root/samples/kprobes/kretprobe_example.c

DEFINITIONS

1. entry_handler
2. ret_handler
3. kretprobe_init
4. kretprobe_exit

/*
 * kretprobe_example.c
 *
 * Here's a sample kernel module showing the use of return probes to
 * report the return value and total time taken for probed function
 * to run.
 *
 * usage: insmod kretprobe_example.ko func=<func_name>
 *
 * If no func_name is specified, do_fork is instrumented
 *
 * For more information on theory of operation of kretprobes, see
 * Documentation/kprobes.txt
 *
 * Build and insert the kernel module as done in the kprobe example.
 * You will see the trace data in /var/log/messages and on the console
 * whenever the probed function returns. (Some messages may be suppressed
 * if syslogd is configured to eliminate duplicate messages.)
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kprobes.h>
#include <linux/ktime.h>
#include <linux/limits.h>

static char func_name[NAME_MAX] = "do_fork";
module_param_string(func, func_name, NAME_MAX, S_IRUGO);
MODULE_PARM_DESC(func, "Function to kretprobe; this module will report the"
                        " function's execution time");

/* per-instance private data */
struct my_data {
        ktime_t entry_stamp;
};

/* Here we use the entry_hanlder to timestamp function entry */
static int entry_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
{
        struct my_data *data;

        if (!current->mm)
                return 1;       /* Skip kernel threads */

        data = (struct my_data *)ri->data;
        data->entry_stamp = ktime_get();
        return 0;
}

/*
 * Return-probe handler: Log the return value and duration. Duration may turn
 * out to be zero consistently, depending upon the granularity of time
 * accounting on the platform.
 */
static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
{
        int retval = regs_return_value(regs);
        struct my_data *data = (struct my_data *)ri->data;
        s64 delta;
        ktime_t now;

        now = ktime_get();
        delta = ktime_to_ns(ktime_sub(now, data->entry_stamp));
        printk(KERN_INFO "%s returned %d and took %lld ns to execute\n",
                        func_name, retval, (long long)delta);
        return 0;
}

static struct kretprobe my_kretprobe = {
        .handler                = ret_handler,
        .entry_handler          = entry_handler,
        .data_size              = sizeof(struct my_data),
        /* Probe up to 20 instances concurrently. */
        .maxactive              = 20,
};

static int __init kretprobe_init(void)
{
        int ret;

        my_kretprobe.kp.symbol_name = func_name;
        ret = register_kretprobe(&my_kretprobe);
        if (ret < 0) {
                printk(KERN_INFO "register_kretprobe failed, returned %d\n",
                                ret);
                return -1;
        }
        printk(KERN_INFO "Planted return probe at %s: %p\n",
                        my_kretprobe.kp.symbol_name, my_kretprobe.kp.addr);
        return 0;
}

static void __exit kretprobe_exit(void)
{
        unregister_kretprobe(&my_kretprobe);
        printk(KERN_INFO "kretprobe at %p unregistered\n",
                        my_kretprobe.kp.addr);

        /* nmissed > 0 suggests that maxactive was set too low. */
        printk(KERN_INFO "Missed probing %d instances of %s\n",
                my_kretprobe.nmissed, my_kretprobe.kp.symbol_name);
}

module_init(kretprobe_init)
module_exit(kretprobe_exit)
MODULE_LICENSE("GPL");