I'm preparing an image file for a linux system. I need to be able to run my script that creates the image and have the output be bit-for-bit identical each time.
I do the normal procedure, by making a large binary file, partition it, create a loop device with the partition and then make the I filesystem. I then
mount the file system, copy the syslinux and initrd stuff over, unmount the partition, delete the loop devices and I have my image file. I can
dd it to a disk and the linux system boots correctly. So I'm making the filesystem correctly.
I run my script that performs the above steps but each time the output differs. Some of it is timestamps in the ext2 data structures. I wrote a program that reads in the ext2 structures and can clear out the timestamps, and
tune2fs can clear out a few more things but some of the bitmap data even differs and it seems the file data isn't even in the same place each time.
So how would I go about creating identical filesystems?
Here's the commands I use to create a filesystem, put a file on it and unmount it. Save the output and run it again, then compare the outputs, the file
a.txt gets put in different locations.
dd if=/dev/zero bs=1024 count=46112 of=cf.bin parted cf.bin <<EOF unit s mklabel msdos mkpart p ext2 63s 45119s set 1 boot on q EOF losetup -o $(expr 63 \* 512) /dev/loop0 cf.bin mke2fs -b 1024 -t ext2 /dev/loop0 22528 #clear some parameters tune2fs -i 0 /dev/loop0 # interval between check tune2fs -L LABEL /dev/loop0 tune2fs -U 00000000-0000-0000-0000-000000000000 /dev/loop0 #uuid tune2fs -c 0 /dev/loop0 #mount count mount /dev/loop0 mnt # make a dummy file echo HELLO > mnt/a.txt umount mnt losetup -d /dev/loop0
If I put the above commands in a script, copy and paste them to run a second time (but save the output between), and even change the date before running the commands a 2nd time (using the
date command), the a.txt gets put in the same disk location. But if you run the script, save the output, and run it again from the command line, compare the outputs and a.txt is in different locations. Very curious behavior. What data is being used to generate the file locations? Clearly it's not the time. The only thing I can think of is the difference between calling the commands twice via calling the script twice vs running the commands twice in the same script would be something like the process ID of the calling process. Ideas anyone?
I gave up on trying to use ext2. So I can't answer my original question about ext2, but I'll describe what I did to get a completely reproducible build of a basic linux system.
- Instead of ext2, use a FAT variant or ISO9660. If you need a partition less than 32MB, use FAT16 for the linux system partition, otherwise use FAT32. Either FAT16 or FAT32 will repeatedly put files in the same locations. But it does have some time stamps in its directory entries.
- Add linux system files needed to boot.
- Write a program to walk the FAT16/32 filesystem directory structures and set all time stamps to 0.
- Clear the disk signature in the mbr. Either do this in your program that clears timestamps, or use dd.
- Since it's a FAT filesystem, I'm using syslinux for a boot loader. cpio will produce identical initrd's from run to run, so there's no issues there. This is all that is needed for a basic bit-for-bit identical linux system.
Issues with FAT file systems
For just booting a linux system, FAT shouldn't cause any problems. But for larger data partitions, there are a couple issues with FAT32 that may crop up.
- It is possible to bump into the maximum number of files in a directory. This isn't likely to be a problem. (but of course, in my case it was)
- FAT32 will store an 8.3 filename for each file. Long file names are shortened to a stem with a tilde and a number appended. But if you have more than 9 files that map to the same short stem, FAT32 uses an undocumented procedure to generate a sort of hash to append to the file name instead. I dug into the linux kernel code for FAT32, and it uses the time as a hash seed (the function
vfat_create_shortname()in file namei_vfat.c). So this field is not reproducible. I don't know how Microsoft's implementation does it. You may get away with just clearing this field, as I don't think the 8.3 names are used for anything other than DOS. Or you could generate your own unique numbers that you can reproduce, it doesn't matter what the numbers are, just that they're unique.
Using ISO9660 for an additional partition
Use genisoimage to create the iso. It will generate identical output from run to run with the exception of time stamps. Using the -l option lets you have file names of up to 31 character. If you need filenames longer than that, use the rock ridge extension. The command is
genisoimage -o gfx.iso -R -l -f assets/files/
Write a program that walks the iso9660 filesystem, clears all time stamps, including the TF field of the rock ridge entries.
- Use fdisk or parted to make a partition in your disk image. 96h is the MBR id number for ISO9660.
- If necessary, patch up the partition table. Parted doesn't support making a partition of type iso9660. Unfortunately, I'm stuck with an older version of both parted and fdisk, and parted is easier to use. So I used parted to make my second partition as fat32. Then used fdisk to change the type to 96.
Use dd to embed the iso in the disk image, using the same numbers you used for making the partition. I used
dd bs=512 seek=$part2_start_lba conv=notrunc if=gfx.iso of=cf.bin
where cf.bin is my disk image file. 6. Mount the iso partition after linux has booted. If the iso is the second partition, it will be /dev/sda2. You may have to use mknod to make the proper device file in /dev first.