Linux Deploy on modern Android

[ See full list of commands here ]


New versions of Android have been nothing but disastrous for those of us who still want to use our phone as a general purpose computer. The stuff Google introduces under the guise of security, while providing dubious security benefits at best, just so coincides with making rooting harder and harder. Magisk startup process is already super complex, involving mount trickery and hot patching of fstab file and init binary to sidestep all of the "security" obstacles. And it's not getting any better...

My advice? Get a phone that can run Ubuntu Touch: https://devices.ubuntu-touch.io/

Linux Deploy and Termux are partially broken since Android 10, and broken even more in Android 11. Linux Deploy is no longer maintained and missing newer distros, but we'll sidestep that and get a copy of Ubuntu 21.04 running.

! Installing into /data
---

Since Android 10, Linux Deploy seems to lose track of the loop device after a while, preventing you from shutting down the container cleanly. We'll be installing Linux directly into a folder on /data partition to avoid using an image file. On one hand this does make backups harder, on the other hand it removes the overhead of an ext4 filesystem in the image.

! What is Ubuntu Base?
---

Ubuntu Base is a small (27.1 MB) distribution of Ubuntu that is intended to run in chroot containers. It lacks its own init system, which is arguably its main advantage (Ubuntu uses systemd).

It comes with a fully functioning APT, so you can extend it to a full Ubuntu system, should you need to.

We'll use Linux Deploy to unpack it into /data/linux.

! Sneaking inside the container
---

Open up Termux and run this:

```
su
chroot /data/linux
/bin/su -
```

The first one will give you a root shell in Termux. You will see a "Grant root?" popup if this is the first time you're doing it.
The second will bring you inside the container, but with all the wrong environment.
The third one will execute the SetUser binary from within the container (Real path /data/linux/bin/su) and ask it to reset the environment (The - argument). It will launch Ubuntu's bash and set up all the environment variables.

! Android Permission System
---

Pretty much the only Android-specific thing Linux Deploy does is to populate the file /etc/group with Android-specific group IDs. Each ID grants a specific permission. Without them, the user will not be able to use the feature in question. We are interested in `aid_inet` in particular, because we have no network access without it.

! Allowing APT to use networking
---

```
usermod -g aid_inet _apt
```

This will switch _apt primary group to aid_inet. For some reason adding it as a secondary group using `useradd _apt aid_inet` /will not work/. It is possible that APT drops all secondary groups with a `setgroups(0, NULL)` call but I can't be sure without digging into the source code.

UPDATE: I went and checked the source code and that's almost exactly what it does! In file apt-pkg/acquire.cc at line 632 it calls `setgroups(1, [_apt gid from /etc/passwd])`. Offending code first appeared in 2015. Adds little security but almost breaks compatibility with Android!

Now, at last, we can run `apt update`

! If you messed up
---

If you messed up, run `rm -rf /data/linux`. It will nuke the folder and you can start over from scratch.

! Building and installing latest QEMU
---

Let's install the dependencies

```
apt install build-essential automake
apt install python3 git libglib2.0-dev libfdt-dev libpixman-1-dev zlib1g-dev
```

It's all mostly the same, but QEMU has adopted a new "ninja" build system since then, which is honestly just bad. Sure, you can just install it from apt now, but that wasn't always the case. If you're ever going to use a custom build system, bundle the sources and the shell script to build it.

`apt install ninja-build`

Download the sources, v6.0.0 branch:

`git clone git://git.qemu.org/qemu.git --branch=v6.0.0 --depth=1`

Oh, and QEMU also relies on git submodules now :(
You will see it start downloading something when we proceed with the build.

Let's also try to enable Link-Time Optimizations and add `-mcpu`; may win us just a hair more speed. Do beware that neither of those optimizations will affect the quality of the recompiled output, only maybe speedup the recompiler (TCG) itself.

```
mkdir build
cd build
../configure --target-list=i386-softmmu --enable-lto --extra-cflags="-mcpu=cortex-a76.cortex-a55+crc+crypto"
```