How a Corrupted USB Drive Was Saved by GNU/Linux
I started looking at the source code for mkfs.msdos, also known as mkdosfs(8), but then came up with a better idea. What if I could create a filesystem with the FAT parameters arranged so that the FATs and the directory in this new filesystem were in the same place where the FATs and directory were in the disk image I already had? The bytes that read LEXAR MEDIA probably were the volume name. Maybe, by giving the right parameters to mkfs.msdos(8), I could create a filesystem image wherein 0x08000 would point to the first FAT, 0x26a00 would point to the second FAT and 0x45400 would point to the volume label.
On the mkdosfs(8) manpage, I found:
SYNOPSIS mkdosfs [ -A ] [ -b sector-of-backup ] [ -c ] [ -l file name ] [ -C ] [ -f number-of-FATs ] [ -F FAT-size ] [ -i volume-id ] [ -I ] [ -m message-file ] [ -n volume-name ] [ -r root-dir-entries ] [ -R number-of-reserved-sectors ] [ -s sectors-per-cluster ] [ -S logical-sector-size ] [ -v ] device [ block-count ]
Therefore, I specified -f 2 for two FATs and -n mkfs__msdos--that is, a string I could find easily--for the volume name. This way I could tell where the vol-name landed.
How about the other parameters? I saw above that the FATs were 0x1ea00 bytes apart; if they landed the wrong distance from each other, I could tweak -F and maybe -s. I found on-line that for a filesystem of this size, the clusters would be 8192 bytes; in other words, there would be 16 512-byte sectors per cluster. The cluster is the file allocation unit described by the FAT. Hence, it would be -s 16.
As for where to create the filesystem, it wouldn't do to put it on the USB drive. Instead, I created a file the same size as the drive image but filled with zeroes:
# dd if=/dev/zero of=/tmp/r2x bs=512 count=1001952
After creating the filesystem, I figured I'd mount it and create a file. The file would have enough data in it that we could see a reasonable allocation chain. To accomplish this, I wrote a script and prepared to call it with parameters until I happened to find everything where I wanted it. I called it b.sh:
#!/bin/bash # parameters added to mkfs.msdos.... ARGS="$*" if mount | grep /tmp/r2d; then umount /tmp/r2d; fi losetup -d /dev/loop2 losetup /dev/loop2 /tmp/r2x mkfs.msdos -n mkfs__msdos -s 16 $ARGS /dev/loop2 mount -t vfat /dev/loop2 /tmp/r2d yes hello | dd bs=8192 count=3 of=/tmp/r2d/foo.txt umount /tmp/r2d
My plan was to try running this script with different parameters until I got it right. 0x8000 is 32KB. In 512-byte sectors, that's 64. Because the first FAT started at 0x8000, I decided to try -R 64, like this:
# sh b.sh -R 64 mkfs.msdos 2.8 (28 Feb 2001) Loop device does not match a floppy size, using default hd params 2+1 records in 2+1 records out #
The surprising thing was my first guess turned out to be right, at least as far as the FAT placement:
# hexdump -C /tmp/r2x | less ... 00008000 f8 ff ff ff 03 00 04 00 f8 ff 00 00 00 00 00 00 |..........| 00008010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00026a00 f8 ff ff ff 03 00 04 00 f8 ff 00 00 00 00 00 00 |..........| 00026a10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00045400 6d 6b 66 73 5f 5f 6d 73 64 6f 73 08 00 00 71 89 |mkfs__msdos...q.| 00045410 0f 31 0f 31 00 00 71 89 0f 31 00 00 00 00 00 00 |.1.1..q..1......| 00045420 41 66 00 6f 00 6f 00 2e 00 74 00 0f 00 65 78 00 |Af.o.o...t...ex.| 00045430 74 00 00 00 ff ff ff ff ff ff 00 00 ff ff ff ff |t.....| 00045440 46 4f 4f 20 20 20 20 20 54 58 54 20 00 00 71 89 |FOO TXT ..q.| 00045450 0f 31 0f 31 00 00 71 89 0f 31 02 00 00 50 00 00 |.1.1..q..1...P..| 00045460 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00049400 68 65 6c 6c 6f 0a 68 65 6c 6c 6f 0a 68 65 6c 6c |hello.hello.hell| 00049410 6f 0a 68 65 6c 6c 6f 0a 68 65 6c 6c 6f 0a 68 65 |o.hello.hello.he| ...
I didn't check the directory size, but it apparently it was okay as well--more on that below.
I now had a boot sector that would tell fsck.msdos to expect the FATs and the root directory at all the right places. So what if I created a filesystem image where the first sector was that one, but all the rest of the sectors contained data from the USB drive? Then, fsck.msdos would read the boot sector; I'd tell it to use FAT#2 to repair everything; and we'd see how it turned out.
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