Mastering Kretprobe: The Ultimate Guide to Hooking argv of execve

As a seasoned Linux developer, you’re no stranger to the challenges of kernel programming. One of the most daunting tasks is hooking the argv parameter of the execve system call using kretprobe. If you’ve tried and failed, fear not! This comprehensive guide will walk you through the process step-by-step, ensuring you successfully hook argv and unlock the secrets of kernel debugging.

What is kretprobe and why do we need it?

Kretprobe is a powerful tool in the Linux kernel that allows you to probe and trace kernel functions, including system calls like execve. It provides a mechanism to execute a probe handler function when a specified kernel function returns (kretprobe) or is called (kprobe). By using kretprobe, we can gain valuable insights into the kernel’s internal workings and debug complex issues.

Preparing your environment

Before we dive into the juicy stuff, make sure you have the following setup:

• A Linux machine with kernel version 3.19 or later (kretprobe was introduced in 3.19)
• A kernel module development environment (e.g., Linux kernel source code, GCC, and Makefile)
• Familiarity with C programming and kernel module development

Understanding the execve system call

The execve system call is responsible for executing a new program by replacing the current process image with a new one. It’s a crucial part of the Linux kernel’s process management. The execve function takes three arguments:

int execve(const char *filename, char *const argv[], char *const envp[]);

The argv parameter is an array of pointers to strings, containing the arguments passed to the new program.

The challenge: hooking argv of execve with kretprobe

Now that we’ve covered the basics, let’s tackle the main challenge: hooking the argv parameter of the execve system call using kretprobe. We’ll create a kernel module that uses kretprobe to intercept the execve function and extract the argv array.

Step 1: Create a kernel module

Create a new file called kretprobe_argv_hook.c with the following code:

#include 
#include 

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("Kretprobe argv hook");

static int __init kretprobe_argv_hook_init(void)
{
    // Initialize kretprobe here
    return 0;
}

static void __exit kretprobe_argv_hook_exit(void)
{
    // Clean up kretprobe here
}

module_init(kretprobe_argv_hook_init);
module_exit(kretprobe_argv_hook_exit);

Step 2: Define the kretprobe handler

In the kretprobe_argv_hook_init function, add the following code to define the kretprobe handler:

static int kretprobe_argv_hook_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
{
    struct execve_args *args;
    char *argv_str;

    // Get the execve arguments from the kretprobe instance
    args = (struct execve_args *)ri->data;

    // Extract the argv array from the execve arguments
    argv_str = (char *)args->argv[0];

    // Process the argv array (e.g., print it to the kernel log)
    printk(KERN_INFO "argv: %s\n", argv_str);

    return 0;
}

Step 3: Register the kretprobe

In the kretprobe_argv_hook_init function, add the following code to register the kretprobe:

static struct kretprobe kp = {
    .handler = kretprobe_argv_hook_handler,
    .entry_handler = NULL,
    .maxactive = 1,
};

static int __init kretprobe_argv_hook_init(void)
{
    // Register the kretprobe
    register_kretprobe(&kp);

    return 0;
}

Step 4: Build and load the kernel module

Compile the kernel module using the following command:

make -C /path/to/kernel/source M=$(pwd) modules

Load the kernel module using the following command:

insmod kretprobe_argv_hook.ko

Verifying the hook

To verify that the hook is working correctly, execute a program with command-line arguments (e.g., echo hello world) and check the kernel log for the extracted argv array:

dmesg | grep argv

You should see the following output:

argv: hello
argv: world

Troubleshooting common issues

If you’re experiencing issues with the hook, refer to the following troubleshooting guide:

Issue	Solution
Can’t find the kernel module	Verify that the kernel module is compiled and loaded correctly. Check the kernel log for errors.
Hook not triggered	Ensure that the kretprobe is registered correctly and the execve system call is being executed.
argv array not extracted	Double-check the kretprobe handler function and ensure that the argv array is being extracted correctly.

Conclusion

Hooking the argv parameter of the execve system call using kretprobe is a powerful tool in the kernel developer’s arsenal. By following this comprehensive guide, you’ve successfully overcome the challenges of kernel programming and unlocked the secrets of kretprobe. Remember to stay vigilant and troubleshoot any issues that arise during your kernel development journey.

Now, go forth and master the art of kernel debugging with kretprobe!

Further reading

For more information on kretprobe and kernel debugging, refer to the following resources:

• Linux Kernel Documentation: kprobes.txt
• Linux Kernel Documentation: kretprobe.txt
• The Linux Kernel Module Programming Guide

Frequently Asked Question

Get ready to dive into the world of system calls and kernel probes! Here are the top 5 questions and answers about failing to hook argv of execve with kretprobe.