linux-kernel - Re: [RFC PATCH 11/28] x86/pvh: Avoid absolute symbol references in .head.text

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Message-ID: <81fb3f6b-4ded-41d1-be66-d86af4f22171@amd.com>
Date: Wed, 25 Sep 2024 17:10:56 -0400
From: Jason Andryuk <jason.andryuk@....com>
To: Ard Biesheuvel <ardb+git@...gle.com>, <linux-kernel@...r.kernel.org>
CC: Ard Biesheuvel <ardb@...nel.org>, <x86@...nel.org>, "H. Peter Anvin"
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Subject: Re: [RFC PATCH 11/28] x86/pvh: Avoid absolute symbol references in
 .head.text

Hi Ard,

On 2024-09-25 11:01, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> The .head.text section contains code that may execute from a different
> address than it was linked at. This is fragile, given that the x86 ABI
> can refer to global symbols via absolute or relative references, and the
> toolchain assumes that these are interchangeable, which they are not in
> this particular case.
> 
> In the case of the PVH code, there are some additional complications:
> - the absolute references are in 32-bit code, which get emitted with
>    R_X86_64_32 relocations, and these are not permitted in PIE code;
> - the code in question is not actually relocatable: it can only run
>    correctly from the physical load address specified in the ELF note.
> 
> So rewrite the code to only rely on relative symbol references: these
> are always 32-bits wide, even in 64-bit code, and are resolved by the
> linker at build time.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>

Juergen queued up my patches to make the PVH entry point position 
independent (5 commits):
https://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git/log/?h=linux-next

My commit that corresponds to this patch of yours is:
https://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git/commit/?h=linux-next&id=1db29f99edb056d8445876292f53a63459142309

(There are more changes to handle adjusting the page tables.)

Regards,
Jason