Gentoo On Android

Gentoo Prefix is a variant of Gentoo meta-distribution which installs under normal user privilege a full Gentoo in a directory offset of a Unix-like OS, such as HPUX, AIX, GNU/Linux, Interix, Mac OS X, Solaris, *BSD, and Cygwin.

Traditionally, Gentoo Prefix uses the kernel/libc and in some cases compiler/linker from the host OS. Problems arise in Linux/glibc based OS when the version of glibc is too old, like RHEL/CentOS 5 and many ancient GNU/Linux still in duty^{1, 2, 3}. Therefore it has been a dream of the Gentoo Prefix community to have a bleeding edge libc to further reduce dependence on host while still being a Prefix (as opposed to a virtual machine). It is technically accessible as Debian multi-arch⁴ and patchelf from nixOS⁵ demonstrated the use of the ELF interp field to locate the dynamic linker.

Around late 2012, Ruud Koolen (redlizard) made a first attempt to have libc in Gentoo Prefix⁶, and successfully made a functional system⁷. It is, however, still in need of much work of maintenance designs to co-exist with the present rpath mechanism.

The present rpath mechanism is implemented with sys-devel/binutils-config in Gentoo Prefix, which includes an ld wrapper injecting LDFLAGS like "-rpath=${EPREFIX}/usr/lib". Though we could patch gcc headers to redefine the default standard run time linker location, the same ld-wrapper mechanism can be used to inject things as "–dynamic-linker=${EPREFIX}/lib/ld-linux.so.2". The latter solution will make switching between rpath and prefix-libc variants of Gentoo Prefix easier.

There has been discussions in the Prefix team on how to treat Gentoo Prefix on ARM Linux.

The most straightforward way is to introduce a new keyword "arm-linux" like before. The problem is that we have too many keywords to look after in Prefix, and adding a new one will make things worse.

To save ourselves from too many keywords, we could interpret keyword "arm" as both Prefix and vanilla Gentoo. Being a new architecture, packages with "arm" keywords are recent and have a higher probability to be trivially Prefix ready. The problem is that there are corner cases, the keyword can become vague when a package can be used in vanilla Gentoo and not Prefix (though there is no such example yet).

Provided we seamlessly support using glibc from Prefix on Linux boxes, it is more favored to have "arm" representing both Prefix and vanilla: under the same glibc, a package is unlikely to break on Prefix but not vanilla or vice versa.

Similarly, a package with has keyword "amd64 x86 amd64-linux", will almost certainly be ready for "x86-linux". So we do have the possibility to reduce the keywords maintained by Prefix by digging out such extrapolation after Prefix begins to use its own libc.

One of the amazing features of Gentoo Prefix is not needing root privilege to use portage, the Gentoo package manager. However in some cases, the user does have root privilege and still want to install a Gentoo Prefix as normal user and give it root privilege when necessary, such as setting suid root to some tools⁸. At present it will result in failure as portage run as normal user will refuse to do anything requiring root.

We want to add such a feature to portage: call sudo/su before performing things requiring root, in such a way that this feature can be disabled by configuration or USE flags. Rooted Android will benefit from this feature via, for example, Superuser App⁹.

Android is a Linux-based mobile operating system developed by Google. It features a Java engine Dalvik as the central. The purpose of Bionic, libc of Android, is to get the Java engine compiled out. Therefore Bionic is very thin and lacks many features expected in a GNU/Linux system. As a consequence, though Android has a Linux kernel, it is non-trivial to have a GNU environment along with it.

Mobile devices are getting more and more powerful. At the time of writing (March 2013), dual/quadro-core GHz-clocked ARM driven mobile devices are widely available in the market. To unleash the full potential, a complete GNU userland would be very useful and crucial to provide desktop level user experience, with laptop docks (Motorola Lapdock¹⁰, ASUS Transformer¹¹) and/or desktop monitor via HDMI¹².

Several attempts exist and are surveyed as followed.

