Date: Wed, 11 Mar 2009 13:13:34 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Nate Eldredge <neldredge@math.ucsd.edu> Cc: freebsd-hackers@freebsd.org, vasanth raonaik <vasanth.raonaik@gmail.com> Subject: Re: Debugging init process. Message-ID: <alpine.BSF.2.00.0903111310280.5537@fledge.watson.org> In-Reply-To: <Pine.GSO.4.64.0903101900160.3173@zeno.ucsd.edu> References: <d9f479c10903100953w4c600775oc06b997b67c472d6@mail.gmail.com> <Pine.GSO.4.64.0903101900160.3173@zeno.ucsd.edu>
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On Tue, 10 Mar 2009, Nate Eldredge wrote:
> On Tue, 10 Mar 2009, vasanth raonaik wrote:
>
>> I need to debug init process. I am not able to attach init to gdb and it
>> throws
>
> As others mentioned, this is explicitly disabled. You could re-enable it by
> hacking the kernel, but it could cause other unexpected problems.
>
> Alternatively, there's always "printf debugging".
>
> What is wrong with init, that you need to debug it? It's a fairly simple
> program that's been around for a long time and should be pretty stable.
One specific concern with debugging init is that the ptracing application will
intercept all signals destined for the target application, and since signals
are used to manage the system run cycle, and init becomes the parent of
orphaned processes, this could lead to unexpected side effects. Also, if init
exits, your system will panic :-). Last time I mucked with init, I found the
best ways to debug it were:
(1) Provide a LD_PRELOAD for libc that replaces getpid(), getppid(), and other
things to return pid 1.
(2) Run init in a jail so that if it exits, the box won't panic.
(3) Use tools like ktrace on the actual init, combined with utrace()
instrumentation of init so you can track its behavior "in the wild"
FYI, if you do want to clear P_SYSTM, one easy way to do that is to attach
kgdb to /dev/mem, and directly manipulate the flags on initproc. This comes
with some risks, of course. :-)
Robert N M Watson
Computer Laboratory
University of Cambridge
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