[PATCH] tracing/osnoise: Don't start a kthread in timerlat user-space mode

In timerlat user-space mode (NO_OSNOISE_WORKLOAD) the workload is a user
task reading timerlat_fd, so there is no per-CPU kernel thread to run.
start_kthread() only checks for that in the osnoise branch:

	if (timerlat_enabled()) {
		snprintf(comm, 24, "timerlat/%d", cpu);
		main = timerlat_main;
	} else {
		/* if no workload, just return */
		if (!test_bit(OSN_WORKLOAD, &osnoise_options)) {
			...
			return 0;
		}
		...
	}

start_per_cpu_kthreads() guards against this mode itself, but
osnoise_hotplug_workfn() calls start_kthread() directly. Bringing a CPU
online in user-space mode therefore starts a timerlat/N thread and sets
its bit in kthread_cpumask while OSN_WORKLOAD is clear.

stop_kthread() uses that bit to decide what the recorded task is:

	if (cpumask_test_and_clear_cpu(cpu, &kthread_cpumask) &&
	    !WARN_ON(!test_bit(OSN_WORKLOAD, &osnoise_options))) {
		kthread_stop(kthread);
	} else if (!WARN_ON(test_bit(OSN_WORKLOAD, &osnoise_options))) {
		kill_pid(kthread->thread_pid, SIGKILL, 1);
		put_task_struct(kthread);
	}

The bit is set but OSN_WORKLOAD is not, so the first WARN_ON() fires and
the && short-circuits into the user-task branch. A running kernel thread
is SIGKILLed and its task_struct is put without ever being stopped, so it
keeps executing until it is freed underneath itself.

Any write to osnoise/options reaches this: osnoise_options_write() stops
the per-CPU threads before applying the new value, so stop_kthread()
still sees OSN_WORKLOAD clear. Re-writing the value that is already set
triggers it just as well as changing it.

  echo NO_OSNOISE_WORKLOAD > osnoise/options
  echo timerlat > current_tracer
  echo 0 > /sys/devices/system/cpu/cpu1/online
  echo 1 > /sys/devices/system/cpu/cpu1/online
  echo NO_OSNOISE_WORKLOAD > osnoise/options

The offline/online pair is required, otherwise no kthread is recorded and
no bit is set. Running this on 7.3-rc7 with KASAN gives a WARNING in
stop_kthread() followed by a slab-use-after-free in update_se(); leaving
out the hotplug step produces neither.

Keep the check in start_kthread(), where the related hotplug bug was
fixed in 0bb0a5c12ecf, instead of adding another copy to the callback.

Fixes: e88ed227f639 ("tracing/timerlat: Add user-space interface")
Cc: <redacted>
Signed-off-by: Chaosheng Qu <redacted>
---
 kernel/trace/trace_osnoise.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index 0e1265acd..9715462ee 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c

@@ -2038,6 +2038,14 @@ static int start_kthread(unsigned int cpu)
 		return 0;
 
 	if (timerlat_enabled()) {
+		/*
+		 * In timerlat user-space mode the workload is the user task
+		 * reading timerlat_fd, so there is no kernel thread to start.
+		 * One started here would set its kthread_cpumask bit, which
+		 * stop_kthread() reads as "osnoise workload kthread".
+		 */
+		if (!test_bit(OSN_WORKLOAD, &osnoise_options))
+			return 0;
 		snprintf(comm, 24, "timerlat/%d", cpu);
 		main = timerlat_main;
 	} else {

-- 
2.43.0