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Kernel: Do not cancel stale timers when servicing sys$alarm

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The sys$alarm() syscall has logic to cache a m_alarm_timer to avoid
allocating a new timer for every call to alarm. Unfortunately that
logic was broken, and there were conditions in which we could have
a timer allocated, but it was no longer on the timer queue, and we
would attempt to cancel that timer again resulting in an infinite
loop waiting for the timers callback to fire.

To fix this, we need to track if a timer is currently in use or not,
allowing us to avoid attempting to cancel inactive timers.

Luke and Tom did the initial investigation, I just happened to have
time to write a repro and attempt a fix, so I'm adding them as the
as co-authors of this commit.

Co-authored-by: Luke <luke.wilde@live.co.uk>
Co-authored-by: Tom <tomut@yahoo.com>
This commit is contained in:
Brian Gianforcaro 2021-07-31 16:27:44 -07:00 • committed by Andreas Kling
parent 2dd6d2121a
commit fc91eb365d
3 changed files with 28 additions and 6 deletions
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5
Kernel/Syscalls/alarm.cpp
View file

@ -15,13 +15,14 @@ KResultOr<FlatPtr> Process::sys$alarm(unsigned seconds)
REQUIRE_PROMISE(stdio);
unsigned previous_alarm_remaining = 0;
if (m_alarm_timer) {
if (TimerQueue::the().cancel_timer(*m_alarm_timer)) {
bool was_in_use = false;
if (TimerQueue::the().cancel_timer(*m_alarm_timer, &was_in_use)) {
// The timer hasn't fired. Round up the remaining time (if any)
Time remaining = m_alarm_timer->remaining() + Time::from_nanoseconds(999'999'999);
previous_alarm_remaining = remaining.to_truncated_seconds();
}
// We had an existing alarm, must return a non-zero value here!
if (previous_alarm_remaining == 0)
if (was_in_use && previous_alarm_remaining == 0)
previous_alarm_remaining = 1;
}