Building the Linux Kernel and Modules for Android

05 Feb 2015 | android

Distributions running through Linux Deploy work with the Android kernel (a modified Linux kernel), and therefore you can change the configuration of the kernel or connect new modules only by rebuilding this kernel, or assembling modules for this version of the kernel.

Playbook

• Download and prepare Android NDK:

  wget http://dl.google.com/android/ndk/android-ndk-r9d-linux-x86.tar.bz2
  tar -jxf android-ndk-r9d-linux-x86.tar.bz2
  export ARCH=arm
  export CROSS_COMPILE=$(pwd)/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86/bin/arm-linux-androideabi-
  

• Download and prepare the Android SDK, you will need the adb and fastboot utilities:

  wget http://dl.google.com/android/android-sdk_r13-linux_x86.tgz
  tar -zxf android-sdk_r13-linux_x86.tgz
  android-sdk-linux_x86/tools/android update sdk -u -t platform-tool
  export PATH=$PATH:$(pwd)/android-sdk-linux_x86/platform-tools
  

• Get the source code of this device (in our case it is tinykernel-flo for Nexus 7 (2013), or download a similar version from kernel.org):

  git clone -b tiny-jb-mr2 https://github.com/tiny4579/tinykernel-flo
  cd tinykernel-flo
  

• Get the kernel configuration file from the device (the kernel must be assembled with support for this feature):

  adb shell cat /proc/config.gz | gzip -d > .config
  

  or from boot.img (how to extract boot is described below):

  scripts/extract-ikconfig boot.img > .config
  

• If you can’t get the kernel configuration file, you can use the preconfiguration from the kernel delivery (arch/arm/configs list of configurations):

  make flo_defconfig
  

• Find out the exact version of the device kernel:

  adb shell cat /proc/version
  

  The result looks like this:

  Linux version 3.4.0-g03485a6 (android-build@vpbs1.mtv.corp.google.com) (gcc version 4.7 (GCC) )
   #1 SMP PREEMPT Tue Mar 18 15:02:27 PDT 2014
  

  In this case, the full version of the kernel will be the string “3.4.0-g03485a6”.

• Install the local version of the kernel (what is displayed after the main version 3.4.0):

  echo "-g03485a6" > .scmversion
  

• Change the kernel configuration in the .config file or by:

  make menuconfig
  

  In our case, the following lines were changed in the .config file (module support is enabled and the binfmt_misc module is enabled):

  CONFIG_MODULES=y
  CONFIG_BINFMT_MISC=m
  

• Start building kernel:

  make
  

  or modules only:

  make modules
  

• Download utilities for working with boot image boot.img:

  git clone https://github.com/meefik/binary-tools-android.git
  cd binary-tools-android
  

• Get a boot image from the device (the kernel is stored on a special boot section):

  adb shell su -с 'dd if=/dev/block/platform/msm_sdcc.1/by-name/boot' > boot.img
  

  The path may be different on other devices, you can define it with the command:

  adb shell su -c 'ls /dev/block/platform/*/by-name/boot'
  

• Get information about the image:

  ./boot_info boot.img 
  

  The result looks like this:

  Page size: 2048 (0x00000800)
  Kernel size: 6722240 (0x006692c0)
  Ramdisk size: 492556 (0x0007840c)
  Second size: 0 (0x00000000)
  Board name: 
  Command line: 'console=ttyHSL0,115200,n8 androidboot.hardware=flo user_debug=31 msm_rtb.filter=0x3F ehci-hcd.park=3'
  Base address: 2149580800 (0x80200000)
  

• Extract kernel and ramdisk from boot.img, replace kernel and pack back:

  ./unmkbootimg boot.img
  ./mkbootimg --kernel ../tinykernel-flo/arch/arm/boot/zImage \
   --ramdisk initramfs.cpio.gz \
   --base 0x80200000 \
   --cmdline 'console=ttyHSL0,115200,n8 androidboot.hardware=flo user_debug=31 msm_rtb.filter=0x3F ehci-hcd.park=3' \
   -o new_boot.img
  

• Flash the device with the new kernel:

  adb reboot bootloader
  fastboot flash boot new_boot.img
  

• Download the module on the device:

  adb push ../tinykernel-flo/fs/binfmt_misc.ko /storage/sdcard0/binfmt_misc.ko
  adb shell
  su
  mount -o rw,remount /system
  mkdir /system/lib/modules
  cp /storage/sdcard0/binfmt_misc.ko /system/lib/modules/binfmt_misc.ko
  chmod 644 /system/lib/modules/binfmt_misc.ko
  insmod /system/lib/modules/binfmt_misc.ko
  exit
  exit
  

• It should be noted that the vermagic of the module must fully comply with the kernel version (to the nearest character), otherwise it will not be possible to load the new module. You need to find out the vermagic of the module and compare it with the modules already present on the device:

  modinfo binfmt_misc.ko
  

  The result looks something like this:

  filename:       /path/to/kernel/fs/binfmt_misc.ko
  license:        GPL
  depends:        
  intree:         Y
  vermagic:       3.4.0-g03485a6 SMP preempt ARMv7