1. busybox. It uses statically linked busybox to provide main tools expected in a GNU environment¹³. It is straightforward to deploy and accessible as an App in Google Play¹⁴. But the features provided are limited.
2. chrooted full distribution. We could install full a GNU/Linux distribution in a chroot environment, and even have a desktop environment within a VNC server. We could access the local VNC server with a VNC client from Android. It is well-received and also available as an App in Google Play^{15, 16}. The drawback of this solution, however, is the overhead imposed by the chroot and VNC server. Furthermore, this approach needs root permission to the Android system.
3. native full distribution. Because the Android system lies in the /system directory, top directories like /usr, /lib, /var, /etc required by a typical GNU/Linux distribution are available. We can install a full distribution to / in parallel to Android. This approach, for example, Motorola Webtop and Debian^{17, 18}, is not yet wide spread yet, but it is at present the best solution we have. It needs root permission to operate.

Gentoo Prefix, if powered with its own glibc, will give another native solution here, and (surprise!) without the need of root permission. It is also clean in the sense that all the GNU programs are installed in a subdirectory.

It could be hard to bootstrap Gentoo Prefix on Android. One possible way is to install a native full distribution or a chrooted one and then bootstrap a Gentoo Prefix with libc from within. We could distribute precompiled Gentoo Prefix on Google Play for the general public.

While users would access Gentoo via Google Play easily, to attract more developer an easy way to get Gentoo Prefix for Android out is very helpful.

At present, Gentoo Prefix can only be bootstrapped on systems on which it will run. It would be hard for users to bootstrap Gentoo Prefix in environments lacking of compiler, linker, and featured libc, like Android.

It is possible, however, to install a native full distribution and then bootstrap a Gentoo Prefix with libc. It would be better if we could cross-compile out a Gentoo Prefix from faster servers.

At present, we can bootstrap a new Gentoo Prefix from an existing one by doing,

env PORTAGE_OVERRIDE_EPREFIX=/new/location/ emerge -e @system

But cross compiling Gentoo Prefix or bootstrap by cross-compilation is still a dream.

redlizard's gentoo-prefix-libc has cross bootstraping in mind, with the hope of running uclibc-based Gentoo Prefix on Android. The details, however, still call for much effort.

In order to cross bootstrap Gentoo Prefix, we should have a set of cross toolchains at the first place.

In vanilla Gentoo, a nice tool, crossdev¹⁹, is readily available. Cross development naturally includes the concept of "prefix" almost by definition, which is usually named "sysroot" and handled by the variable ${ROOT} in Gentoo. This "prefix", however, is easily confused with ${EPREFIX} used in Gentoo Prefix. (See? I am saying prefix too many times already.) Therefore it is not trivial to port crossdev and glibc to Gentoo Prefix in a consistent manner.

The author has tried to make cross toolchains manually²⁰. It would be very nice if crossdev could produce cross toolchains in Gentoo Prefix by a single command as

$ crossdev -t armv7a-hardfloat-linux-gnueabi

In the mean time, there is still confusion around cross compiling and Gentoo Prefix^{21, 22}. A firm policy is needed to carry cooperative effort on.

The tasks in this project can be categorized into two types.

1. Work involving intensive coding

   They are tagged as coding. They require concentration and relatively easy to predict timetime.

2. Discussion of policy, clearification of concepts with a wide audience in Gentoo community

   They are tagged as policy. They call for consensus with cooreponding teams. Discussion time is hard to control, and should be done in parallel to coding as soon as possible.

Dates in the following items represent deadlines.

I am born in China and now studying particle physics in Japan as a graduate student. I like tweaking electronic devices and computer programs. Right now I am really fond of travelling around the world and learning different languages.

As a GNU/Linux user for 9 years since 2003, I use Debian for desktop and Gentoo Prefix for servers. I live in Emacs and stumpwm. I started to love Gentoo Prefix in 2010 and joined gentoo in Dec. 2011²³.

1. GenTop2 http://forum.xda-developers.com/showthread.php?t=1617684
2. linux-on-android: Port linux distributions to android-based phones http://code.google.com/p/linux-on-android/