linux-kernel - Re: [PATCH 1/2] x86_64: Reflect the relocatability of the kernel in the ELF header.

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Date:	Tue, 1 May 2007 10:36:57 +0530
From:	Vivek Goyal <vgoyal@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andi Kleen <ak@...e.de>,
	Kexec Mailing List <kexec@...ts.infradead.org>,
	linux-kernel@...r.kernel.org, Jurriaan <thunder7@...all.nl>,
	Helge Hafting <helgehaf@...el.hist.no>,
	Horms <horms@...ge.net.au>,
	Andrew Morton <akpm@...ux-foundation.org>, vgoyal@...ibm.com
Subject: Re: [PATCH 1/2] x86_64: Reflect the relocatability of the kernel in the ELF header.

On Mon, Apr 30, 2007 at 10:20:53PM -0600, Eric W. Biederman wrote:
> Vivek Goyal <vgoyal@...ibm.com> writes:
> 
> > On Mon, Apr 30, 2007 at 05:17:07PM +0200, Andi Kleen wrote:
> >> On Monday 30 April 2007 17:12:39 Eric W. Biederman wrote:
> >> > 
> >> > Currently because vmlinux does not reflect that the kernel is relocatable
> >> > we still have to support CONFIG_PHYSICAL_START.  So this patch adds a small
> >> > c program to do what we cannot do with a linker script set the elf header
> >> > type to ET_DYN.
> >> > 
> >> > Since last time I have fixed the type to be in my code ET_DYN (oops),
> >> > and verified this works with kexec.  I realized while testing that we
> >> > don't have anyway of identifying a kernel vmlinux as linux so we
> >> > probably want to add an ELF note but that will be another patch.
> >> 
> >> The patch is ok for me, but does it pass Vivek's usual testing?
> >
> > I am facing one issue with this patch. gdb can not analyze the
> > resulting kernel core file. Looks like gdb treats vmlinux differently if
> > ELF header type is "ET_DYN". It reads the symbol values incorrectly.
> 
> Weird.
> 
> > For example, symbol value of "panic_timeout" is 0xffffffff808a1fa8 but
> > gdb somehow things that it is 0xffffffff008aaebf. Looks like it is
> > performing some relocation.
> >
> > I am using GNU gdb Red Hat Linux (6.5-5.fc6rh).
> 
> Does it take a kernel core file to reproduce this problem?
> Or can you just open up gdb on a vmlinux and look at the symbol
> address?

It takes a core file to reproduce the problem. Without core file gdb can
get right symbol addresses.

> 
> At least without a core file it is working on with gdb 6.4.
>

This seems to be a problem with gdb 6.5. I transferred the dump to a
different machine having GNU gdb 6.4, and it works fine there.

Thanks
Vivek
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