Re: [PATCH 4/8] x86/EFI: redo .reloc section bounds determination

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.04.2021 11:46, Roger Pau Monné wrote:
> On Thu, Apr 01, 2021 at 11:45:38AM +0200, Jan Beulich wrote:
>> There's no need to link relocs-dummy.o into the ELF binary. The two
>> symbols needed can as well be provided by the linker script. Then our
>> mkreloc tool also doesn't need to put them in the generated assembler
>> source.
> 
> Maybe I'm just dense today, but I think the message needs to be
> expanded a bit to mention that while the __base_relocs_{start,end} are
> not used when loaded as an EFI application, they are used by the EFI
> code in Xen when booted using the multiboot2 protocol for example, as
> they are used by efi_arch_relocate_image.
> 
> I think relocation is not needed when natively loaded as an EFI
> application, as then the load address matches the one expected by
> Xen?

It's quite the other way around: The EFI loader applies relocations
to put the binary at its loaded _physical_ address (the image gets
linked for the final virtual address). Hence we need to apply the
same relocations a 2nd time (undoing what the EFI loader did)
before we can branch from the physical (identity mapped) address
range where xen.efi was loaded to the intended virtual address
range where we mean to run Xen from.

For the ELF binary the symbols are needed solely to make ld happy.

> I also wonder, at some point there where plans for providing a single
> binary that would work as multiboot{1,2} and also be capable of being
> loaded as an EFI application (ie: have a PE/COFF header also I assume
> together with the ELF one), won't the changes here make it more
> difficult to reach that goal or require reverting later on, as I feel
> they are adding more differences between the PE binary and the ELF
> one.

There were such plans, yes, but from the last round of that series
I seem to recall that there was at least one issue breaking this
idea. So no, at this point I'm not intending to take precautions to
make that work easier (or not further complicate it). This said, I
don't think the change here complicates anything there.

> The code LGTM, but I think at least I would like the commit message to
> be expanded.

Well, once I know what exactly you're missing there, I can certainly
try to expand it.

Jan