How do you create a loadable kernel module for Android?

Creating Loadable Kernel Modules for Android

This article guides you through the process of creating and loading kernel modules for Android, enhancing functionality and customizing your device.

Understanding Kernel Modules

What are Kernel Modules?

Kernel modules are small programs that extend the functionality of the Linux kernel. They offer a modular approach, allowing developers to add features without recompiling the entire kernel.

Benefits of Kernel Modules:

• Flexibility: Add or remove functionality without recompiling the entire kernel.
• Customization: Tailored modifications for specific needs.
• Reduced Kernel Size: Only load modules when needed, saving space and resources.

Setting Up the Environment

1. Obtain Source Code:

Download the Android source code from https://source.android.com/.

2. Configure and Build:

source build/envsetup.sh
lunch aosp_arm-eng
make -j$(nproc)

Writing Your Module

1. Create the Module Source:

Create a new directory for your module inside the kernel source tree:

mkdir -p drivers/mymodule

2. Write the Module Code:

Example: A simple “hello world” module:

// drivers/mymodule/mymodule.c

#include 
#include 

static int __init mymodule_init(void) {
  printk(KERN_ALERT "Hello, world from mymodule!\n");
  return 0;
}

static void __exit mymodule_exit(void) {
  printk(KERN_ALERT "Goodbye, world from mymodule!\n");
}

module_init(mymodule_init);
module_exit(mymodule_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("Simple Hello World Module");

3. Add a Kconfig Entry:

Create a new Kconfig file for your module:

// drivers/mymodule/Kconfig

config MYMODULE
	bool "Load the mymodule module"
	default n
	help
	  This option loads the mymodule module.

4. Add a Makefile:

// drivers/mymodule/Makefile

obj-$(CONFIG_MYMODULE) += mymodule.o

Building and Installing the Module

1. Rebuild the Kernel:

make -j$(nproc)

2. Install the Module:

Copy the compiled module to your Android device:

adb push drivers/mymodule/mymodule.ko /system/lib/modules

3. Load the Module:

Use the `insmod` command to load the module:

adb shell insmod /system/lib/modules/mymodule.ko

You should see the “Hello, world…” message in your Android’s kernel log.

4. Unloading the Module:

Unload the module using the `rmmod` command:

adb shell rmmod mymodule

Module Structure

Module Entry Points:

• module_init(your_init_function): Called when the module is loaded.
• module_exit(your_exit_function): Called when the module is unloaded.

Essential Macros:

• MODULE_LICENSE("GPL"): Specifies the module’s license (GPL is common).
• MODULE_AUTHOR("Your Name"): Sets the author’s name.
• MODULE_DESCRIPTION("Module description"): Provides a short description.

Example: A Device Driver

This section demonstrates creating a module that interacts with hardware (replace placeholders with your specific device and functionality).

1. Header File:

// drivers/mymodule/mymodule.h

#ifndef _MYMODULE_H_
#define _MYMODULE_H_

#include 
#include 
#include 

// ... your device-specific header files ...

#endif // _MYMODULE_H_

2. Device Driver Source:

// drivers/mymodule/mymodule.c

#include 
#include 
#include 
#include "mymodule.h"

// ... device-specific data structures and functions ...

// Platform device structure
static struct platform_device mymodule_device = {
  .name  = "mymodule",
  .id    = -1, // Automatic ID
};

static int __init mymodule_init(void) {
  // Register the platform device
  platform_device_register(&mymodule_device);

  // ... your device initialization logic ...

  return 0;
}

static void __exit mymodule_exit(void) {
  // ... your device cleanup logic ...
  // Deregister the platform device
  platform_device_put(&mymodule_device);
}

module_init(mymodule_init);
module_exit(mymodule_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("MyModule Device Driver");

3. Kconfig Entry:

// drivers/mymodule/Kconfig

config MYMODULE
	bool "Load the mymodule module"
	default n
	depends on PLATFORM_DEVICE_MYMODULE
	help
	  This option loads the mymodule module.

4. Add Device Tree Entry:

Modify your device’s Device Tree Overlay (DTO) to add the platform device:

// ... your-device-name.dtsi
&mymodule {
  compatible = "your-company,mymodule";
  // ... your device-specific properties ...
};

Testing and Debugging

• Kernel Log: Use adb logcat to view kernel messages.
• Debug Messages: Add `printk()` statements for debugging.
• System Logs: Check system logs for errors using adb logcat.
• Memory Debugging: Use `kmalloc()` and `kfree()` for memory allocation and deallocation. Consider using `kmemcheck` for memory leaks.

Important Considerations

• Compatibility: Ensure your module is compatible with the Android version and kernel used on your device.
• Android Compatibility Definition (ACD): Follow the ACD guidelines for kernel module development.
• Security: Thoroughly test and review your module for security vulnerabilities.
• Documentation: Provide clear documentation for your module’s functionality.

This article offers a fundamental guide to kernel module development for Android. For further learning, delve into Android kernel documentation, sample modules, and advanced debugging techniques.