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 * kretprobe_example.c
 * Here's a sample kernel module showing the use of return probes to
 * report the return value and total time taken for probed function
 * to run.
 * usage: insmod kretprobe_example.ko func=<func_name>
 * If no func_name is specified, do_fork is instrumented
 * For more information on theory of operation of kretprobes, see
 * Documentation/kprobes.txt
 * Build and insert the kernel module as done in the kprobe example.
 * You will see the trace data in /var/log/messages and on the console
 * whenever the probed function returns. (Some messages may be suppressed
 * if syslogd is configured to eliminate duplicate messages.)

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kprobes.h>
#include <linux/ktime.h>
#include <linux/limits.h>
#include <linux/sched.h>

static char func_name[NAME_MAX] = "do_fork";
module_param_string(func, func_name, NAME_MAX, S_IRUGO);
MODULE_PARM_DESC(func, "Function to kretprobe; this module will report the"
			" function's execution time");

/* per-instance private data */
struct my_data {
	ktime_t entry_stamp;

/* Here we use the entry_hanlder to timestamp function entry */
static int entry_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
	struct my_data *data;

	if (!current->mm)
		return 1;	/* Skip kernel threads */

	data = (struct my_data *)ri->data;
	data->entry_stamp = ktime_get();
	return 0;

 * Return-probe handler: Log the return value and duration. Duration may turn
 * out to be zero consistently, depending upon the granularity of time
 * accounting on the platform.
static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
	int retval = regs_return_value(regs);
	struct my_data *data = (struct my_data *)ri->data;
	s64 delta;
	ktime_t now;

	now = ktime_get();
	delta = ktime_to_ns(ktime_sub(now, data->entry_stamp));
	printk(KERN_INFO "%s returned %d and took %lld ns to execute\n",
			func_name, retval, (long long)delta);
	return 0;

static struct kretprobe my_kretprobe = {
	.handler		= ret_handler,
	.entry_handler		= entry_handler,
	.data_size		= sizeof(struct my_data),
	/* Probe up to 20 instances concurrently. */
	.maxactive		= 20,

static int __init kretprobe_init(void)
	int ret; = func_name;
	ret = register_kretprobe(&my_kretprobe);
	if (ret < 0) {
		printk(KERN_INFO "register_kretprobe failed, returned %d\n",
		return -1;
	printk(KERN_INFO "Planted return probe at %s: %p\n",,;
	return 0;

static void __exit kretprobe_exit(void)
	printk(KERN_INFO "kretprobe at %p unregistered\n",;

	/* nmissed > 0 suggests that maxactive was set too low. */
	printk(KERN_INFO "Missed probing %d instances of %s\n",