Re: [PATCH v2 1/2] x86: Add support for building a multiboot2 PE binary

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On Wed, Apr 17, 2024 at 3:15 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 10.04.2024 11:41, Ross Lagerwall wrote:
> > On Mon, Apr 8, 2024 at 11:25 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >> On 28.03.2024 16:11, Ross Lagerwall wrote:
> >>> * The image base address is set to 0 since it must necessarily be below
> >>>   4 GiB and the loader will relocate it anyway.
> >>
> >> While technically okay, what is the reason for this adjustment?
> >
> > The multiboot2 spec generally uses 32 bit addresses for everything and
> > says:
> >
> > "The bootloader must not load any part of the kernel, the modules, the
> > Multiboot2 information structure, etc. higher than 4 GiB - 1."
> >
> > An image base address above 4 GiB causes trouble because multiboot2
> > wasn't designed for this.
>
> Yet mb2 doesn't care about that PE header field at all, does it? In which
> case my question remains: What purpose does this particular modification
> of the image have?
>

With the currently published version of mb2, it doesn't look at the PE
header field since it has no knowledge about PE binaries.

With the proposal on the grub-devel list [1], mb2 would use the PE
header to load the new xen-mbi binary in which case, the image base
address is indeed relevant.

Ross

[1] https://lists.gnu.org/archive/html/grub-devel/2024-03/msg00081.html