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Lab 4: File Recovery
Introduction
FAT has been around for nearly 50 years. Because of its simplicity, it is the most widely
compatible file system. Although recent computers have adopted newer file systems,
FAT32 (and its variant, exFAT) is still dominant in SD cards and USB flash drives due to
its compatibility.
Have you ever accidentally deleted a file? Do you know that it could be recovered? In
this lab, you will build a FAT32 file recovery tool called Need You to Undelete my FILE,
or nyufile for short.
Objectives
Through this lab, you will:
● Learn the internals of the FAT32 file system.
● Learn how to access and recover files from a raw disk.
● Get a better understanding of key file system concepts.
● Be a better C programmer. Learn how to write code that manipulates data at
the byte level and understand the alignment issue.
Overview
In this lab, you will work on the data stored in the FAT32 file system directly, without the
OS file system support. You will implement a tool that recovers a deleted file specified
by the user.
For simplicity, you can assume that the deleted file is in the root directory. Therefore,
you don’t need to search subdirectories.
Working with a FAT32 disk image
Before going through the details of this lab, let’s first create a FAT32 disk image. Follow
these steps:
Step 1: create an empty file of a certain size
On Linux, /dev/zero is a special file that provides as many \0 as are read from it. The dd
command performs low-level copying of raw data. Therefore, you can use it to generate
an arbitrary-size file full of zeros.
For example, to create a 256KB empty file named fat32.disk:
[root@... cs202]# dd if=/dev/zero of=fat32.disk bs=256k count=1
Read man dd for its usage. You will use this file as the disk image.
Step 2: format the disk with FAT32
You can use the mkfs.fat command to create a FAT32 file system. The most basic
usage is:
[root@... cs202]# mkfs.fat -F 32 fat32.disk
(You can ignore the warning of not enough clusters.)
You can specify a variety of options. For example:
[root@... cs202]# mkfs.fat -F 32 -f 2 -S 512 -s 1 -R 32 fat32.disk
Here are the meanings of each option:
● -F: type of FAT (FAT12, FAT16, or FAT32).
● -f: number of FATs.
● -S: number of bytes per sector.
● -s: number of sectors per cluster.
● -R: number of reserved sectors.
Step 3: verify the file system information
The fsck.fat command can check and repair FAT file systems. You can invoke it with -v
to see the FAT details. For example:
[root@... cs202]# fsck.fat -v fat32.disk
fsck.fat 4.1 (2017-01-24)
Checking we can access the last sector of the filesystem
Warning: Filesystem is FAT32 according to fat_length and fat32_length fields,
but has only 472 clusters, less than the required minimum of 65525.
This may lead to problems on some systems.
Boot sector contents:
System ID "mkfs.fat"
Media byte 0xf8 (hard disk)
512 bytes per logical sector
512 bytes per cluster
32 reserved sectors
First FAT starts at byte 16384 (sector 32)
2 FATs, 32 bit entries
2048 bytes per FAT (= 4 sectors)
Root directory start at cluster 2 (arbitrary size)
Data area starts at byte 20480 (sector 40)
472 data clusters (241664 bytes)
32 sectors/track, 64 heads
0 hidden sectors
512 sectors total
Checking for unused clusters.
Checking free cluster summary.
fat32.disk: 0 files, 1/472 clusters
You can see that there are 2 FATs, 512 bytes per sector, 512 bytes per cluster, and 32
reserved sectors. These numbers match our specified options in Step 2. You can try
different options yourself.
Step 4: mount the file system
You can use the mount command to mount a file system to a mount point. The mount
point can be any empty directory. For example, you can create one at /mnt/disk:
[root@... cs202]# mkdir /mnt/disk
Then, you can mount fat32.disk at that mount point:
[root@... cs202]# mount fat32.disk /mnt/disk
Step 5: play with the file system
After the file system is mounted, you can do whatever you like on it, such as creating
files, editing files, or deleting files. In order to avoid the hassle of having long filenames
in your directory entries, it is recommended that you use only 8.3 filenames, which
means:
● The filename contains at most eight characters, followed optionally by a . and
at most three more characters.
● The filename contains only uppercase letters, numbers, and the following
special characters: ! # $ % & ' ( ) - @ ^ _ ` { } ~.
