1 /*
2  * kretprobe_example.c
3  *
4  * Here's a sample kernel module showing the use of return probes to
5  * report the return value and total time taken for probed function
6  * to run.
7  *
8  * usage: insmod kretprobe_example.ko func=<func_name>
9  *
10  * If no func_name is specified, do_fork is instrumented
11  *
12  * For more information on theory of operation of kretprobes, see
13  * Documentation/kprobes.txt
14  *
15  * Build and insert the kernel module as done in the kprobe example.
16  * You will see the trace data in /var/log/messages and on the console
17  * whenever the probed function returns. (Some messages may be suppressed
18  * if syslogd is configured to eliminate duplicate messages.)
19  */
20 
21 #include <linux/kernel.h>
22 #include <linux/module.h>
23 #include <linux/kprobes.h>
24 #include <linux/ktime.h>
25 #include <linux/limits.h>
26 #include <linux/sched.h>
27 
28 static char func_name[NAME_MAX] = "do_fork";
29 module_param_string(func, func_name, NAME_MAX, S_IRUGO);
30 MODULE_PARM_DESC(func, "Function to kretprobe; this module will report the"
31 			" function's execution time");
32 
33 /* per-instance private data */
34 struct my_data {
35 	ktime_t entry_stamp;
36 };
37 
38 /* Here we use the entry_hanlder to timestamp function entry */
39 static int entry_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
40 {
41 	struct my_data *data;
42 
43 	if (!current->mm)
44 		return 1;	/* Skip kernel threads */
45 
46 	data = (struct my_data *)ri->data;
47 	data->entry_stamp = ktime_get();
48 	return 0;
49 }
50 
51 /*
52  * Return-probe handler: Log the return value and duration. Duration may turn
53  * out to be zero consistently, depending upon the granularity of time
54  * accounting on the platform.
55  */
56 static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
57 {
58 	int retval = regs_return_value(regs);
59 	struct my_data *data = (struct my_data *)ri->data;
60 	s64 delta;
61 	ktime_t now;
62 
63 	now = ktime_get();
64 	delta = ktime_to_ns(ktime_sub(now, data->entry_stamp));
65 	printk(KERN_INFO "%s returned %d and took %lld ns to execute\n",
66 			func_name, retval, (long long)delta);
67 	return 0;
68 }
69 
70 static struct kretprobe my_kretprobe = {
71 	.handler		= ret_handler,
72 	.entry_handler		= entry_handler,
73 	.data_size		= sizeof(struct my_data),
74 	/* Probe up to 20 instances concurrently. */
75 	.maxactive		= 20,
76 };
77 
78 static int __init kretprobe_init(void)
79 {
80 	int ret;
81 
82 	my_kretprobe.kp.symbol_name = func_name;
83 	ret = register_kretprobe(&my_kretprobe);
84 	if (ret < 0) {
85 		printk(KERN_INFO "register_kretprobe failed, returned %d\n",
86 				ret);
87 		return -1;
88 	}
89 	printk(KERN_INFO "Planted return probe at %s: %p\n",
90 			my_kretprobe.kp.symbol_name, my_kretprobe.kp.addr);
91 	return 0;
92 }
93 
94 static void __exit kretprobe_exit(void)
95 {
96 	unregister_kretprobe(&my_kretprobe);
97 	printk(KERN_INFO "kretprobe at %p unregistered\n",
98 			my_kretprobe.kp.addr);
99 
100 	/* nmissed > 0 suggests that maxactive was set too low. */
101 	printk(KERN_INFO "Missed probing %d instances of %s\n",
102 		my_kretprobe.nmissed, my_kretprobe.kp.symbol_name);
103 }
104 
105 module_init(kretprobe_init)
106 module_exit(kretprobe_exit)
107 MODULE_LICENSE("GPL");
108