tk->tkr.clock->max_cycles); + tk->tkr.xtime_nsec += delta * tk->tkr.mult; /* If arch requires, add in get_arch_timeoffset() */ -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
