Linux Character Device Driver - OS Assignment

Overview

This project implements a Linux kernel character device driver that enforces ordered execution of operations within a time limit.

The driver ensures that:

• The kernel version matches before loading
• A READ operation is performed before WRITE
• Both operations occur within a specified time
• The result is validated when the module is removed

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Features

• Kernel version validation using LINUX_VERSION_CODE
• Dynamic character device allocation
• Waitqueue-based synchronization (read before write)
• Timer-based timeout enforcement
• Safe user-kernel communication using copy APIs
• Success/failure validation during module removal

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Requirements

Install required packages:

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

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Build Instructions

make

• builds the driver.ko module

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Check Kernel Version

uname -r

Example:

6.8.0-106-generic → use 6,8

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Load Module

sudo insmod driver.ko kernel_version=6,8 time=30

Parameters:

• kernel_version → kernel major and minor version
• time → timeout in seconds

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Check Kernel Logs

dmesg | tail -10

You should see:

• Major number
• Minor number
• Timer value

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Create Device File

sudo mknod /dev/my_device c 240 0
sudo chmod 666 /dev/my_device

Explanation:

• mknod creates the device file using major and minor numbers
• chmod 666 allows all users to read and write

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Execution (Correct Order)

cat /dev/my_device
whoami > /dev/my_device

Explanation:

• cat triggers the read() function
• whoami > triggers the write() function

Important:

-If write is executed before read, it will block -It will resume only after read or timeout

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Remove Module

sudo rmmod driver
dmesg | tail -10

Expected Results

SUCCESS

• Successfully completed the actions within time. Username:

FAILURE

• FAILURE: Conditions not met

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Test Cases

1. Success Case

cat /dev/my_device
whoami > /dev/my_device

• Read → Write within time → SUCCESS

2. Wrong Order

whoami > /dev/my_device
cat /dev/my_device

• Write blocks → FAIL

3. Timeout Case

cat /dev/my_device
sleep 35
whoami > /dev/my_device

• Write after timeout → FAIL

4. Only Read

cat /dev/my_device

• No write → FAIL

5. Only Write

whoami > /dev/my_device

• Blocks until timeout → FAIL

6. Version Mismatch

sudo insmod driver.ko kernel_version=5,4 time=30

• Module not inserted

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Technical Details

Waitqueue

• Used to enforce ordering:
  • wait_event_interruptible() blocks write
  • wake_up() resumes execution after read or timeout

Timer

• Implemented using timer_list
• Uses jiffies to track timeout
• On expiry:
  • Sets time_up = 1
  • Wakes blocked processes

Memory Transfer

• copy_to_user() → kernel → user
• copy_from_user() → user → kernel

Debugging

dmesg -w              # Live logs
dmesg -c              # Clear logs
lsmod | grep driver   # Check module loaded

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Notes

• Device file is created manually using mknod
• Write operation blocks until read or timeout
• Ensure correct kernel version is passed during insertion

Author

Manav Sharma