/*
   * 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
  ",
  			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;
  
  	my_kretprobe.kp.symbol_name = func_name;
  	ret = register_kretprobe(&my_kretprobe);
  	if (ret < 0) {
  		printk(KERN_INFO "register_kretprobe failed, returned %d
  ",
  				ret);
  		return -1;
  	}
  	printk(KERN_INFO "Planted return probe at %s: %p
  ",
  			my_kretprobe.kp.symbol_name, my_kretprobe.kp.addr);
  	return 0;
  }
  
  static void __exit kretprobe_exit(void)
  {
  	unregister_kretprobe(&my_kretprobe);
  	printk(KERN_INFO "kretprobe at %p unregistered
  ",
  			my_kretprobe.kp.addr);
  
  	/* nmissed > 0 suggests that maxactive was set too low. */
  	printk(KERN_INFO "Missed probing %d instances of %s
  ",
  		my_kretprobe.nmissed, my_kretprobe.kp.symbol_name);
  }
  
  module_init(kretprobe_init)
  module_exit(kretprobe_exit)
  MODULE_LICENSE("GPL");