Re: [RFC PATCH 0/3] efi/x86: add support for generic EFI mixed mode boot

[Arvind Sankar <nivedita@alum.mit.edu>]

On Thu, Feb 13, 2020 at 10:36:14PM +0000, Ard Biesheuvel wrote:
> On Thu, 13 Feb 2020 at 19:47, Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > On Thu, Feb 13, 2020 at 05:55:44PM +0000, Ard Biesheuvel wrote:
> > > On Thu, 13 Feb 2020 at 18:53, Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > > > As an alternative to the new section, how about having a CONFIG option
> > > > to emit the 64-bit kernel with a 32-bit PE header instead, which would
> > > > point to efi32_pe_entry? In that case it could be directly loaded by
> > > > existing firmware already. You could even have a tool that can mangle an
> > > > existing bzImage's header from 64-bit to 32-bit, say using the newly
> > > > added kernel_info structure to record the existence and location of
> > > > efi32_pe_entry.
> > > >
> > >
> > > That wouldn't work with, say, signed distro kernels.
> >
> > No, the idea would be that the distro would distribute two signed
> > images, one 32-bit and one 64-bit, which are identical except for the
> > header. At install time, the installer chooses based on the system's
> > firmware bit-ness.
> >
> 
> I guess it would be possible, but then we'd need two different images
> while today, we can run the same image on both kinds of firmwares. The
> only thing I am trying to do is remove all the quirky bootparams stuff
> from the loader so that we can switch to LoadImage

Yeah, but doing that will allow you to boot directly from firmware on
existing machines, and only one image needs to be chosen at install
time, so it just adds a few MiB to the package. I guess most people will
still use a boot manager or loader that can be easily enhanced to use
LoadImage and the new section, but it would be nice to have the option
to avoid that.

> 
> > >
> > > > Also, the PE header can live anywhere inside the image, right? Is there
> > > > any reason to struggle to shoehorn it into the "boot sector"?
> > >
> > > It cannot. It must live outside a region described by the section headers.
> >
> > It could still be inserted after .setup, or at the very end of the file, no?
> 
> The PE/COFF spec mentions that the COFF header needs to follow the
> signature. Also, the SizeOfHeaders field would become somewhat
> ambiguous if the header is split up like that.


Ah, the definition of SizeOfHeaders doesn't make much sense if the
headers weren't contiguous with the MS-DOS stub. I guess they just
wanted the MS-DOS stub to potentially vary in size, but still want the
header to immediately follow it, drat.