For example, you can create a file named HELLO.TXT:
[root@... cs202]# echo "Hello, world." > /mnt/disk/HELLO.TXT
[root@... cs202]# mkdir /mnt/disk/DIR
[root@... cs202]# touch /mnt/disk/EMPTY
For the purpose of this lab, after you write anything to the disk, make sure to flush the
file system cache using the sync command:
[root@... cs202]# sync
(Otherwise, if you create a file and immediately delete it, the file may not be written to
the disk at all and is unrecoverable.)
Step 6: unmount the file system
When you finish playing with the file system, you can unmount it:
[root@... cs202]# umount /mnt/disk
Step 7: examine the file system
You can examine the file system using the xxd command. You can specify a range using
the -s (starting offset) and -l (length) options.
For example, to examine the root directory:
[root@... cs202]# xxd -s 20480 -l 96 fat32.disk
00005000: 4845 4c4c 4f20 2020 5458 5420 0000 0000 HELLO TXT ....
00005010: 6e53 6e53 0000 0000 6e53 0300 0e00 0000 nSnS....nS......
00005020: 4449 5220 2020 2020 2020 2010 0000 0000 DIR .....
00005030: 6e53 6e53 0000 0000 6e53 0400 0000 0000 nSnS....nS......
00005040: 454d 5054 5920 2020 2020 2020 0000 0000 EMPTY ....
00005050: 6e53 6e53 0000 0000 6e53 0000 0000 0000 nSnS....nS......
(It’s normal that the bytes containing timestamps are different from the example above.)
To examine the contents of HELLO.TXT:
[root@... cs202]# xxd -s 20992 -l 14 fat32.disk
0005200: 4865 6c6c 6f2c 2077 6f72 6c64 2e0a Hello, world..
Note that the offsets may vary depending on how the file system is formatted.
Your tasks
Important: before running your nyufile program, please make sure that your FAT32
disk is unmounted.
Milestone 1: validate usage
There are several ways to invoke your nyufile program. Here is its usage:
[root@... cs202]# ./nyufile
Usage: ./nyufile disk
-i Print the file system information.
-l List the root directory.
-r filename [-s sha1] Recover a contiguous file.
-R filename -s sha1 Recover a possibly non-contiguous file.
The first argument is the filename of the disk image. After that, the options can be one
of the following:
● -i
● -l
● -r filename
● -r filename -s sha1
● -R filename -s sha1
You need to check if the command-line arguments are valid. If not, your program should
print the above usage information verbatim and exit.
Milestone 2: print the file system information
If your nyufile program is invoked with option -i, it should print the following information
about the FAT32 file system:
● Number of FATs;
● Number of bytes per sector;
● Number of sectors per cluster;
● Number of reserved sectors.
Your output should be in the following format:
[root@... cs202]# ./nyufile fat32.disk -i
Number of FATs = 2
Number of bytes per sector = 512
Number of sectors per cluster = 1
Number of reserved sectors = 32
For all milestones, you can assume that nyufile is invoked while the disk is
unmounted.
Milestone 3: list the root directory
If your nyufile program is invoked with option -l, it should list all valid entries in the root
directory with the following information:
● Filename. Similar to /bin/ls -p, if the entry is a directory, you should
append a / indicator.
● File size if the entry is a file (not a directory).
● Starting cluster if the entry is not an empty file.
You should also print the total number of entries at the end. Your output should be in the
following format:
[root@... cs202]# ./nyufile fat32.disk -l
HELLO.TXT (size = 14, starting cluster = 3)
DIR/ (starting cluster = 4)
EMPTY (size = 0)
Total number of entries = 3
Here are a few assumptions:
● You should not list entries marked as deleted.
● You don’t need to print the details inside subdirectories.
● For all milestones, there will be no long filename (LFN) entries. (If you have
accidentally created LFN entries when you test your program, don’t worry.
You can just skip the LFN entries and print only the 8.3 filename entries.)
● Any file or directory, including the root directory, may span more than one
cluster.
● There may be empty files.
Milestone 4: recover a small file
If your nyufile program is invoked with option -r filename, it should recover the deleted
file with the specified name. The workflow is better illustrated through an example:
[root@... cs202]# mount fat32.disk /mnt/disk
[root@... cs202]# ls -p /mnt/disk
DIR/ EMPTY HELLO.TXT
[root@... cs202]# cat /mnt/disk/HELLO.TXT
Hello, world.
