linux-kernel - Re: [RFC] ELF Relocatable x86 and x86

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Message-ID: <m1r6zzsbka.fsf@ebiederm.dsl.xmission.com>
Date:	Wed, 02 Aug 2006 03:59:17 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Magnus Damm" <magnus.damm@...il.com>
Cc:	fastboot@...l.org, linux-kernel@...r.kernel.org,
	Horms <horms@...ge.net.au>,
	"Jan Kratochvil" <lace@...kratochvil.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Vivek Goyal" <vgoyal@...ibm.com>, "Linda Wang" <lwang@...hat.com>
Subject: Re: [RFC] ELF Relocatable x86 and x86_64 bzImages

"Magnus Damm" <magnus.damm@...il.com> writes:

> On 8/2/06, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> "Magnus Damm" <magnus.damm@...il.com> writes:
>> > Eric, could you please list the advantages of your run-time relocation
>> > code over my incomplete relocate-in-userspace prototype posted to
>> > fastboot a few weeks ago?
>>
>> If you watch an architecture evolve one thing you will notice is that
>> the kinds of relocations keep growing.  An ever growing list of things
>> to for the bootloader to do is a pain.  Especially when bootloaders
>> generally need to be as simple and as fixed as possible because bootloaders
>> are not something you generally want to update.
>
> I agree that updating bootloaders is something you want to avoid. I'm
> not however sure that I would call kexec-tools a bootloader...

On the truly insane possibilities.  It is actually possible to load
a relocated bzImage.  run setup16.S below 1M and have it jump
to the kernel at any address below 4G.

>> Beyond that if you look at head.S the code to process the relocations
>> (after I have finished post processing them at build time) is 9 instructions.
>> Which is absolutely trivial, at least for now.
>
> Yeah, but the 33 patches are touching more than 9 instructions. =)

True.  I at of that is general clean ups to allow the kernel to be
relocated though.  Not to actually perform the relocation.

> One binary to rule them all... If that is true, is there any simple
> way then to extract vmlinux from the bzImage?

Unfortunately the process is a little lossy :(

So that still means you still need the vmlinux to get the debug
symbols.


Eric
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