To split iniramfs from the boot image, you need an utility such as
abootimg or unmkbootimg or split_boot. I recommend using
abootimg since it's available in the Debian and Ubuntu package
repository. In addition to those programs, you can use unpackbootimg
program generated from CyanogenMod source with make otatools.
To split boot image, use the following commands. I assume the boot image
filename is boot.img and it's located in the current working
directory.
mkdir boot
cd boot
abootimg -x ../boot.img
The first 512 bytes needs to be skipped, so that gzip recognizes the format and able to extract the initrd.img file.
dd if=initrd.img bs=1 skip=512 of=initrd.cpio.gz
The 512 bytes header can be extracted from initrd.img using the following command.
dd if=initrd.img bs=1 count=512 of=initrd.headers
Decompress the initrd.cpio.gz file using GNU gzip or Busybox gzip.
gunzip initrd.cpio.gz
This will delete initrd.cpio.gz and create an uncompressed cpio archive
initrd.cpio.
Use the cpio utility to extract the initrd.cpio archive.
With GNU cpio and bsdcpio
mkdir initrd
cd initrd
# if your shell allows redirection (unrestricted shell)
# cpio -ivd < ../initrd.cpio
dd if=../initrd.cpio | cpio -ivd
With bsdtar (from libarchive)
mkdir initrd
cd initrd
bsdtar -xvf ../initrd.cpio
Note that bsdtar understands archive compression, so that you can directly extract gzip'ed initramfs using bsdtar.
With Busybox cpio
# This is simpler to type if your shell is unrestricted shell.
# busybox cpio -ivd < ../initrd.cpio
busybox dd if=../initrd.cpio | busybox cpio -ivd
The cpio archive will be extracted in the current working directory.
Make any necessary changes to the extracted initrd file, such as
modifying the default.prop and other modifications. If you're done with
the modification, repack the initrd by following next instruction.
The factory default cpio format is newc, which is SVR4 portable format
with no crc. It's possible to use crc format, which is the same as
newc format plus checksum for data integrity. I recommend to just use
the default factory to ensure compatibility.
To create the newc cpio archive compressed with gzip, use the
following command in the initramfs working directory. GNU cpio, bsdcpio,
bsdtar, and busybox cpio can be used to achieve this goal. Please use
newc format to make it bootable.
Using GNU cpio
find . | cpio -ov -H newc | gzip | dd of=../initrd.new.cpio.gz
To use the 'crc' cpio format, use the following command (untested).
find . | cpio -ov -H crc | gzip | dd of=../initrd.new.cpio.gz
Using bsdcpio (from libarchive package)
find . | cpio -ov --format newc | gzip | dd of=../initrd.new.cpio.gz
Using bsdtar (from libarchive)
bsdtar -cvf - --format newc . | gzip | dd of=../initrd.new.cpio.gz
If you are using Busybox cpio instead of GNU cpio, you must specify flag
-H newc to create the cpio archive.
busybox find . | busybox cpio -ivd -H newc | \
busybox gzip | busybox dd of=../initrd.new.cpio.gz
If you don't specify -H flags, the default output for GNU cpio is binary
cpio format, which isn't supported by busybox. Use the newc or crc
format to ensure that the boot loader can use the image.
After finishing initrd.cpio.gz modification, the 512 bytes headers should be combined with the new initrd.cpio.gz file.
cat initrd.headers initrd.new.cpio.gz > initrd.img.new
If you don't make any modification to the kernel, the modified initramfs can used to create a new boot image file with the existing kernel image.
abootimg --create ../boot-new.img \
-f ./bootimg.cfg \
-k zImage \
-r ./initrd.img
Done. Your new boot image file is ready to use. The new boot image file is ../boot-new.img
You can create an update.zip package to install the new boot image file.
Since there is a simple tool to unpack and repack boot image, including those made for MTK devices, it's good to use it. Just go to the Carliv Image Kitchen XDA and you are good to go.
Happy hacking and have a good day!
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