README.md

Exercise 2: Readable miscdev module in Rust

Overview

In this exercise you will inspect a kernel module written in Rust that registers a miscdev device and provides a read implementation.

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_read.ko

Task 5: Check for presence of device

Inspect the list of miscdev devices:

    cat /proc/misc

You should see a messages as follows (lkp_miscdev_read is our device):

123 lkp_miscdev_read
124 cpu_dma_latency
125 vmbus/hv_kvp
126 vmbus/hv_vss
236 device-mapper
237 loop-control
144 nvram
228 hpet
235 autofs
231 snapshot
183 hw_random
127 vga_arbiter
242 rfkill

You can also see it in /dev:

    ls -l /dev/lkp_miscdev_read

As follows:

crw------- 1 root root 10, 123 Nov 30 14:14 /dev/lkp_miscdev_read

Task 6: Read from device

Read from device using cat:

    sudo cat /dev/lkp_miscdev_read

You should see:

my file data

Since the file is opened before it's read, you should also inspect the system log:

    sudo dmeg

You should see a message as follows:

[38097.623850] lkp_miscdev_read: open on device 1

Task 7: Remove the module

Remove the module using rmmod:

    sudo rmmod miscdev_read

Task 8: Check device is removed

Inspect the list of miscdev devices:

    cat /proc/misc

You should see a messages as follows (lkp_miscdev_read is not there anymore):

124 cpu_dma_latency
125 vmbus/hv_kvp
126 vmbus/hv_vss
236 device-mapper
237 loop-control
144 nvram
228 hpet
235 autofs
231 snapshot
183 hw_random
127 vga_arbiter
242 rfkill

You can also try to find it in /dev:

    ls -l /dev/lkp_miscdev_read

You should get the following error:

ls: cannot access '/dev/lkp_miscdev_read': No such file or directory

Summary

In this exercise, you got familiar with registering miscdev devices from Rust modules, and how unregistration is automatic when the module is unloaded. You also saw how a read implementation is exercised from the shell.