(Focus Keyphrase: Kprobes in Linux kernel)

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Introduction – When You Cannot Modify the Code

Imagine this situation:

• You are debugging a kernel issue
• The problem happens deep inside the kernel
• You don’t want to modify kernel source
• You don’t want to rebuild the kernel
• And you definitely don’t want to reboot repeatedly

Now ask yourself:

💭 How do I trace what is happening inside a kernel function?
💭 How do I observe function arguments and return values at runtime?

This is exactly where Kprobes and Kretprobes become incredibly powerful.

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What Are Kprobes?

Kprobes allow you to dynamically insert probes into running kernel code.

👉 You can hook into almost any kernel function
👉 You can execute your handler when that function is hit
👉 No kernel recompilation required

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Use Cases of Kprobes

Kprobes are useful when you want to:

✔ Trace execution of a specific function
✔ Inspect function arguments
✔ Monitor kernel behavior in real-time
✔ Debug issues without modifying source code

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How Kprobes Work

Kprobes insert a breakpoint instruction at a target address.

When the kernel hits that instruction:

1. Execution pauses
2. Your handler function is invoked
3. Control returns to original execution

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Example: Simple Kprobe

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

static struct kprobe kp;

static int handler_pre(struct kprobe *p, struct pt_regs *regs)
{
    pr_info("Kprobe hit at %s\n", p->symbol_name);
    return 0;
}

static int __init kprobe_init(void)
{
    kp.symbol_name = "do_sys_open";
    kp.pre_handler = handler_pre;

    register_kprobe(&kp);
    pr_info("Kprobe registered\n");
    return 0;
}

static void __exit kprobe_exit(void)
{
    unregister_kprobe(&kp);
    pr_info("Kprobe unregistered\n");
}

module_init(kprobe_init);
module_exit(kprobe_exit);
MODULE_LICENSE("GPL");

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What Are Kretprobes?

While Kprobes trigger when a function is entered,
Kretprobes trigger when a function returns.

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Use Cases of Kretprobes

Kretprobes are useful when you want to:

✔ Capture return values of functions
✔ Measure execution time
✔ Analyze function outcomes

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Example: Kretprobe

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

static struct kretprobe krp;

static int handler_ret(struct kretprobe_instance *ri, struct pt_regs *regs)
{
    pr_info("Function returned\n");
    return 0;
}

static int __init kretprobe_init(void)
{
    krp.kp.symbol_name = "do_sys_open";
    krp.handler = handler_ret;
    krp.maxactive = 20;

    register_kretprobe(&krp);
    pr_info("Kretprobe registered\n");
    return 0;
}

static void __exit kretprobe_exit(void)
{
    unregister_kretprobe(&krp);
    pr_info("Kretprobe unregistered\n");
}

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

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Kprobes vs Kretprobes

Feature	Kprobe	Kretprobe
Trigger Point	Function entry	Function return
Use Case	Trace execution	Capture return value
Timing	Before execution	After execution

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When Should You Use Kprobes?

Use Kprobes when:

• You need quick insights into kernel behavior
• You cannot modify kernel source
• You want dynamic tracing

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Limitations

❌ Can introduce overhead
❌ Requires careful handling
❌ Not suitable for production-heavy tracing

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Summary

Kprobes and Kretprobes give you:

✔ Dynamic kernel instrumentation
✔ Real-time debugging
✔ Zero recompilation

They allow you to observe the kernel without touching its source —
which makes them one of the most powerful debugging tools.

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Build Strong Kernel Debugging Skills

Debugging tools are powerful —
but they are only as effective as your understanding of the system.

If you want to master:

✔ Embedded Linux fundamentals
✔ Kernel internals
✔ Device driver development
✔ Real-world debugging techniques

Explore:

👉 https://embitude.in/embedded-linux-bundle/

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🎥 Learn more on YouTube:
👉 https://www.youtube.com/@PradeepTewani

🤝 Join the community of like-minded professionals:
👉 https://embitudeinfotech.graphy.com/s/community