[Galaxy 3 Android Development] mizch:
repack-zImage.sh is a bash script for Linux which allows you to unpack a kernel image (zImage) for modification and repack it afterwards into a new working kernel image.

You don't need a kernel tree for this program nor a compiler. It should work with any zImage that contains an initramfs, for whatever phone, operating system or CPU architecture you like.

My main purpose when I wrote it was to modify the initramfs of leaked Samsung i5800 firmware for which no kernel source is available.

Usage:

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Put the unzipped script into some directory along your $PATH (e.g., /usr/local/bin). Put the unpacked files from initramfs_utils.zip into /usr/local/bin.

Then simply run 'repack-zImage.sh -u' with your zImage in the current directory and it will create a directory named 'zImage_unpacked' which contains the unpacked blocks of your zImage. Refer to the comments near the start of the program to identify which file corresponds to which fragment of the original zImage. (The file name of the zImage should be "zImage". If it isn't, pass it as the only non-option argument. The subdirectory's name will change accordingly.)

Most notably, there will be a directory 'initramfs' in there, which contains all files from the original initramfs in their original tree. You can modify the contents as you like, but keep in mind that your initramfs cannot grow larger than the space reserved for it in the original zImage. So you're restricted to relatively small changes which should, however, satisfy many needs. You always can call a script or executable on some other partition (including the SD card if already mounted) if you need more room for your modifications.

After your modifications are done, cd back to the directory which contains zImage and zImage_unpacked and run 'repack-zImage.sh -p' to start the packing process.

This will create a directory called 'zImage_packing' which contains your new zImage (and a zImage.tar for loaders like ODIN). It will emit (between others) one or two messages about a padding being done and about how many bytes were padded. This number (or the lower number of the two) is an indication about how many compressed bytes are left for further additions to the initrd.

If your initramfs (or some other modified part) grows too large, the script will abort with an appropriate error message.

In initramfs-utils.zip, three programs are provided. They should be copied to /usr/local/bin:

* cpio_set0. This is a slightly modified cpio (compiled for 32 bit Linux). repack-zImage.sh will run without it, but there may be slightly more room in your initramfs if you use the modified one. It sets all file times in the archive to 0 (epoch), thus yielding better and consistent compression results. Else, the size of the compressed initramfs will differ from invocation to invocation due to differing atimes. Put it somewhere along your $PATH (e.g., /usr/local/bin).

* gen_init_cpio and

* gen_initramfs_list.sh. These are utilities copied from a kernel tree and used to support creation of an initramfs (in certain modes).

'repack-zImage.sh --help' will output usage information.

Happy hacking,

mizch

Current Version: 6

2011-05-03

('repack-zImage.sh --version' will output version information.)

- added support for lzma compressed ramdisks (both directions)

Version 4

2011-02-17

- Workaround for ambiguous gunzip result, see post #20

- Some code cleanup + CLI cleanup

- better error detection

Version 3

2011-01-06

- now also works with unzipped initramfs withing gzipped zImage part (i.e., all kinds of zImages)

Version 1

2011-01-05

- initial version. Works only for gzipped initramfs within gzipped zImage (e.g., G3 Eclair kernels)

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repack-zImage.zip contains version 4 of the script.

For the newest version, download repack-zImage.v6.zip and initramfs-utils.zip.