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

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Date:	Tue, 10 Aug 2010 08:12:15 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...shcourse.ca>
To:	Jason Wessel <jason.wessel@...driver.com>
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 Tue, 10 Aug 2010, Jason Wessel wrote:

> 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.

  i've used that technique before as a simple way to examine kernel
data on a running system, and it always worked fine before.

> 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.

  i can give that a shot later, certainly.

> 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.

  i haven't looked at this issue for a while, but i'd certainly like
to figure out why what used to work before has stopped working.  this
is something i like to show my beginner kernel programming students,
and it's moderately frustrating that i can't get it to work anymore.

  i'm sure it's something idiotic i'm doing.  and i'm around linuxcon
all week, so yes, let's bump into each other.

rday


-- 

========================================================================
Robert P. J. Day                               Waterloo, Ontario, CANADA

        Top-notch, inexpensive online Linux/OSS/kernel courses
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Twitter:                                       http://twitter.com/rpjday
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