From a558cd021d83b65c47ee5b9bec1fcfe5298a769f Mon Sep 17 00:00:00 2001 From: John Stultz Date: Wed, 11 Mar 2015 21:16:33 -0700 Subject: [PATCH] timekeeping: Add checks to cap clocksource reads to the 'max_cycles' value When calculating the current delta since the last tick, we currently have no hard protections to prevent a multiplication overflow from occuring. This patch introduces infrastructure to allow a cap that limits the clocksource read delta value to the 'max_cycles' value, which is where an overflow would occur. Since this is in the hotpath, it adds the extra checking under CONFIG_DEBUG_TIMEKEEPING=y. 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 mis-scheduled 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. Signed-off-by: John Stultz Cc: Dave Jones Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Prarit Bhargava Cc: Richard Cochran Cc: Stephen Boyd Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1426133800-29329-6-git-send-email-john.stultz@linaro.org Signed-off-by: Ingo Molnar --- kernel/time/timekeeping.c | 49 ++++++++++++++++++++++++++++----------- 1 file changed, 35 insertions(+), 14 deletions(-) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index acf049144cf6..657414cf2e46 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -126,9 +126,9 @@ static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset) const char *name = tk->tkr.clock->name; if (offset > max_cycles) { - printk_deferred("WARNING: timekeeping: Cycle offset (%lld) is larger than allowed by the '%s' clock's max_cycles value (%lld): time overflow\n", + printk_deferred("WARNING: timekeeping: Cycle offset (%lld) is larger than allowed by the '%s' clock's max_cycles value (%lld): time overflow danger\n", offset, name, max_cycles); - printk_deferred(" timekeeping: Your kernel is sick, but tries to cope\n"); + printk_deferred(" timekeeping: Your kernel is sick, but tries to cope by capping time updates\n"); } else { if (offset > (max_cycles >> 1)) { printk_deferred("INFO: timekeeping: Cycle offset (%lld) is larger than the the '%s' clock's 50%% safety margin (%lld)\n", @@ -137,10 +137,39 @@ static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset) } } } + +static inline cycle_t timekeeping_get_delta(struct tk_read_base *tkr) +{ + cycle_t cycle_now, delta; + + /* read clocksource */ + cycle_now = tkr->read(tkr->clock); + + /* 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 */ + if (unlikely(delta > tkr->clock->max_cycles)) + delta = tkr->clock->max_cycles; + + return delta; +} #else static inline void timekeeping_check_update(struct timekeeper *tk, cycle_t offset) { } +static inline cycle_t timekeeping_get_delta(struct tk_read_base *tkr) +{ + cycle_t cycle_now, delta; + + /* read clocksource */ + cycle_now = tkr->read(tkr->clock); + + /* calculate the delta since the last update_wall_time */ + delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask); + + return delta; +} #endif /** @@ -218,14 +247,10 @@ static inline u32 arch_gettimeoffset(void) { return 0; } static inline s64 timekeeping_get_ns(struct tk_read_base *tkr) { - cycle_t cycle_now, delta; + cycle_t delta; s64 nsec; - /* read clocksource: */ - cycle_now = tkr->read(tkr->clock); - - /* calculate the delta since the last update_wall_time: */ - delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask); + delta = timekeeping_get_delta(tkr); nsec = delta * tkr->mult + tkr->xtime_nsec; nsec >>= tkr->shift; @@ -237,14 +262,10 @@ static inline s64 timekeeping_get_ns(struct tk_read_base *tkr) static inline s64 timekeeping_get_ns_raw(struct timekeeper *tk) { struct clocksource *clock = tk->tkr.clock; - cycle_t cycle_now, delta; + cycle_t delta; s64 nsec; - /* read clocksource: */ - cycle_now = tk->tkr.read(clock); - - /* calculate the delta since the last update_wall_time: */ - delta = clocksource_delta(cycle_now, tk->tkr.cycle_last, tk->tkr.mask); + delta = timekeeping_get_delta(&tk->tkr); /* convert delta to nanoseconds. */ nsec = clocksource_cyc2ns(delta, clock->mult, clock->shift); -- 2.20.1