Marcin's rootprompt blog – /dev/web0

Stumbling upon this page you probably came up with the same idea that I have – to run Gentoo on your Samsung Galaxy S2 device. It is definitely possible in a chroot environment. I have not tested dual booting to date, but if you’re into a chrooted Gentoo – read on. There are a few “gotchas” in the process I have come across – described below.

Base system installation

First of all – decide where will you put it. Your natural choice might be one of the SD cards, but see this:

# grep /mnt/sdcard /proc/mounts
/dev/block/vold/179:11 /mnt/sdcard vfat rw,dirsync,nosuid,nodev,noexec,noatime,nodiratime,[...]
/dev/block/vold/179:17 /mnt/sdcard/external_sd vfat rw,dirsync,nosuid,nodev,noexec,noatime,[...]
tmpfs /mnt/sdcard/external_sd/.android_secure tmpfs ro,relatime,size=0k,mode=000 0 0

You might try remounting these without the restrictions with your on-board busybox mount, but the internal SD is a vfat mount, so don’t expect things to work well in a chroot on that. My external SD is also contains a vfat filesystem, so I did not explore further for now. One good approach would be to create an ext2/3/4 filesystem on the external card – and do it SDHC-friendly (block alignment, wear leveling, etc) and hope the phone mounts it properly at boot (proper fs, and proper options – if not, a Tasker “on-boot” scenario could fix it). But again – YMMV, I did not explore this option myself.

Because of the above I put Gentoo  in /data/gentoo mounted as ext4 – not much of neither block space nor inodes, but will work for a small base system without portage tree (for now).

The location is defined, mkdir done, so now grab your stage3 archive for ARM and unpack it there. Back in the day I went with:

8d4c0d6b6f52140acc97b22ae370f770  stage3-armv7a_hardfp-20111108.tar.bz2

Check your favorite mirror at releases/arm/autobuilds/current-stage3-armv7a/ and refer to http://distfiles.gentoo.org/releases/arm/autobuilds/current-stage3-armv7a/ if your mirror does not rsync it.

Do the bind mounts and test it:

# mount -o bind /dev ./dev
# mount -o bind /dev/pts ./dev/pts
# mount -o bind /proc ./proc
# mount -o bind /sys ./sys
# chroot /data/gentoo /bin/bash

Then, set up your make.conf (SYNC, GENTOO_MIRRORS). You might want to add these too:

USE="-X -cups -ipv6"
PORTAGE_NICENESS=1

Set up /etc/resolv.conf (I suggest to use some universal DNS server entry) and your timezone. Optionally a hostname like sgs2 🙂

A note on user accounts: Android’s underlying Linux generates UIDs dynamically over 1024. Care must be taken with these UIDs and from now on I will use the root account. Other UIDs <1024 and >0 may also expose issues.

Set up root’s password and PermitRootLogin in /etc/ssh/sshd_config.

Try starting sshd:

# /etc/init.d/sshd --nodeps start

You might try to login via network now; the sshd will be running there until you reboot. If you encounter “no route to host” – let the device’s wifi wake up after a few taps if it was not active.

As usual for a chrooted install you need to take care of your bind mounts. Below is a simplified version of my script for doing that. It also includes other mounts in the /data/gentoo/android directory (create it beforehand!) for convenience. Root mount is not replicated there:

# mount -o bind / /data/gentoo/android/
mount: Invalid argument

The do-mounts script:

#!/bin/sh -x

# do we really need to do it?
grep -q /data/gentoo /proc/mounts && exit 0
# mount list. /data excluded to avoid mount loops; /* directories included
# but do not work:
MOUNTS="`(cat /proc/mounts |awk '{ print $2 }' ; \
  busybox find / -type d -maxdepth 1) |grep -v ^/data |sort |uniq`"

# basic bind mounts:
mount -o bind /dev ./dev
mount -o bind /dev/pts ./dev/pts
mount -o bind /proc ./proc
mount -o bind /sys ./sys

# loops come handy right in /dev:
( cd ./dev; for f in block/loop?; do busybox ln -s $f;done )

# the convenience mounts:
for f in $MOUNTS; do
  busybox mkdir -p ./android/$f
  mount -o bind $f ./android/$f
done

You might want to run this script from Tasker at boot for example. I chose to run it from a wrapper for running a command in Gentoo chroot. It is run every time, but I don’t mind speed and latency in favour of serialization (Tasker currently seems to have issues with that so you may encounter surprises if you create two tasks – one for mounts and one for sshd – a race condition may ruin your day).

You might also find handy a wrapper (mentioned above) for running commands in the Gentoo chroot. Here is a version of mine:

#!/bin/shsu -c /data/gentoo/do-mounts
chroot /data/gentoo /bin/bash -c ". /etc/profile ; \
  env-update --no-ldconfig >/dev/null; . /etc/profile ; \
  export TERM=vt100 ; export HOME=/root ; export USER=root ; \
  export SHELL=/bin/sh ; $*"

Just remember to take care of ldconfig manually when required.

Beyond Gentoo base system – portage

Probably your hands almost typed emerge –sync somewhere above or unpacked a portage snapshot. Maybe not even almost 🙂 And most probably you suffered from inode exhaustion. A loop mount to the rescue.

Get a free loop device. The output losetup -f may be inaccurate on Android (at least I have had issues with it lying). I chose minor device 7. And dd an 1G empty file:

dd if=/dev/zero of=/mnt/sdcard/file bs=1M count=1024
losetup /dev/loop7 /mnt/sdcard/file
mkfs.ext2 -O dir_index -i 4096 /dev/loop7   # many small files, right?
busybox mount /dev/loop7 /data/gentoo/usr/portage

Don’t forget to add the losetup and mount to your do-mounts script.

And now you can get a portage snapshot from your favourite mirror and then emerge –sync.

You will however encounter another issue in a few minutes – portage will not be able to download any distfile. It seems that uid 250 on Android is deprived of network connectivity. I am fully aware (and you should be too) what the following hack means. Works for me with all security implications.

sed your /etc/passwd:

s/^portage:x:250:250:/portage:x:0:0:/

And you are done. emerge screen or vim and watch it compile 🙂 An alternative would be to emerge -p –fetchonly and wget distfiles as root first. But this would not solve distcc issues…

Notes

Speaking of it – how about a distcc helper on a different box? Possible, I did it. I will share some details soon.

Good luck!

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Related articles

• Samsung Galaxy S2 – rootprompt’s first things first (rootprompt.apatsch.net)
• Gentoo on Android – cross compilation with distcc (rootprompt.apatsch.net)