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 */
entry_handler(struct kretprobe_instance * ri,struct pt_regs * regs)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 */
ret_handler(struct kretprobe_instance * ri,struct pt_regs * regs)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
kretprobe_init(void)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
kretprobe_exit(void)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