Android/SharkBait/Porter Guide

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This article is intended for porters who want to add SharkBait support for a device that's not currently supported. In this article, you will be given the general idea on how to adapt parts that are crucial for SharkBait system to work on a device. The following components will be covered:

Preinit

Preinit performs early initialization of the device to load the Gentoo init (OpenRC as of current). The following directory structure is required for a supported device (angler as an example here) is as follows:

CODE preinit directory structure
angler
├── bootimg.cfg
├── initramfs
│   ├── init
│   └── (other contents in initramfs)
└── Makefile
  • bootimg.cfg defines offsets for kernel and initramfs, as well as kernel commandline options. Porters should adapt what they get when dissecting boot.img for their device and make the following modifications:
    • remove bootsize option so that abootimg does not complain about a boot.img bigger than the original one (though this is unlikely to be the case).
    • add androidboot.selinux=permissive to the boot commandline to set SELinux to permissive.
  • initramfs holds the minimal initramfs that correctly mounts filesystems and switch_root to load Gentoo init.
    • initramfs/init is required. The angler init is a shell script that does the necessary jobs, but any executable file should work.
    • Mind the permissions: kernel won't be able to execute an init that's not executable, which would result in a boot failure.
  • Makefile enables processing the initramfs programmatically before installkernel packs it up.
    • This will be useful for things like busybox installation, runtime-specific things, etc.
    • installkernel will call make in the device directory (angler in the directory hierarchy). Read the example Makefile so that you handle the paths correctly.

SharkBait-setup

SharkBait-setup handles the setup of the Android container. The following directory structure is required for a supported device (angler as an example here) is as follows:

CODE sharkbait-setup directory structure
angler
├── disable_encryption.sh
├── fstab.android
├── patches
│   ├── fstab.angler.patch
│   └── (other patches to apply to Android rootfs)
└── serial-consoles (if any)
  • disable_encryption.sh is ran on the helper workstation and disables encryption for the partition where the Gentoo root will reside in.
    • For devices that does not support encryption or have the Gentoo root in a partition that's not encrypted, this script should just print a notice and return 0.
    • Important
      Warn users about data wipe and wait for 10 seconds for a Ctrl-C!
  • fstab.android is appended to the Gentoo fstab and contain mountpoints that are necessary for Android.
    • Android's vold should not handle any internal mounts any more (external mounts such as OTG or SD card may still be managed by vold); make sure all required mounts are present.
    • It is strongly recommended to mount to /var/lib/android and then bind into the LXC rootfs for ease when accessing the Android partitions and extra security. Refer to the example fstab.android file for angler to get a better understanding of this.
  • patches holds patches that will be applied to the real Android rootfs via patch -p0, extracted from the current boot.img present on the device.
    • Disable all partition mounts in fstab.$DEVICE.patch or equivalent file.
    • Put more patches that are required here, such as patches on init.rc to properly handle cgroups issues introduced by containerization. Refer to init.rc.patch for angler for more information on this topic.
  • serial-consoles defines serial consoles that are available on the device, if there is any. This file will be appended to /etc/inittab.

Kernel sources

The kernel source package of a device packages the source tree from the vendor. A ebuild for sys-kernel/${BOARD_NAME}-sources is required; read the ebuild for angler-sources for reference. Only the package name, version, and the git repo URL would require adapting; the other parts of the ebuild do not need modification.

The following modifications on the source tree are required for a working kernel:

  • Apply this patch on the tree to enable installkernel function.
  • Supply a valid defconfig with LXC features enabled. See the example for angler and LXC#Kernel_options_required LXC for reference.
  • Make sure that the kernel compiles and boots correctly with (relatively) new compilers from Gentoo.
  • Regularly merge upstream changes.

Kernel headers

Kernel headers that match the device kernel source ease the process of compiling cross-compile toolchains for Android targets. The ebuild for angler should be a clear example, with only the need to modify the version number and the git repo URL.

Test the port

Make sure the port boots correctly. You may need a serial console to debug boot failures. Also, check that all the hardware functions work properly (camera, bluetooth, etc.); if that's not the case point it out when submitting a merge request so that we can look into the issue. Happy hacking!