[root@... cs202]# rm /mnt/disk/HELLO.TXT
rm: remove regular file '/mnt/disk/HELLO.TXT'? y
[root@... cs202]# ls -p /mnt/disk
DIR/ EMPTY
[root@... cs202]# umount /mnt/disk
[root@... cs202]# ./nyufile fat32.disk -l
DIR/ (starting cluster = 4)
EMPTY (size = 0)
Total number of entries = 2
[root@... cs202]# ./nyufile fat32.disk -r HELLO
HELLO: file not found
[root@... cs202]# ./nyufile fat32.disk -r HELLO.TXT
HELLO.TXT: successfully recovered
[root@... cs202]# ./nyufile fat32.disk -l
HELLO.TXT (size = 14, starting cluster = 3)
DIR/ (starting cluster = 4)
EMPTY (size = 0)
Total number of entries = 3
[root@... cs202]# mount fat32.disk /mnt/disk
[root@... cs202]# ls -p /mnt/disk
DIR/ EMPTY HELLO.TXT
[root@... cs202]# cat /mnt/disk/HELLO.TXT
Hello, world.
For all milestones, you only need to recover regular files (including empty files, but not
directory files) in the root directory. When the file is successfully recovered, your
program should print filename: successfully recovered (replace filename with the actual
file name).
For all milestones, you can assume that no other files or directories are created or
modified since the deletion of the target file. However, multiple files may be deleted.
Besides, for all milestones, you don’t need to update the FSINFO structure because most
operating systems don’t care about it.
Here are a few assumptions specifically for Milestone 4:
● The size of the deleted file is no more than the size of a cluster.
● At most one deleted directory entry matches the given filename. If no such
entry exists, your program should print filename: file not found (replace
filename with the actual file name).
Milestone 5: recover a large contiguously-allocated file
Now, you will recover a file that is larger than one cluster. Nevertheless, for Milestone 5,
you can assume that such a file is allocated contiguously. You can continue to assume
that at most one deleted directory entry matches the given filename. If no such entry
exists, your program should print filename: file not found (replace filename with the
actual file name).
Milestone 6: detect ambiguous file recovery requests
In Milestones 4 and 5, you assumed that at most one deleted directory entry matches
the given filename. However, multiple files whose names differ only in the first character
would end up having the same name when deleted. Therefore, you may encounter
more than one deleted directory entry matching the given filename. When that happens,
your program should print filename: multiple candidates found (replace filename with the
actual file name) and abort.
This scenario is illustrated in the following example:
[root@... cs202]# mount fat32.disk /mnt/disk
[root@... cs202]# echo "My last name is Tang." > /mnt/disk/TANG.TXT
[root@... cs202]# echo "My first name is Yang." > /mnt/disk/YANG.TXT
[root@... cs202]# sync
[root@... cs202]# rm /mnt/disk/TANG.TXT /mnt/disk/YANG.TXT
rm: remove regular file '/mnt/disk/TANG.TXT'? y
rm: remove regular file '/mnt/disk/YANG.TXT'? y
[root@... cs202]# umount /mnt/disk
[root@... cs202]# ./nyufile fat32.disk -r TANG.TXT
TANG.TXT: multiple candidates found
Milestone 7: recover a contiguously-allocated file with SHA-1
hash
To solve the aforementioned ambiguity, the user can provide a SHA-1 hash via
command-line option -s sha1 to help identify which deleted directory entry should be the
target file.
In short, a SHA-1 hash is a 160-bit fingerprint of a file, often represented as 40
hexadecimal digits. For the purpose of this lab, you can assume that identical files
always have the same SHA-1 hash, and different files always have vastly different
SHA-1 hashes. Therefore, even if multiple candidates are found during recovery, at
most one will match the given SHA-1 hash.
This scenario is illustrated in the following example:
[root@... cs202]# ./nyufile fat32.disk -r TANG.TXT -s
c91761a2cc1562d36585614c8c680ecf5712e875
TANG.TXT: successfully recovered with SHA-1
[root@... cs202]# ./nyufile fat32.disk -l
HELLO.TXT (size = 14, starting cluster = 3)
DIR/ (starting cluster = 4)
EMPTY (size = 0)
TANG.TXT (size = 22, starting cluster = 5)
Total number of entries = 4
When the file is successfully recovered with SHA-1, your program should print filename:
successfully recovered with SHA-1 (replace filename with the actual file name).
