How to SPLIT BOOT.IMG file into kernel and ramdisk.gz (Android)

This article shows how to split a boot.img file into kernel and ramdisk parts.

In Android systems kernel and initial ramdisk filesystem are usually joined into an only file, named boot.img, and flashed into the device.

Recover image files have same structure too.


mkbootimg is the tool which builds a boot.img file from its kernel and ramdisk parts.

Boot.img file could also contain a second filesystem within it.

Split boot.img file using a python script

Copy split_boot_img.py code at the end of this article in a file named split_boot_img.py

Then execute (you need to already have a boot.img file):
$ python split_boot_img.py -i boot.img -o parts # This splits boot.img, create parts directory and stores there zImage (kernel) and ramdisk.gz.

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Swapping on a File in Linux

This article shows how to create a standard file and configure your linux based system to activate swapping or paging on it.

CREATE THE SWAP FILE

First we create the swap file using dd command. cp is not useful here because swap file has not to contain holes.
# dd if=/dev/zero of=/mnt/swapfile bs=1M count=200 # We create a swap file of 200MByte in size.

For safety we disallow other users to read and modify the file:
# chown root.root /mnt/swapfile
# chmod 600 /mnt/swapfile

Swap size is unlimited for linux kernels after 2.3.3
Kernels after 2.4.10 support up to 32 swap areas.

SET UP THE SWAP FILE

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How to BUILD the LINUX KERNEL for the ANDROID EMULATOR (Eclair version)

DOWNLOAD THE KERNEL SOURCE CODE

First we download the kernel source code from https://android.googlesource.com

Within that page there are kernels for other platforms too. We choose to download kernel/goldfish project from there.

$ git clone https://android.googlesource.com/kernel/goldfish

A goldfish directory appears:
$ cd goldfish
This directory does not contain any source code in it.

We check which branch we have downloaded:
$ git branch
it shows * master , not the one we are searching for:

To list all remote available branches:
$ git branch -r
or
$ git remote show origin

origin/HEAD -> origin/master
  origin/android-goldfish-2.6.29
  origin/android-goldfish-3.4
  origin/linux-goldfish-3.0-wip
  origin/master

What does goldfish mean? (from android-kernel mail list)
Goldfish is the kernel hacked branch that supports the qemu based arm emulator for android, so it is the one we need.

Download GOLDFISH kernel version
Choose the version that suits you.
$ git checkout --track -b android-goldfish-2.6.29 origin/android-goldfish-2.6.29
$ git branch

* android-goldfish-2.6.29
  master

RUNNING THE EMULATOR

Within this link we will find how to get the android emulator, and launch it.
Building Android in Debian Sid

Showing the kernel version running in the emulator
$ adb shell
# cat /proc/version

Linux version 2.6.29-00261-g0097074 (digit@digit.mtv.corp.google.com) (gcc version 4.4.0 (GCC) ) #14 Tue Feb 2 15:49:02 PST 2010

OBTAINING KERNEL CONFIGURATION

We are going to obtain the kernel configuration .config file from within our running emulator.

$ cd /path/to/goldfish # we enter in the kernel source directory.
$ adb pull /proc/config.gz . # get compressed .config file from the emulator.

$ gunzip config.gz # uncompress it.
$ cp config .config # rename it into .config

Now you can edit .config file the way it suits you the most.

BUILDING AND COMPILING THE KERNEL

CROSS_COMPILE environment variable stores the path to the arm cross compiling toolchain. I use the one which comes with android source code.

$ ARCH=arm CROSS_COMPILE=/path/to/mydroid/prebuilt/linux-x86/toolchain/arm-eabi-4.4.0/bin/arm-eabi- make

Executing make will build the kernel.

Last lines will show:

Kernel: arch/arm/boot/Image is ready
Kernel: arch/arm/boot/zImage is ready

So we have obtained Image and zImage kernel binary files.

RUN THE EMULATOR USING THE NEW COMPILED KERNEL IMAGE

We need -kernel option:
$ emulator -kernel /path/to/goldfish/arch/arm/boot/zImage -show-kernel -verbose

We can check now the kernel version:
$ adb shell
# cat /proc/version

Linux version 2.6.29-00262-gb0d93fb (user@myPC) (gcc version 4.4.0 (GCC) ) #1 Sun May 2 14:27:31 CEST 2010

If we do not specify kernel option it usually uses the prebuilt one:
$ emulator -show-kernel -verbose

emulator: argv[01] = "-kernel"
emulator: argv[02] = "/path/to/mydroid/prebuilt/android-arm/kernel/kernel-qemu"

ACTIVATING MODULE LOADING SUPPORT IN THE KERNEL

Module loading support is previously disabled in the kernel, if we want to load modules in the kernel we have to enable it:

edit .config file and set:
CONFIG_MODULES=y

$ ARCH=arm CROSS_COMPILE=/path/to/mydroid/prebuilt/linux-x86/toolchain/arm-eabi-4.4.0/bin/arm-eabi- make

I am asked about some options when executing make. I ask yes for module related options.

After compiling I see several modules have been built.

MODPOST 6 modules
CC      drivers/video/fb_sys_fops.mod.o
LD [M]  drivers/video/fb_sys_fops.ko
CC      drivers/video/syscopyarea.mod.o
LD [M]  drivers/video/syscopyarea.ko
CC      drivers/video/sysfillrect.mod.o
LD [M]  drivers/video/sysfillrect.ko
CC      drivers/video/sysimgblt.mod.o
LD [M]  drivers/video/sysimgblt.ko
CC      drivers/video/vfb.mod.o
LD [M]  drivers/video/vfb.ko

We can upload the modules in the emulator and install them:
$ adb push drivers/video/fb_sys_fops.ko /data
$ adb shell
# insmod /data/fb_sys_fops.ko

REFERENCE

http://www.cianer.com/androidg1/28-building-android-kernel-images

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Building Android in Debian Sid

How to Compile the Linux Kernel in GNU/Debian and Ubuntu

NOTE: Tested in GNU/Debian Sid. Ubuntu versions should also work.


OBTAIN KERNEL SOURCE CODE:

Obviously the first step is obtaining the source code.

It is recommended to place and build linux kernel source code in /usr/src directory, although it is not required.

We create our "Linux_Kernel_Source" directory for this task.
$mkdir Linux_Kernel_Source
$cd Linux_Kernel_Source


Linux kernel source code is freely available in Internet:
http://www.kernel.org
specifically, version 2.6:
http://www.kernel.org/pub/linux/kernel/v2.6

We download and uncompress latest source code (2.6.31 when this article was written):
#aptitude install wget bzip2 # packages needed to get the kernel source code and uncompress it.
$wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.31.tar.bz2

Uncompress the source:
$tar xvjf linux-2.6.31.tar.bz2

/usr/src/linux is a symlink to linux sources or at least linux kernel headers. It is required.
#ln -s linux-2.6.31 /usr/src/linux

$cd linux-2.6.31


KERNEL CONFIGURATION:

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