Linux Kernel Character Device Driver

A simple Linux kernel module that implements a character device driver with read/write functionality. This driver creates a device file that allows userspace programs to communicate with kernel space through a 100-byte buffer.

Overview

This project demonstrates fundamental kernel module development concepts:

• Character device registration with a fixed major number (90)
• File operations (open, release, read, write)
• User-kernel space data transfer using copy_to_user() and copy_from_user()
• Kernel buffer management for storing and retrieving data
• Module signing for Secure Boot environments

Project Structure

├── src/                    # Source code
│   └── hello.c            # Main driver implementation
├── docs/                   # Documentation
│   ├── dependencies.md    # Required packages and build tools
│   ├── resources.md       # Useful references and kernel APIs
│   ├── signing_driver.md  # Module signing instructions for Secure Boot
│
├── Makefile               # Build configuration
├── .gitignore             # Git ignore rules
└── README.md              # This file

Prerequisites

Install the required build tools and kernel headers:

sudo apt-get install build-essential linux-headers-$(uname -r)

For Secure Boot systems, you'll also need:

sudo apt-get install mokutil

Building the Module

make

This will generate hello.ko in the project root directory.

Installation and Usage

1. Load the Module

sudo insmod ./hello.ko

Verify it loaded successfully:

dmesg | tail -n 1
# Output: [ xxxx.xxx] Hello World!

2. Create the Device File

sudo mknod /dev/mydev c 90 0
sudo chmod 666 /dev/mydev

3. Interact with the Device

Write data to the device:

echo "Hello from userspace!" > /dev/mydev

Read data from the device:

cat /dev/mydev
# Output: Hello from userspace!

4. Unload the Module

sudo rmmod hello

Check kernel logs:

dmesg | tail -n 1
# Output: [ xxxx.xxx] Goodbye World!

5. Cleanup

sudo rm /dev/mydev
make clean

Module Signing (Secure Boot)

If you're running a system with Secure Boot enabled, you'll need to sign the module. See docs/signing_driver.md for detailed instructions.

Quick steps:

1. Generate a key pair: openssl req -new -x509 -newkey rsa:2048 -keyout MOK.priv -outform DER -out MOK.der -nodes -days 36500 -subj "/CN=MyCustomKey/"
2. Sign the module: sudo /usr/src/linux-headers-$(uname -r)/scripts/sign-file sha256 ./MOK.priv ./MOK.der hello.ko
3. Enroll the key: sudo mokutil --import MOK.der (requires reboot)

How It Works

The driver creates a kernel buffer that acts as shared storage between userspace and kernel space:

• Write Operation: Data from userspace is copied into the kernel buffer (max 99 bytes + null terminator)
• Read Operation: Data from the kernel buffer is copied back to userspace
• Open/Release: Logs device access events to the kernel log

All operations include proper error handling and bounds checking to prevent buffer overflows and invalid memory access.

Technical Details

• Major Number: 90 (statically assigned)
• Device Type: Character device
• Buffer Size: 100 bytes
• Supported Operations: open, release, read, write
• Error Codes: EFAULT (copy failures), EINVAL (invalid input), EIO (registration failure)