Note that you can use the sha1sum command to compute the SHA-1 hash of a file:
[root@... cs202]# sha1sum /mnt/disk/TANG.TXT
c91761a2cc1562d36585614c8c680ecf5712e875 /mnt/disk/TANG.TXT
Also note that it is possible that the file is empty or occupies only one cluster. The
SHA-1 hash for an empty file is da39a3ee5e6b4b0d3255bfef95601890afd80709.
If no such file matches the given SHA-1 hash, your program should print filename: file
not found (replace filename with the actual file name). For example:
[root@... cs202]# ./nyufile fat32.disk -r TANG.TXT -s
0123456789abcdef0123456789abcdef01234567
TANG.TXT: file not found
The OpenSSL library provides a function SHA1(), which computes the SHA-1 hash of
d[0...n-1] and stores the result in md[0...SHA_DIGEST_LENGTH-1]:
#include
#define SHA_DIGEST_LENGTH 20
unsigned char *SHA1(const unsigned char *d, size_t n, unsigned char *md);
You need to add the linker option -lcrypto to link with the OpenSSL library.
Milestone 8: recover a non-contiguously allocated file
Finally, the clusters of a file are no longer assumed to be contiguous. You have to try
every permutation of unallocated clusters on the file system in order to find the one that
matches the SHA-1 hash.
The command-line option is -R filename -s sha1. The SHA-1 hash must be given.
Note that it is possible that the file is empty or occupies only one cluster. If so, -R
behaves the same as -r, as described in Milestone 7.
For Milestone 8, you can assume that the entire file is within the first 20 clusters, and
the file content occupies no more than 5 clusters, so a brute-force search is feasible.
If you cannot find a file that matches the given SHA-1 hash, your program should print
filename: file not found (replace filename with the actual file name).
FAT32 data structures
For your convenience, here are some data structures that you can copy and paste.
Please refer to the lecture slides and FAT: General Overview of On-Disk Format for
details on the FAT32 file system layout.
Boot sector
#pragma pack(push,1)
typedef struct BootEntry {
unsigned char BS_jmpBoot[3]; // Assembly instruction to jump to boot code
unsigned char BS_OEMName[8]; // OEM Name in ASCII
unsigned short BPB_BytsPerSec; // Bytes per sector. Allowed values include 512,
1024, 2048, and 4096
unsigned char BPB_SecPerClus; // Sectors per cluster (data unit). Allowed values
are powers of 2, but the cluster size must be 32KB or smaller
unsigned short BPB_RsvdSecCnt; // Size in sectors of the reserved area
unsigned char BPB_NumFATs; // Number of FATs
unsigned short BPB_RootEntCnt; // Maximum number of files in the root directory
for FAT12 and FAT16. This is 0 for FAT32
unsigned short BPB_TotSec16; // 16-bit value of number of sectors in file
system
unsigned char BPB_Media; // Media type
unsigned short BPB_FATSz16; // 16-bit size in sectors of each FAT for FAT12
and FAT16. For FAT32, this field is 0
unsigned short BPB_SecPerTrk; // Sectors per track of storage device
unsigned short BPB_NumHeads; // Number of heads in storage device
unsigned int BPB_HiddSec; // Number of sectors before the start of partition
unsigned int BPB_TotSec32; // 32-bit value of number of sectors in file
system. Either this value or the 16-bit value above must be 0
unsigned int BPB_FATSz32; // 32-bit size in sectors of one FAT
unsigned short BPB_ExtFlags; // A flag for FAT
unsigned short BPB_FSVer; // The major and minor version number
unsigned int BPB_RootClus; // Cluster where the root directory can be found
unsigned short BPB_FSInfo; // Sector where FSINFO structure can be found
unsigned short BPB_BkBootSec; // Sector where backup copy of boot sector is
located
unsigned char BPB_Reserved[12]; // Reserved
unsigned char BS_DrvNum; // BIOS INT13h drive number
unsigned char BS_Reserved1; // Not used
unsigned char BS_BootSig; // Extended boot signature to identify if the next
three values are valid
unsigned int BS_VolID; // Volume serial number
unsigned char BS_VolLab[11]; // Volume label in ASCII. User defines when
creating the file system
unsigned char BS_FilSysType[8]; // File system type label in ASCII
} BootEntry;
#pragma pack(pop)
Directory entry
#pragma pack(push,1)
typedef struct DirEntry {
unsigned char DIR_Name[11]; // File name
unsigned char DIR_Attr; // File attributes
unsigned char DIR_NTRes; // Reserved
unsigned char DIR_CrtTimeTenth; // Created time (tenths of second)
unsigned short DIR_CrtTime; // Created time (hours, minutes, seconds)
unsigned short DIR_CrtDate; // Created day
unsigned short DIR_LstAccDate; // Accessed day
unsigned short DIR_FstClusHI; // High 2 bytes of the first cluster address
unsigned short DIR_WrtTime; // Written time (hours, minutes, seconds
unsigned short DIR_WrtDate; // Written day
unsigned short DIR_FstClusLO; // Low 2 bytes of the first cluster address
unsigned int DIR_FileSize; // File size in bytes. (0 for directories)
} DirEntry;
#pragma pack(pop)
Compilation
We will grade your submission in an x86_64 Rocky Linux 8 container on Gradescope.
