README.md

Exercise 3: Readable and writable miscdev module in Rust

Overview

In this exercise you will inspect an update to the previous module where we also have a write implementation for the miscdev device.

Task 1: Connect to your lab VM

1. Connect to your VM using ssh.

Task 2: Setup tmux

Start tmux so that connection issues do not prevent you from being able to get back to your shell:

    tmux -u attach -d || tmux -u

Task 3: Build the module

Build the module with make:

    make

Task 4: Load the module

Load the module using insmod:

    sudo insmod miscdev.ko

Task 5: Check for presence of device

You can also see it in /dev:

    ls -l /dev/lkp_miscdev

As follows:

crw-rw-rw- 1 root root 10, 123 Nov 30 14:53 /dev/lkp_miscdev

Note that the permissions are different: now everyone is allowed to read from and write to the device.

Task 6: Write to device

Write to device using redirection:

    echo testing > /dev/lkp_miscdev

Task 7: Read from device

Read from device using cat:

    cat /dev/lkp_miscdev

You should see:

testing

This is the text that was written in the previous task. You can repeat tasks 6 and 7 with different contents.

Challenge: Several writes

If we issue multiple writes to the device (e.g., using cat > /dev/lkp_miscdev and writing multiple lines), read will only return the last one. Why? How can we fix this?

Summary

In this exercise, you built on previous one and got familiar with how Rust ensures that memory remains available when in use, and how it forces the use of synchronization primitives when needed. You also saw how a write implementation can be exercised from the shell.