Re: [PATCH 0/4] ArmVirtPkg: use PE/COFF metadata for self relocation

[Julien Grall <julien@xen.org>]

Hi Ard,

On 08/06/2020 18:34, Ard Biesheuvel wrote:
> As suggested by Jiewen in response to Ilias RFC [0], it is better to use
> the PE/COFF metadata for self-relocating executables than to rely on ELF
> metadata, given how the latter is only available when using ELF based
> toolchains. Also, we have had some maintenance issues with this code in
> the past, as PIE linking of non-position independent objects is not a well
> tested code path in toolchains in general.
> 
> So implement this for the self-relocating PrePi in ArmVirtPkg first.
> 
> First, we need to ensure that the module in question is emitted with its
> PE/COFF relocation metadata preserved, by creating a special FDF rule.
> 
> We also need to provide a way for the code to refer to the start of the
> image directly, by adding it to the linker script.
> 
> Then, it is simply a matter of swapping out the two assembly routines,
> and adding the C code that serves the same purpose but based on PE/COFF
> base relocations.
> 
> Note that PE/COFF relocations are considerably more compact than ELF RELA
> relocations, so this does not impact the memory footprint of the resulting
> image adversely.

I have tested the change in a Xen guest. No issues reported.

Tested-by: Julien Grall <julien@xen.org>

Cheers,

-- 
Julien Grall