Solved COP4600 Ex6: IO Device Files

Overview

This exercise should begin to help you understand the Linux File System and how to

interact with it. Linux originally used Minix’s file system, but it has gone through four iterations

of extended file systems over the course of the kernel’s development, your kernel’s file system is

using the ext4 file system (fourth extended file system).

In Unix-based systems (like Linux) and many other operating systems, devices in the

system are represented using device files – that is, virtual files present in the virtual filesystem

presented by the OS that do not present data records on a storage medium (as traditional files do).

Instead, device files act as handles to read from and write to connected devices. These devices are

often physical ones – such as solid-state drives, mice, or keyboards – but they can also be virtual

devices, which simulate hardware via a software mechanism.

In this exercise, you will create and attach a virtual storage drive via a loopback device, which

allows a regular file to masquerade as a device… which will then to represented using a virtual

file (Unix be cray like that). Loopback devices are commonly used on Unix systems, especially

for creating, reading, and writing storage images (such disc images, i.e. “ISOs”). You’ll connect

the storage, mount it, write to it, unmount it, and then verify that the data has been written.

Mouse

Joystick

SSD

Virtual

Filesystem

Drivers

/dev/input/mouse0

/dev/input/js0

/dev/block/sda

File2.txt

Image File

Device File

/mnt/ex

File1.txt

6

Loopback

Device

Structure

The exercise is broken into these main parts:

1) Create a storage device image file.

2) Connect the file to a loopback device, format it, and mount a filesystem on the device.

3) Write data to the virtual device by creating files on it after mounting

4) Unmount the virtual device and disconnect the loopback device

5) Verify the data has been written to the image file

Creating the Image

Follow these steps to complete the exercise:

1. Create an ex6.img file, filled with zeroes, of size one mebibyte (1 MiB) using dd:

$ dd if=

of= bs= count=

The dd command’s input can come from any source, a regular file or a special (device) file.

A special file, /dev/zero, will have an infinite number of zeroes available for reading. The

output data’s size is defined by the block-size and block-count – you could create a 1KiB file

by using a size of 512 and a count of 2, for example.

2. Create a loopback device connected to the image file using losetup, format the image in

ext4 format using mkfs, and then mount the loopback device on /mnt/ex6 via mount.

Take a screenshot of the commands you used in this step.

1. You will need to use sudo for the losetup command.

3. In this step you will be writing data to the virtual device by creating files on it after

mounting.

In the mounted directory (/mnt/ex6), create two text files:

1. one empty,

2. and one containing a message.

Display the contents of each file using the cat command and take a screenshot.

4. Unmount the filesystem (umount) and disconnect the loopback device (losetup).

5. Lastly, you need to verify if the data has been written to the image file.

Install hexedit (sudo apt-get install hexedit) and use it to examine the image –

you should be able to find your filenames and text. Take a screenshot of the file text.

Just for fun: try the strings command and see what happens!

Submissions

You will submit the following at the end of this exercise:

⚫ Screenshot of loopback setup, formatting, and mounting of the image file (step2.png)

⚫ Screenshot of the text files’ contents (using cat) in the mounted directory (step3.png)

⚫ Screenshot of image file in the hexedit program showing your file text (step5.png)