linux-kernel - Re: why can't i use gdb to dump module variables anymore?

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Message-ID: <4C613F95.8020505@windriver.com>
Date:	Tue, 10 Aug 2010 07:01:25 -0500
From:	Jason Wessel <jason.wessel@...driver.com>
To:	"Robert P. J. Day" <rpjday@...shcourse.ca>
CC:	Jon Masters <jonathan@...masters.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: why can't i use gdb to dump module variables anymore?

On 08/09/2010 11:06 AM, Robert P. J. Day wrote:
> On Sun, 8 Aug 2010, Jon Masters wrote:
>
>   
>> On Jul 8, 2010, at 7:51 AM, Robert P. J. Day wrote:
>>     
>>> (gdb) p rpjday_2
>>> Cannot access memory at address 0xffffffffa007c0b4
>>> (gdb) p rpjday_3
>>> Cannot access memory at address 0xffffffffa007c0b8
>>>       
>> Did you ever get a reply to this one? Were you trying to do this
>> locally, by loading a kernel core or using kgdb?
>>     
>
>   never figured out what was happening here, used to work just fine
> for simple gdb debugging of the kernel; nowadays (at least on ubuntu),
> not so much.  anyway, i'm in bahstahn for linuxcon, staying at the
> onyx hotel for first 3 days.  my girlfriend is doing a bofs session:
>
> http://events.linuxfoundation.org/linuxcon2010/mckinnon
>
>   anyway, regarding gdb, this was based on a column i wrote last
> year:
>
> http://www.linux.com/learn/linux-training/33991-the-kernel-newbie-corner-kernel-and-module-debugging-with-gdb
>
> where it worked just fine -- it's the technique described in LDD3.
> but now, on ubuntu 10.04, weirdness.  i haven't looked at it closely
> again for a while, but if you have any ideas, i'm open to suggestions.
> maybe some odd security/selinux/??? setting?
>   

I took a brief look at the instructions you had a the URL.  It seemed
like something that should probably work ok unless that memory address
is really not accessible via /proc/kcore.

Depending on the age of the ubuntu system and if you have a serial port
or not, kgdb is actually enabled in the kernel and you could certainly
try the same test there.

It should also be possible to try out a gdb with kernel module awareness
on the /proc/kcore.  If you want to try and debug it at some point
perhaps we'll cross paths at LinuxCON.

Jason
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