linux-kernel - Re: Documentation for init

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Message-ID: <53FC7EC1.9070401@gmail.com>
Date:	Tue, 26 Aug 2014 08:34:09 -0400
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Shea Levy <shea@...alevy.com>, linux-kernel@...r.kernel.org
Subject: Re: Documentation for init

On 2014-08-26 01:48, Shea Levy wrote:
> Hi all,
> 
> Is there any official documentation of the init process? I'm
> specifically interested in the process state at kernel handoff (argv,
> envp, open fds, etc.) as well as any special properties pid 1 has
> (parent of all orphans, anything else?).
> 
> Thanks,
> Shea Levy
> 
> P.S. I am not subscribed to LKML, please CC me in responses
This is following is just my understanding based on what I have seen and
read, and I may well be totally wrong on some points (if that is the
case, I would love to know about it, I'm always trying to learn more).

As far as I can tell, the argv that gets passed to the init process is
the concatenation of all arguments on the kernel command-line that the
kernel doesn't recognize or parse.  A lot of LiveCD's make use of this
to control hardware detection and module loading.  The only open file
descriptors (i believe, I may be wrong) are 0, 1, and 2, all pointing at
/dev/console.

As for special properties:
 * Parent of all orphans
 * Doesn't have a session ID until it calls setsid() (not certain about
this one)
 * Calling exit() will cause either a reboot or possibly a panic (I
think that this is dependent on the argument passed to exit())
 * Not catching a fatal signal will cause a panic (this means that
sending SIGKILL and SIGABRT to PID 1 will always cause a panic).
 * Has a PPID of 0, only other process like this is kthreadd
 * Becomes the parent of most X programs (almost all of them dissociate
very quickly from whatever started them.
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