Debugging Linux kernel using kretprobes - Getting Function Return Value

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Consider the scenario, where you want to find out the return value of a particular kernel function without modifying the kernel, kretprobes is the mechanism to go.

What are kretprobes?

 kretprobes allows you to call a user-defined function when the function returns.

Sample Code which will give the return value of fork() system call:

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

MODULE_LICENSE("GPL");
static struct kretprobe retprobe;

static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
{
    int retval = regs_return_value(regs);
    printk(KERN_INFO "%s returned %d\n", "_do_fork", retval);
    return 0;
}

static int test_kretprobe_init(void)
{
    int retval = 0;
    printk(KERN_INFO"%s: In init\n", __func__);
    retprobe.kp.symbol_name = "_do_fork";
    retprobe.handler = ret_handler;
    retval = register_kretprobe(&retprobe);
    if (retval < 0) {
        printk(KERN_INFO"register_kretprobe failed, returned:%d\n",
                retval);
    }
    return retval;
}

static void test_kretprobe_exit(void)
{
    unregister_kretprobe(&retprobe);
    printk(KERN_INFO"%s: In exit\n", __func__);
}
module_init(test_kretprobe_init);

module_exit(test_kretprobe_exit);

Output:


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