Root/samples/kprobes/kretprobe_example.c

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
28static char func_name[NAME_MAX] = "do_fork";
29module_param_string(func, func_name, NAME_MAX, S_IRUGO);
30MODULE_PARM_DESC(func, "Function to kretprobe; this module will report the"
31            " function's execution time");
32
33/* per-instance private data */
34struct my_data {
35    ktime_t entry_stamp;
36};
37
38/* Here we use the entry_hanlder to timestamp function entry */
39static 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 */
56static 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
70static 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
78static 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
94static 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
105module_init(kretprobe_init)
106module_exit(kretprobe_exit)
107MODULE_LICENSE("GPL");
108

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