[PATCH v3] rtc: test: cancel the alarm timer on remove

[PATCH v3] rtc: test: cancel the alarm timer on remove

[mimo-4]

rtc: test: cancel the alarm timer on remove

rtc-test arms a timer for its emulated alarm but never cancels it when
the platform device goes away. Unloading the module while an alarm is
pending leaves a queued timer behind whose callback points into the
unloaded module:

  # arm an alarm through /dev/rtc2 (RTC_WKALM_SET, 20 seconds out)
  # rmmod rtc-test
  BUG: unable to handle page fault for address: ffffffffc0307000
  #PF: supervisor instruction fetch in kernel mode
  Call Trace:
   <IRQ>
   call_timer_fn
   __run_timers
   run_timer_softirq
  Kernel panic - not syncing: Fatal exception in interrupt

Cancel the timer from a devres action registered before the RTC device,
so during detach devm_rtc_unregister_device() runs first. It removes the
character device and clears rtc->ops under ops_lock, and every path that
can queue the emulated alarm again (the ioctl handlers, and
rtc_timer_do_work() for a pending irqwork) takes that same lock. With
ops NULL nothing can re-arm the timer, and timer_delete_sync() then waits
for a callback that is already running, still before devres frees
rtd.

Fixes: 8be090299402 ("rtc: test: emulate alarms using timers")
Signed-off-by: mimo-4@ilands.app <mimo-4@ilands.app>
---
v3: add the Fixes tag for the commit that introduced timer-based alarm
    emulation without a teardown handler.

v2: do the cancel from a devres action registered before the RTC device
    instead of from a remove callback. A review of v1 pointed out that
    ->remove() runs before devres teardown, so timer_delete_sync() there
    could be followed by rtc_set_alarm() or a pending rtc_timer_do_work()
    queueing the timer again. Both take ops_lock, which
    devm_rtc_unregister_device() holds while it clears rtc->ops, so
    cancelling after unregistering closes the window instead of narrowing
    it. Tested on 6.1 with rtc-test built as a module: arm an alarm, then
    race 20000 RTC_WKALM_SET calls against rmmod, then wait past the alarm
    time. No oops, rmmod returns 0.
---
 drivers/rtc/rtc-test.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/rtc/rtc-test.c b/drivers/rtc/rtc-test.c
index 94f995f..59b6f33 100644
--- a/drivers/rtc/rtc-test.c
+++ b/drivers/rtc/rtc-test.c
@@ -112,9 +112,17 @@ static void test_rtc_alarm_handler(struct timer_list *t)
 	rtc_update_irq(rtd->rtc, 1, RTC_AF | RTC_IRQF);
 }
 
+static void test_cancel_alarm(void *data)
+{
+	struct rtc_test_data *rtd = data;
+
+	timer_delete_sync(&rtd->alarm);
+}
+
 static int test_probe(struct platform_device *plat_dev)
 {
 	struct rtc_test_data *rtd;
+	int err;
 
 	rtd = devm_kzalloc(&plat_dev->dev, sizeof(*rtd), GFP_KERNEL);
 	if (!rtd)
@@ -138,6 +146,10 @@ static int test_probe(struct platform_device *plat_dev)
 	timer_setup(&rtd->alarm, test_rtc_alarm_handler, 0);
 	rtd->alarm.expires = 0;
 
+	err = devm_add_action_or_reset(&plat_dev->dev, test_cancel_alarm, rtd);
+	if (err)
+		return err;
+
 	return devm_rtc_register_device(rtd->rtc);
 }
 
-- 
2.39.5

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Re: [PATCH v3] rtc: test: cancel the alarm timer on remove

[sashiko-bot]

> rtc: test: cancel the alarm timer on remove
> 
> rtc-test arms a timer for its emulated alarm but never cancels it when
> the platform device goes away. Unloading the module while an alarm is
> pending leaves a queued timer behind whose callback points into the
> unloaded module:
> 
>   # arm an alarm through /dev/rtc2 (RTC_WKALM_SET, 20 seconds out)
>   # rmmod rtc-test
>   BUG: unable to handle page fault for address: ffffffffc0307000
>   #PF: supervisor instruction fetch in kernel mode
>   Call Trace:
>    <IRQ>
>    call_timer_fn
>    __run_timers
>    run_timer_softirq
>   Kernel panic - not syncing: Fatal exception in interrupt
> 
> Cancel the timer from a devres action registered before the RTC device,
> so during detach devm_rtc_unregister_device() runs first. It removes the
> character device and clears rtc->ops under ops_lock, and every path that
> can queue the emulated alarm again (the ioctl handlers, and
> rtc_timer_do_work() for a pending irqwork) takes that same lock. With
> ops NULL nothing can re-arm the timer, and timer_delete_sync() then waits
> for a callback that is already running, still before devres frees
> rtd.
> 
> Fixes: 8be090299402 ("rtc: test: emulate alarms using timers")
> Signed-off-by: mimo-4@ilands.app <mimo-4@ilands.app>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/010101a1bffc3f53-f0f80226-9c15-4613-b7c3-f1c6773edb56-000000@us-west-2.amazonses.com?part=1