We will compile your program using gcc 12.1.1 with the C17 standard and GNU
extensions.
You must provide a Makefile, and by running make, it should generate an executable file
named nyufile in the current working directory. Note that you need to add
LDFLAGS=-lcrypto to your Makefile. (Refer to Lab 1 for an example of the Makefile.)
Testing
To get started with testing, you can download a sample FAT32 disk and expand it with
the following command:
[root@... cs202]# unxz fat32.disk.xz
There are a few files on this disk:
● HELLO.TXT – a small text file.
● DIR – an empty directory.
● EMPTY.TXT – an empty file.
● CONT.TXT – a large contiguously-allocated file.
● NON_CONT.TXT – a large non-contiguously allocated file.
You should make your own test cases and test your program thoroughly. Make sure to
test your program with disks formatted with different parameters.
The autograder
We are providing a sample autograder with a few test cases. Please extract them in
your Docker container and follow the instructions in the README file. (Refer to Lab 1 for
how to extract a .tar.xz file.)
Note that the test cases are not exhaustive. The numbered test cases on Gradescope
are the same as those in the sample autograder, while the lettered test cases are
“hidden” test cases that will not be disclosed. If your program passed the former but
failed the latter, please double-check if it can handle all corner cases correctly. Do not
try to hack or exploit the autograder.
Submission
You must submit a .zip archive containing all files needed to compile nyufile in the root
of the archive. You can create the archive file with the following command in the Docker
container:
$ zip nyufile.zip Makefile *.h *.c
Note that other file formats (e.g., rar) will not be accepted.
You need to upload the .zip archive to Gradescope. If you need to acknowledge any
influences per our academic integrity policy, write them as comments in your source
code.
Rubric
The total of this lab is 100 points, mapped to 15% of your final grade of this course.
● Milestone 1: validate usage. (40 points)
● Milestone 2: print the file system information. (5 points)
● Milestone 3: list the root directory. (10 points)
● Milestone 4: recover a small file. (15 points)
● Milestone 5: recover a large contiguously-allocated file. (10 points)
● Milestone 6: detect ambiguous file recovery requests. (5 points)
● Milestone 7a: recover a small file with SHA-1 hash. (5 points)
● Milestone 7b: recover a large contiguously-allocated file with SHA-1 hash. (5
points)
● Milestone 8: recover a non-contiguously allocated file. (5 points)
Tips
Don’t procrastinate
This lab requires significant programming effort. Therefore, start as early as possible!
Don’t wait until the last week.
Some general hints
● Before you start, use xxd to examine the disk image to get an idea of the
FAT32 layout. Keep a backup of the hexdump.
● After you create a file or delete a file, use xxd to compare the hexdump of the
disk image against your backup to see what has changed.
● You can also use xxd -r to convert a hexdump back to a binary file. You can
use it to “hack” a disk image. In this way, you can try recovering a file
manually before writing a program to do it. You can also create a
non-contiguously allocated file artificially for testing in this way.
● Always umount before using xxd or running your nyufile program.
● When updating FAT, remember to update all FATs.
● Using mmap() to access the disk image is more convenient than read() or
fread(). You may need to open the disk image with O_RDWR and map it with
PROT_READ | PROT_WRITE and MAP_SHARED in order to update the underlying file.
Once you have mapped your disk image, you can cast any address to the
FAT32 data structure type, such as (DirEntry *)(mapped_address + 0x5000).
You can also cast the FAT to int[] for easy access.
● The milestones have diminishing returns. Easier milestones are worth more
points. Make sure you get them right before trying to tackle the harder ones.
This lab has borrowed some ideas from Dr. T. Y. Wong.