archlinux-steamlink

Running Arch Linux ARM on the Steam Link. Yes, running - no puny chroot.

The Steam Link has ~256MiB of RAM (unlike what Wikipedia says), and a Marvell ARMv7 CPU (1 Core, 1 Thread) (MV88DE3108).

Directly booting a custom image has been made /harder/ due to contractual requirements. This repository explores alternative methods.

Requirements

• Ethernet connectivity to the Steam Link
• A USB Stick

Build requirements

• bsdtar
• curl
• sudo
• e2fsprogs

Preparing the USB

• partition the USB to one big partition
• sudo mkfs.ext4 /dev/sdXY
• sudo tune2fs -O ^has_journal /dev/sdXY - disable journaling (no kernel support)

Working

• Full userspace replacement
• Handoff to systemd

Technical explanation

Three scripts - factory_test, init-trampoline and handoff - work together to replace the running userspace (including PID 1) with a different one.

• factory_test:
  • convince steam link that it is being QA/QC'd, and make it exec handoff
• handoff:
  • creates a copy of /sbin/init in a tmpfs
  • masks /sbin/init, overlaying init-trampoline
  • calls the previously-copied init binary as telinit u to reexec init
    • this causes PID 1 to exec init-trampoline
• init-trampoline:
  • pivot_roots to the new userspace rootfs
  • chroots to the new userspace rootfs to work around an issue causing the caller to still have their cwd in the old rootfs
  • creates necessary system mountpoints
  • execs lib/systemd/systemd to hand off PID 1 to systemd

Todo

• Maybe open a fd to /sbin/init in handoff before mounting it, saves us from copying it to tmpfs
• A patched kexec works with limitations
  • Restructure repo into two methods: trampoline/kexec
  • Potentially submit patched kconfig to archlinux arm upstream, or provide unofficial builds

Special thanks

• grawity: handoff idea, debugging