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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 fi le system. Although recent computers have adopted newer fi le systems, FAT32 (and its variant, exFAT) is still dominant in SD cards and USB fl ash drives due to its compatibility. Have you ever accidentally deleted a fi le? Do you know that it could be recovered?In this lab, you will build a FAT32 fi le 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 fi le system. Learn how to access and recover fi les from a raw disk. Get a better understanding of key fi le 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 fi le system directly, without the OS fi le system support. You will implement a tool that recovers a deleted file specifi ed by the user. For simplicity, you can assume thatthe deleted fi le is in the root directory. Therefore, you don t need to search subdirectories. Lab 4: File Recovery 1/18 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 fi le that provides as many � 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 fi le full of zeros. For example, to create a 256KB empty fi le 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 fi le as the disk image. Step 2: format the disk with FAT32 You can use the mkfs.fat command to create a FAT32 fi le 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). Lab 4: File Recovery 2/18 -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 fi le 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 specifi ed options in Step 2. You can try different options yourself. Lab 4: File Recovery 3/18 Step 4: mount the file system You can use the mount command to mount a fi le 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 fi le system is mounted, you can do whatever you like on it, such as creating fi les, editing fi les, or deleting fi les. In order to avoid the hassle of having long fi lenames in your directory entries, it is recommended that you use only 8.3 fi lenames, which means: The fi lename contains at most eight characters, followed optionally by a . and at most three more characters. The fi lename contains only uppercase letters, numbers, and the following special characters: ! # $ % & ' ( ) - @ ^ _ ` { } ~ . For example, you can create a fi le 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 fl ush the fi le system cache using the sync command: Lab 4: File Recovery 4/18 [root@... cs202]# sync (Otherwise, if you create a fi le and immediately delete it, the fi le may not be written to the disk at all and is unrecoverable.) Step 6: unmount the file system When you fi nish playing with the fi le system, you can unmount it: [root@... cs202]# umount /mnt/disk Step 7: examine the file system You can examine the fi le 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.. Lab 4: File Recovery 5/18 Note that the offsets may vary depending on how the fi le 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 fi rst argument is the fi lename 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 fi le system: Lab 4: File Recovery 6/18 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 fi le (not a directory). Starting cluster if the entry is not an empty fi le. 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: Lab 4: File Recovery 7/18 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 fi lename (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 fi lename entries.) Any fi le or directory, including the root directory, may span more than one cluster. There may be empty fi les. Milestone 4: recover a small file If your nyufile program is invoked with option -r filename , it should recover the deleted file with the specifi ed name. The workfl ow is better illustrated through an example: Lab 4: File Recovery 8/18 [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 fi les (including empty fi les, but not directory fi les) in the root directory. When the fi le is successfully recovered, your program should print filename: successfully recovered (replace filename with the actual fi le name). For all milestones, you can assume that no other fi les or directories are created or modifi ed since the deletion of the target fi le. However, multiple fi les 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. Lab 4: File Recovery 9/18 Here are a few assumptions specifi cally 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 fi lename. If no such entry exists, your program should print filename: file not found (replace filename with the actual fi le name). Milestone 5: recover a large contiguously-allocated file Now, you will recover a fi le that is larger than one cluster. Nevertheless, for Milestone 5, you can assume that such a fi le is allocated contiguously. You can continue to assume that at most one deleted directory entry matches the given fi lename. If no such entry exists, your program should print filename: file not found (replace filename with the actual fi le 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 fi lename. However, multiple fi les whose names differ only in the fi rst character would end up having the same name when deleted. Therefore, you may encounter more than one deleted directory entry matching the given fi lename. When that happens, your program should print filename: multiple candidates found (replace filename with the actual