[yocto] RFC: poky-tiny: init procedure

[Darren Hart]

For those of you using poky-tiny or are interested in building very very
small systems, I would appreciate your thoughts here.

Currently poky-tiny images will boot and run /bin/sh, which results in
error messages to the console about being unable to open the tty and job
control being disabled (this is a common problem, if not commonly
understood).

The shell must be session leader to open the tty, and the tty must not
be /dev/console (it should be a vt or a physical tty like ttyS0), the
tty is required for job control (handling signals, etc.).

The Problem:
poky-tiny should boot to a shell without error messages and have
job-control.

The Vision:
The goals of poky-tiny are to be an initial starting point from which to
build a distribution that does what you want, and NOTHING more.

The Proposal:
o Do not include the standard Busybox init
o Do provide a basic /init script that can be easily modified
o Do provide a basic shell
  o Without init, this requires setsid and cttyhack to easily find the
    right device
o Do not provide inittab functionality

I can accomplish this by updating busybox to include:
CONFIG_SETSID=y
CONFIG_CTTYHACK=y

And providing a basic init script as follows:

#!/bin/sh
mount none -t proc /proc
mount none -t sysfs /sys
mkdir /dev/pts
mount none -t devpts /dev/pts
ifup lo

# Become session leader and try to find a real tty (e.g. ttyS0)
setsid cttyhack sh

This results in a system that boots with the virtual filesystems
mounted, the local network interface up, and a shell with job control
running. Nothing else.

Potential Problems:
Should the script be /init (for initramfs images) or /sbin/init for
images on block devices. Note that the default method of running
poky-tiny is with an initramfs as it doesn't support any block devices
by default.

Adding services like dropbear to your image will not get started on boot
as their init scripts will not be run. I'm OK with documenting this
behavior as part of creating your own distribution based on tiny.

If the shell crashes, or you exit, the kernel panics (because init returns).

Thoughts/Comments/Criticism welcome.

Thanks!

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel