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 
52 /*
53  * Return-probe handler: Log the return value and duration. Duration may turn
54  * out to be zero consistently, depending upon the granularity of time
55  * accounting on the platform.
56  */
57 static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
58 {
59 	unsigned long retval = regs_return_value(regs);
60 	struct my_data *data = (struct my_data *)ri->data;
61 	s64 delta;
62 	ktime_t now;
63 
64 	now = ktime_get();
65 	delta = ktime_to_ns(ktime_sub(now, data->entry_stamp));
66 	pr_info("%s returned %lu and took %lld ns to execute\n",
67 			func_name, retval, (long long)delta);
68 	return 0;
69 }
70 
71 static struct kretprobe my_kretprobe = {
72 	.handler		= ret_handler,
73 	.entry_handler		= entry_handler,
74 	.data_size		= sizeof(struct my_data),
75 	/* Probe up to 20 instances concurrently. */
76 	.maxactive		= 20,
77 };
78 
79 static int __init kretprobe_init(void)
80 {
81 	int ret;
82 
83 	my_kretprobe.kp.symbol_name = func_name;
84 	ret = register_kretprobe(&my_kretprobe);
85 	if (ret < 0) {
86 		pr_err("register_kretprobe failed, returned %d\n", ret);
87 		return -1;
88 	}
89 	pr_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 	pr_info("kretprobe at %p unregistered\n", my_kretprobe.kp.addr);
98 
99 	/* nmissed > 0 suggests that maxactive was set too low. */
100 	pr_info("Missed probing %d instances of %s\n",
101 		my_kretprobe.nmissed, my_kretprobe.kp.symbol_name);
102 }
103 
104 module_init(kretprobe_init)
105 module_exit(kretprobe_exit)
106 MODULE_LICENSE("GPL");
107