[yocto] RFC: poky-tiny: init procedure

[Darren Hart]

On 06/13/2012 06:20 PM, Darren Hart wrote:
> 
> 
> On 06/13/2012 06:09 PM, Tim Bird wrote:
>> On 6/13/2012 5:33 PM, Darren Hart wrote:
>>> 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.
>>>
>>
>> This sounds like a great approach.  What I've found at Sony is that it's 
>> nice to
>> have a template for turning on individual services, that users can 
>> decide to enable or not.
>> You don't want to include everything under the sun, but it's nice to 
>> have a few basic
>> things spelled out (but disabled) so that users can easily enable them.
>>
>> How about adding something like:
>> if [ -f /etc/rc.local ] ; then
>>   exec /etc/rc.local
>> fi
>>
>> You could have this be as is, or the lines could be commented out.
>>
>> Inside /etc/rc.local you have a bunch of commented-out lines for turning on
>> services like telnetd, dropbear, mounting debugfs, starting networking 
>> (ifup eth0),
>> etc.  If the user wants any of these individual items, they uncomment the
>> appropriate lines in /etc/rc.local.
>>
>> IMHO, you DON'T want to create some fancy directory-based system like 
>> sysVinit
>> that automatically handles anything that could possibly be included.  But
>> having a couple of common cases that the user can easily enable is 
>> pretty handy.
>>
>> Alternatively, you could have these commented-out lines in the init 
>> script itself.
>> This saves you an additional file in the rootfs, as well as the fork to exec
>> /etc/rc.local.  (I suppose you could also 'source' this, to avoid the fork.)
>>
>> Just some ideas.
> 
> I like the rc.local idea as that makes it easily extendable without even
> having to modify the original distro definition.
> 
> Excellent suggestions, thank you!
> 

Updated init with Tim's recommendations and a loop to avoid the kernel
panic on shell exit:

#!/bin/sh

# Mount the Linux kernel virtual filesystems
mount none -t proc /proc
mount none -t sysfs /sys
mkdir /dev/pts
mount none -t devpts /dev/pts

ifup lo

# Allow for distro or local customizations
if [ -f /etc/rc.local ] ; then
        source /etc/rc.local
fi

# Become session leader and try to find a real tty (e.g. ttyS0)
while true; do
        setsid cttyhack sh
        echo "Console sh exited with $?, respawning..."
        sleep 1
done


I believe this is a reasonable set of things to expect to be working on
ANY system built with the Yocto Project (currently poky-tiny is defined
in meta-yocto).

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