tracing/probes: Add a new testcase for BTF typecasts

With the introduction of container_of-style BTF typecasting and
per-CPU variable access support in trace probes, we need a way to
verify their functionality and prevent regressions.

Add a new ftrace kselftest and update the trace event sample module
to test and validate these features.

Specifically, update the trace-events-sample module to set up a
periodic timer whose callback accesses a per-CPU counter. Introduce
a new sample trace event, foo_timer_fn, to trace this callback
and log the current counter value.

Then, add a new test case, btf_probe_event.tc, which defines a
dynamic probe on the timer callback. The probe uses BTF typecasting
to recover the parent structure from the timer argument and
this_cpu_read() to fetch the per-CPU counter. The test verifies
the integrity of the implementation by ensuring the values
recorded by the dynamic probe match those from the static tracepoint.

Link: https://lore.kernel.org/all/178271368625.1176915.13502654445483530822.stgit@devnote2/

Assisted-by: Antigravity:gemini-3.5-flash
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

2 files changed, 69 insertions, 5 deletions

diff --git a/samples/trace_events/trace-events-sample.c b/samples/trace_events/trace-events-sample.c
index 0b7a6efdb247..ca5d98c360cb 100644
--- a/samples/trace_events/trace-events-sample.c
+++ b/samples/trace_events/trace-events-sample.c
@@ -94,6 +94,20 @@ static int simple_thread_fn(void *arg)
 static DEFINE_MUTEX(thread_mutex);
 static int simple_thread_cnt;
 
+static struct foo_timer_data *foo_timer_data;
+
+static void sample_timer_cb(struct timer_list *t)
+{
+	struct foo_timer_data *data = container_of(t, struct foo_timer_data, timer);
+
+	get_cpu();
+	trace_foo_timer_fn(data);
+	(*this_cpu_ptr(data->counter))++;
+	put_cpu();
+
+	mod_timer(t, jiffies + HZ);
+}
+
 int foo_bar_reg(void)
 {
 	mutex_lock(&thread_mutex);
@@ -132,9 +146,27 @@ void foo_bar_unreg(void)
 
 static int __init trace_event_init(void)
 {
+	foo_timer_data = kzalloc_obj(*foo_timer_data, GFP_KERNEL);
+	if (!foo_timer_data)
+		return -ENOMEM;
+
+	foo_timer_data->name = "sample_timer_counter";
+	foo_timer_data->counter = alloc_percpu(int);
+	if (!foo_timer_data->counter) {
+		kfree(foo_timer_data);
+		return -ENOMEM;
+	}
+
+	timer_setup(&foo_timer_data->timer, sample_timer_cb, 0);
+	mod_timer(&foo_timer_data->timer, jiffies + HZ);
+
 	simple_tsk = kthread_run(simple_thread, NULL, "event-sample");
-	if (IS_ERR(simple_tsk))
-		return -1;
+	if (IS_ERR(simple_tsk)) {
+		timer_shutdown_sync(&foo_timer_data->timer);
+		free_percpu(foo_timer_data->counter);
+		kfree(foo_timer_data);
+		return PTR_ERR(simple_tsk);
+	}
 
 	return 0;
 }
@@ -147,6 +179,10 @@ static void __exit trace_event_exit(void)
 		kthread_stop(simple_tsk_fn);
 	simple_tsk_fn = NULL;
 	mutex_unlock(&thread_mutex);
+
+	timer_shutdown_sync(&foo_timer_data->timer);
+	free_percpu(foo_timer_data->counter);
+	kfree(foo_timer_data);
 }
 
 module_init(trace_event_init);
diff --git a/samples/trace_events/trace-events-sample.h b/samples/trace_events/trace-events-sample.h
index 1a05fc153353..816848a456a2 100644
--- a/samples/trace_events/trace-events-sample.h
+++ b/samples/trace_events/trace-events-sample.h
@@ -247,12 +247,14 @@
  */
 
 /*
- * It is OK to have helper functions in the file, but they need to be protected
- * from being defined more than once. Remember, this file gets included more
- * than once.
+ * It is OK to have helper functions and data structures in the file, but they
+ * need to be protected from being defined more than once. Remember, this file
+ * gets included more than once.
  */
 #ifndef __TRACE_EVENT_SAMPLE_HELPER_FUNCTIONS
 #define __TRACE_EVENT_SAMPLE_HELPER_FUNCTIONS
+#include <linux/timer.h>
+
 static inline int __length_of(const int *list)
 {
 	int i;
@@ -270,6 +272,13 @@ enum {
 	TRACE_SAMPLE_BAR = 4,
 	TRACE_SAMPLE_ZOO = 8,
 };
+
+struct foo_timer_data {
+	const char		*name;
+	struct timer_list	timer;
+	int __percpu		*counter;
+};
+
 #endif
 
 /*
@@ -595,6 +604,25 @@ TRACE_EVENT(foo_rel_loc,
 		  __get_rel_bitmask(bitmask),
 		  __get_rel_cpumask(cpumask))
 );
+
+TRACE_EVENT(foo_timer_fn,
+
+	TP_PROTO(struct foo_timer_data *data),
+
+	TP_ARGS(data),
+
+	TP_STRUCT__entry(
+		__string(	name,			data->name	)
+		__field(	int,			count		)
+	),
+
+	TP_fast_assign(
+		__assign_str(name);
+		__entry->count	= *this_cpu_ptr(data->counter);
+	),
+
+	TP_printk("name=%s count=%d", __get_str(name), __entry->count)
+);
 #endif
 
 /***** NOTICE! The #if protection ends here. *****/