1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * kretprobe_example.c
4  *
5  * Here's a sample kernel module showing the use of return probes to
6  * report the return value and total time taken for probed function
7  * to run.
8  *
9  * usage: insmod kretprobe_example.ko func=<func_name>
10  *
11  * If no func_name is specified, _do_fork is instrumented
12  *
13  * For more information on theory of operation of kretprobes, see
14  * Documentation/kprobes.txt
15  *
16  * Build and insert the kernel module as done in the kprobe example.
17  * You will see the trace data in /var/log/messages and on the console
18  * whenever the probed function returns. (Some messages may be suppressed
19  * if syslogd is configured to eliminate duplicate messages.)
20  */
21 
22 #include <linux/kernel.h>
23 #include <linux/module.h>
24 #include <linux/kprobes.h>
25 #include <linux/ktime.h>
26 #include <linux/limits.h>
27 #include <linux/sched.h>
28 
29 static char func_name[NAME_MAX] = "_do_fork";
30 module_param_string(func, func_name, NAME_MAX, S_IRUGO);
31 MODULE_PARM_DESC(func, "Function to kretprobe; this module will report the"
32 			" function's execution time");
33 
34 /* per-instance private data */
35 struct my_data {
36 	ktime_t entry_stamp;
37 };
38 
39 /* Here we use the entry_hanlder to timestamp function entry */
40 static int entry_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
41 {
42 	struct my_data *data;
43 
44 	if (!current->mm)
45 		return 1;	/* Skip kernel threads */
46 
47 	data = (struct my_data *)ri->data;
48 	data->entry_stamp = ktime_get();
49 	return 0;
50 }
51 NOKPROBE_SYMBOL(entry_handler);
52 
53 /*
54  * Return-probe handler: Log the return value and duration. Duration may turn
55  * out to be zero consistently, depending upon the granularity of time
56  * accounting on the platform.
57  */
58 static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
59 {
60 	unsigned long retval = regs_return_value(regs);
61 	struct my_data *data = (struct my_data *)ri->data;
62 	s64 delta;
63 	ktime_t now;
64 
65 	now = ktime_get();
66 	delta = ktime_to_ns(ktime_sub(now, data->entry_stamp));
67 	pr_info("%s returned %lu and took %lld ns to execute\n",
68 			func_name, retval, (long long)delta);
69 	return 0;
70 }
71 NOKPROBE_SYMBOL(ret_handler);
72 
73 static struct kretprobe my_kretprobe = {
74 	.handler		= ret_handler,
75 	.entry_handler		= entry_handler,
76 	.data_size		= sizeof(struct my_data),
77 	/* Probe up to 20 instances concurrently. */
78 	.maxactive		= 20,
79 };
80 
81 static int __init kretprobe_init(void)
82 {
83 	int ret;
84 
85 	my_kretprobe.kp.symbol_name = func_name;
86 	ret = register_kretprobe(&my_kretprobe);
87 	if (ret < 0) {
88 		pr_err("register_kretprobe failed, returned %d\n", ret);
89 		return -1;
90 	}
91 	pr_info("Planted return probe at %s: %p\n",
92 			my_kretprobe.kp.symbol_name, my_kretprobe.kp.addr);
93 	return 0;
94 }
95 
96 static void __exit kretprobe_exit(void)
97 {
98 	unregister_kretprobe(&my_kretprobe);
99 	pr_info("kretprobe at %p unregistered\n", my_kretprobe.kp.addr);
100 
101 	/* nmissed > 0 suggests that maxactive was set too low. */
102 	pr_info("Missed probing %d instances of %s\n",
103 		my_kretprobe.nmissed, my_kretprobe.kp.symbol_name);
104 }
105 
106 module_init(kretprobe_init)
107 module_exit(kretprobe_exit)
108 MODULE_LICENSE("GPL");
109