Tutorial Exercise - Ext2 file system structure

Introduction

For Assignment 3, you will be manipulating an Ext2 file system image. The next few exercises are intended to help you get started on the assignment by helping you understand the structure of the file system. Please start early to fully take advantage of the tutorial time to ask questions.

Task 1

Please read the Assignment 3 handout. It has links to some resources that you will need to read to fully understand the ext2 structure, especially sections "Learning about the Filesystem" and "Mounting a filesystem".

You are given two files as starter code:

• ext2.h
• readimage.c

There are also a bunch of disk images available to you:

• emptydisk: An empty virtual disk.
• onefile: A single text file has been added to emptydisk.
• deletedfile: The file from onefile has been removed.
• onedirectory: A single directory containing a text file has been added to emptydisk.
• hardlink: A hard link to the textfile in onedirectory was added.
• deleteddirectory: A recursive remove was used to remove the directory and file from onedirectory.
• twolevel: The root directory contains a directory called level1 and a file called afile. level1 contains a directory called level2, and level2 contains a file called bfile.
• largefile: A file larger than 13KB (13440 bytes) is in the root directory. This file requires the single indirect block in the inode.

Note that readimage uses mmap to map the disk image file into memory. The superblock is in the second block on the disk starting at byte 1024, so we can interpret these bytes as the super block struct.

You should also look at these bytes of the one of the images, for example emptydisk.img by running xxd emptydisk.img > emptydisk.txt and using an editor to view the contents of the file. Try the following:

• Figure out where each block starts.
• See what the inode bit map and block bitmaps look like.
• Find other non-empty blocks to see if you can see what might be in them.

Your task for this exercise is simple. Add to readimage to print out the following fields from the block group descriptor. For emptydisk.img your output should look exactly like the following. In other words, we should be able to use diff to compare your output to this and see that it is identical.

Inodes: 32
Blocks: 128
Block group:
    block bitmap: 3
    inode bitmap: 4
    inode table: 5
    free blocks: 105
    free inodes: 21
    used_dirs: 2

Task2

Using the same code you wrote, add the code to print out the inode and block bitmaps, and the inodes.

For emptydisk.img your output should exactly match the following. In other words, we should be able to use diff to compare your output to this and see that it is identical.

Inodes: 32
Blocks: 128
Block group:
    block bitmap: 3
    inode bitmap: 4
    inode table: 5
    free blocks: 105
    free inodes: 21
    used_dirs: 2
Block bitmap: 11111111 11111111 11111100 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
Inode bitmap: 11111111 11100000 00000000 00000000

Inodes:
[2] type: d size: 1024 links: 3 blocks: 2
[2] Blocks:  9

For onefile.img your output should match the following:

Inodes: 32
Blocks: 128
Block group:
    block bitmap: 3
    inode bitmap: 4
    inode table: 5
    free blocks: 104
    free inodes: 20
    used_dirs: 2
Block bitmap: 11111111 11111111 11111100 00000000 00100000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
Inode bitmap: 11111111 11110000 00000000 00000000

Inodes:
[2] type: d size: 1024 links: 3 blocks: 2
[2] Blocks:  9
[12] type: f size: 128 links: 1 blocks: 2
[12] Blocks:  35

Task 3

Your final task in this series of exercises is to print the directory block contents. While you are doing this, you should work out how the directory structure creates a linked list of directory entries by using rec_len to show where the next directory entry begins. Note that rec_len in the last directory entry takes you to the end of the block.

For emptydisk.img your output should exactly match the following. In other words, we should be able to use diff to compare your output to this and see that it is identical.

Inodes: 32
Blocks: 128
Block group:
    block bitmap: 3
    inode bitmap: 4
    inode table: 5
    free blocks: 105
    free inodes: 21
    used_dirs: 2
Block bitmap: 11111111 11111111 11111100 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
Inode bitmap: 11111111 11100000 00000000 00000000

Inodes:
[2] type: d size: 1024 links: 3 blocks: 2
[2] Blocks:  9

Directory Blocks:
   DIR BLOCK NUM: 9 (for inode 2)
Inode: 2 rec_len: 12 name_len: 1 type= d name=.
Inode: 2 rec_len: 12 name_len: 2 type= d name=..
Inode: 11 rec_len: 1000 name_len: 10 type= d name=lost+found

For twolevel.img your output should match the following:

Inodes: 128
Blocks: 128
Block group:
    block bitmap: 3
    inode bitmap: 4
    inode table: 5
    free blocks: 91
    free inodes: 113
    used_dirs: 4
Block bitmap: 11111111 11111111 11111111 11111111 11100000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000011 
Inode bitmap: 11111111 11111110 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 

Inodes:
[2] type: d size: 1024 links: 4 blocks: 2
[2] Blocks:  21
[12] type: d size: 1024 links: 3 blocks: 2
[12] Blocks:  127
[14] type: d size: 1024 links: 2 blocks: 2
[14] Blocks:  35

Directory Blocks:
   DIR BLOCK NUM: 21 (for inode 2)
Inode: 2 rec_len: 12 name_len: 1 type= d name=.
Inode: 2 rec_len: 12 name_len: 2 type= d name=..
Inode: 11 rec_len: 20 name_len: 10 type= d name=lost+found
Inode: 12 rec_len: 16 name_len: 6 type= d name=level1
Inode: 13 rec_len: 964 name_len: 5 type= f name=afile
   DIR BLOCK NUM: 127 (for inode 12)
Inode: 12 rec_len: 12 name_len: 1 type= d name=.
Inode: 2 rec_len: 12 name_len: 2 type= d name=..
Inode: 14 rec_len: 1000 name_len: 6 type= d name=level2
   DIR BLOCK NUM: 35 (for inode 14)
Inode: 14 rec_len: 12 name_len: 1 type= d name=.
Inode: 12 rec_len: 12 name_len: 2 type= d name=..
Inode: 15 rec_len: 1000 name_len: 5 type= f name=bfile