[PATCH AUTOSEL 6.9 10/35] clocksource: Make watchdog and suspend-timing multiplication overflow safe

Sasha Levin
2024-05-27T14:12:32Z

From: Adrian Hunter <adrian.hunter@intel.com>

[ Upstream commit d0304569fb019d1bcfbbbce1ce6df6b96f04079b ]

Kernel timekeeping is designed to keep the change in cycles (since the last
timer interrupt) below max_cycles, which prevents multiplication overflow
when converting cycles to nanoseconds. However, if timer interrupts stop,
the clocksource_cyc2ns() calculation will eventually overflow.

Add protection against that. Simplify by folding together
clocksource_delta() and clocksource_cyc2ns() into cycles_to_nsec_safe().
Check against max_cycles, falling back to a slower higher precision
calculation.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-20-adrian.hunter@intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/time/clocksource.c | 42 +++++++++++++++++++--------------------
1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index e5b260aa0e02c..4d50d53ac719f 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -20,6 +20,16 @@
#include "tick-internal.h"
#include "timekeeping_internal.h"

+static noinline u64 cycles_to_nsec_safe(struct clocksource *cs, u64 start, u64 end)
+{
+ u64 delta = clocksource_delta(end, start, cs->mask);
+
+ if (likely(delta < cs->max_cycles))
+ return clocksource_cyc2ns(delta, cs->mult, cs->shift);
+
+ return mul_u64_u32_shr(delta, cs->mult, cs->shift);
+}
+
/**
* clocks_calc_mult_shift - calculate mult/shift factors for scaled math of clocks
* @mult: pointer to mult variable
@@ -222,8 +232,8 @@ enum wd_read_status {
static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
{
unsigned int nretries, max_retries;
- u64 wd_end, wd_end2, wd_delta;
int64_t wd_delay, wd_seq_delay;
+ u64 wd_end, wd_end2;

max_retries = clocksource_get_max_watchdog_retry();
for (nretries = 0; nretries <= max_retries; nretries++) {
@@ -234,9 +244,7 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
wd_end2 = watchdog->read(watchdog);
local_irq_enable();

- wd_delta = clocksource_delta(wd_end, *wdnow, watchdog->mask);
- wd_delay = clocksource_cyc2ns(wd_delta, watchdog->mult,
- watchdog->shift);
+ wd_delay = cycles_to_nsec_safe(watchdog, *wdnow, wd_end);
if (wd_delay <= WATCHDOG_MAX_SKEW) {
if (nretries > 1 || nretries >= max_retries) {
pr_warn("timekeeping watchdog on CPU%d: %s retried %d times before success\n",
@@ -254,8 +262,7 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
* report system busy, reinit the watchdog and skip the current
* watchdog test.
*/
- wd_delta = clocksource_delta(wd_end2, wd_end, watchdog->mask);
- wd_seq_delay = clocksource_cyc2ns(wd_delta, watchdog->mult, watchdog->shift);
+ wd_seq_delay = cycles_to_nsec_safe(watchdog, wd_end, wd_end2);
if (wd_seq_delay > WATCHDOG_MAX_SKEW/2)
goto skip_test;
}
@@ -366,8 +373,7 @@ void clocksource_verify_percpu(struct clocksource *cs)
delta = (csnow_end - csnow_mid) & cs->mask;
if (delta < 0)
cpumask_set_cpu(cpu, &cpus_ahead);
- delta = clocksource_delta(csnow_end, csnow_begin, cs->mask);
- cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift);
+ cs_nsec = cycles_to_nsec_safe(cs, csnow_begin, csnow_end);
if (cs_nsec > cs_nsec_max)
cs_nsec_max = cs_nsec;
if (cs_nsec < cs_nsec_min)
@@ -398,8 +404,8 @@ static inline void clocksource_reset_watchdog(void)

static void clocksource_watchdog(struct timer_list *unused)
{
- u64 csnow, wdnow, cslast, wdlast, delta;
int64_t wd_nsec, cs_nsec, interval;
+ u64 csnow, wdnow, cslast, wdlast;
int next_cpu, reset_pending;
struct clocksource *cs;
enum wd_read_status read_ret;
@@ -456,12 +462,8 @@ static void clocksource_watchdog(struct timer_list *unused)
continue;
}

- delta = clocksource_delta(wdnow, cs->wd_last, watchdog->mask);
- wd_nsec = clocksource_cyc2ns(delta, watchdog->mult,
- watchdog->shift);
-
- delta = clocksource_delta(csnow, cs->cs_last, cs->mask);
- cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift);
+ wd_nsec = cycles_to_nsec_safe(watchdog, cs->wd_last, wdnow);
+ cs_nsec = cycles_to_nsec_safe(cs, cs->cs_last, csnow);
wdlast = cs->wd_last; /* save these in case we print them */
cslast = cs->cs_last;
cs->cs_last = csnow;
@@ -832,7 +834,7 @@ void clocksource_start_suspend_timing(struct clocksource *cs, u64 start_cycles)
*/
u64 clocksource_stop_suspend_timing(struct clocksource *cs, u64 cycle_now)
{
- u64 now, delta, nsec = 0;
+ u64 now, nsec = 0;

if (!suspend_clocksource)
return 0;
@@ -847,12 +849,8 @@ u64 clocksource_stop_suspend_timing(struct clocksource *cs, u64 cycle_now)
else
now = suspend_clocksource->read(suspend_clocksource);

- if (now > suspend_start) {
- delta = clocksource_delta(now, suspend_start,
- suspend_clocksource->mask);
- nsec = mul_u64_u32_shr(delta, suspend_clocksource->mult,
- suspend_clocksource->shift);
- }
+ if (now > suspend_start)
+ nsec = cycles_to_nsec_safe(suspend_clocksource, suspend_start, now);

/*
* Disable the suspend timer to save power if current clocksource is
--
2.43.0