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clocksource: Make watchdog and suspend-timing multiplication overflow…
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… safe

[ Upstream commit d030456 ]

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>
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@ahunter6 @gregkh
ahunter6 authored and gregkh committed Jun 27, 2024
1 parent 82116ea commit 13b5e2b
Showing 1 changed file with 20 additions and 22 deletions.
42 changes: 20 additions & 22 deletions kernel/time/clocksource.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down Expand Up @@ -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++) {
Expand All @@ -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",
Expand All @@ -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;
}
Expand Down Expand Up @@ -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)
Expand Down Expand Up @@ -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;
Expand Down Expand Up @@ -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;
Expand Down Expand Up @@ -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;
Expand All @@ -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
Expand Down

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