Booting Gentoo on Nexus 6P

Fri 25 May 2018

Filed under Gentoo

Preface

As we now have preinit and the crucial UART console available, we can start bringing up the real GNU/Linux system--Gentoo Linux in this case. This article will focus on the following topics:

Filesystem structure and mounting procedure (in details)
Launch sequence to bring up OpenRC
Crafting a preinit initramfs
Tweaks in Gentoo to get ready to start Android in LXC later on

Filesystem Structure and Mounting Procedure

We won't be changing the partition table, as altering that will prevent fastboot flash commands from working properly, and we won't be able to recover unless we re-flash the EMMC chip if we messed the partition table up. The partition we'll use for GNU/Linux root is userdata, but not the entire filesystem--we can retain compatibility with stock Android boot.img by preserving userdata as the partition to be mounted for Android /data. The Gentoo root filesystem sits at userdata:/gnu, and gets binded to / in preinit.

After OpenRC boots, userdata, system, vendor, persist, and cache gets mounted to their mountpoints under /var/lib/android for launching Android later on. This is done by the localmount service in OpenRC's sysinit runlevel, by reading /etc/fstab in Gentoo root. These partitions later get bind-mounted into Android's LXC rootfs, which will be the topic for my next article.

Launch Sequence to Bring up OpenRC

Once kernel finishes its early initializations, preinit, a busybox sh script, gets called. The source is available here in the preinit repository. Preinit basically does the following:

Mount pseudo-filesystems for preinit's proper functioning
Mount userdata partition at /mnt/userdata
Bind-mount /mnt/userdata/gnu to itself, making it a mountpoint, which was mandatory for switch_root
exec switch_root into /mnt/userdata/gnu and call /sbin/init

This procedure does not differ much from standard initramfses, except that the desired root is not the filesystem root. We can add other things into preinit later on to support alternative root layouts, e.g. LUKS.

Crafting a Preinit initramfs

As we're not launching anything other than /init, and, unlike Android, we have a real root filesystem instead of the entire root on tmpfs, we can safely trim unnecessary components inside the initramfs. What we really need in the initramfs is as follows:

init: busybox sh script (duh)
busybox: static executable
symlinks to busybox applets under /bin
mountpoints

The complete initramfs layout can be found in the preinit repository. Repack boot.img with the Makefile rules at the root of preinit repository. Remember to turn SELinux to permissive in kernel commandline: though OpenRC won't respect that commandline option, Android init will load the policies and enforce it later on, and as we're not SELinux-ready now, that would certainly break things.

Boot the phone into fastboot mode, but don't hurry to flash that boot.img as the boot partition yet--use the following commandline to boot the new boot.img temporarily, so that we still have the regular Android boot.img for debug and further actions that require a working Internet connection:

fastboot boot <modded boot.img>

Attach the UART cable to see what's going on. If the kernel can't launch init, double-check that the init script is executable, and the shebang line is correct. Also check if the symlinks to busybox are placed correctly. If everything goes as expected, you should see our dear Preinit and OpenRC waving hello to you:

[   12.602544] new_era: Preinit started
mount: /etc/mtab: No such file or directory
mount: /etc/mtab:[   12.606728] new_era: Trying to mount /dev/mmcblk0p44 on /mnt/userdata...
 No such file or directory
[   12.735390] EXT4-fs (mmcblk0p44): mounted filesystem with ordered data mode. Opts: (null)
mount: /etc/mtab: No such file or directory
[   12.743008] new_era: Setting up /mnt/userdata/gnu as mountpoint...
mount: /etc/mtab: No such file or directory
[   12.767932] new_era: Cleaning up mounts, switching root to /mnt/userdata/gnu, and launching /sbin/init...
mount: /etc/mtab: No such file or directory
mount: /etc/mtab: No such file or directory
mount: /etc/mtab: No such file or directory
INIT: version 2.88 booting

   OpenRC 0.34.11 is starting up Gentoo Linux (aarch64)

 * /proc is already mounted
 * Mounting /run ...
 * /run/openrc: creating directory
 * /run/lock: creating directory
 * /run/lock: correcting owner
 * Caching service dependencies ...

Tweaks in Gentoo to get ready to start Android in LXC later on

Though the system is up and running, we need to tweak a few things to make it suit our later launching Android in LXC. In case a shell is not yet available on serial console (which is most likely the case by default), boot into Android and chroot into Gentoo root as described in this article to configure the following.

You'll notice that we don't have a getty on the serial console: sysvinit does not start getty on ttyHSL0 by default. Edit /etc/inittab in Gentoo root, comment out tty* lines (which Android devices definitely don't have) and add the following line:

s0:12345:respawn:/sbin/agetty -L 115200 ttyHSL0 vt100

Remove udev from sysinit runlevel. It inteferes with Android's ueventd, and sometimes it takes ages for udev to process the uevents from the kernel. keymaps and termencoding are useless as well, as we'll only use a serial console, on which these services don't make sense.

for a in {udev,keymaps,termencoding}; do rc-update del $a boot; done

Though we do not have networking by now, add sshd to the default runlevel. Add syslog-ng as well.

for a in {sshd,syslog-ng}; do rc-update add $a default; done

Reboot. The system should now be running properly. In the next article, we'll boot back into Android to install LXC in Gentoo root, and we'll then launch Android in LXC.