fi le 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 Lab 4: File Recovery 10/18 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 fi le. In short, a SHA-1 hash is a 160-bit fi ngerprint of a fi le, often represented as 40 hexadecimal digits. For the purpose of this lab, you can assume that identical fi les always have the same SHA-1 hash, and different fi les 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 c91761a2cc1562d36585614c 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 fi le is successfully recovered with SHA-1, your program should print filename: successfully recovered with SHA-1 (replace filename with the actual fi le name). Note that you can use the sha1sum command to compute the SHA-1 hash of a fi le: [root@... cs202]# sha1sum /mnt/disk/TANG.TXT c91761a2cc1562d36585614c8c680ecf5712e875 /mnt/disk/TANG.TXT Lab 4: File Recovery 11/18 Also note that it is possible that the fi le is empty or occupies only one cluster. The SHA-1 hash for an empty fi le is da39a3ee5e6b4b0d3255bfef95601890afd80709 . If no such fi le matches the given SHA-1 hash, your program should print filename: file not found (replace filename with the actual fi le name). For example: [root@... cs202]# ./nyufile fat32.disk -r TANG.TXT -s 0123456789abcdef01234567 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 fi le are no longer assumed to be contiguous. You have to try every permutation of unallocated clusters on the fi le system in order to fi nd 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 fi le is empty or occupies only one cluster. If so, -R behaves the same as -r , as described in Milestone 7. Lab 4: File Recovery 12/18 For Milestone 8, you can assume that the entire fi le is within the fi rst 20 clusters, and the fi le content occupies no more than 5 clusters, so a brute?force search is feasible. If you cannot fi nd a fi le that matches the given SHA-1 hash, your program should print filename: file not found (replace filename with the actual fi le 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 fi le system layout. Lab 4: File Recovery 13/18 Boot sector #pragma pack(push,1) typedef struct BootEntry { unsigned char BS_jmpBoot[3]; // Assembly instruction to jump to boot co unsigned char BS_OEMName[8]; // OEM Name in ASCII unsigned short BPB_BytsPerSec; // Bytes per sector. Allowed values includ unsigned char BPB_SecPerClus; // Sectors per cluster (data unit). Allowe 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 dir unsigned short BPB_TotSec16; // 16-bit value of number of sectors in fi unsigned char BPB_Media; // Media type unsigned short BPB_FATSz16; // 16-bit size in sectors of each FAT for 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 p unsigned int BPB_TotSec32; // 32-bit value of number of sectors in fi 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 unsigned short BPB_FSInfo; // Sector where FSINFO structure can be fo unsigned short BPB_BkBootSec; // Sector where backup copy of boot sector 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 unsigned int BS_VolID; // Volume serial number unsigned char BS_VolLab[11]; // Volume label in ASCII. User defines whe unsigned char BS_FilSysType[8]; // File system type label in ASCII } BootEntry; #pragma pack(pop) Lab 4: File Recovery 14/18 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 addre 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 addres 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 fi le 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 Lab 4: File Recovery 15/18 There are a few fi les on this disk: HELLO.TXT C a small text fi le. DIR C an empty directory. EMPTY.TXT C an empty fi le. CONT.TXT C a large contiguously-allocated file. NON_CONT.TXT C 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 fi le. (Refer to Lab 1 for how to extract a .tar.xz fi le.) 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 fi les needed to compile nyufile in the root of the archive. You can create the archive fi le with the following command in the Docker container: $ zip nyufile.zip Makefile *.h *.c Note that other fi le formats (e.g., rar ) will not be accepted. Lab 4: File Recovery 16/18 You need to upload the .zip archive to Gradescope. If you need to acknowledge any infl uences 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 fi nal grade of this course. Milestone 1: validate usage. (40 points) Milestone 2: print the fi le system information. (5 points) Milestone 3: list the root directory. (10 points) Milestone 4: recover a small fi le. (15 points) Milestone 5: recover a large contiguously-allocated file. (10 points) Milestone 6: detect ambiguous fi le recovery requests. (5 points) Milestone 7a: recover a small fi le 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 signifi cant 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 fi le or delete a fi le, 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 fi le. You can use it to hack a disk image. In this way, you can try recovering a fi le Lab 4: File Recovery 17/18 manually before writing a program to do it. You can also create a non?contiguously allocated file artifi cially 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 fi le. 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. Thislab has borrowed some ideasfrom Dr. T. Y. Wong. Lab 4: File Recovery 18/18