Re: [PATCH v2 02/16] x86/compressed: efi-mixed: move 32-bit entrypoint code into .text section

[Ard Biesheuvel]

On Thu, 6 Oct 2022 at 13:13, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Thu, Oct 06, 2022 at 12:56:09PM +0200, Ard Biesheuvel wrote:
> > efi32_pe_entry() preserves and restores the caller's value of %ebx,
> > because from there, we might actually return control to the firmware.
> > The value it keeps in %ebx itself is not live when it jumps to
> > efi32_entry - it stores its value into image_offset, which is reloaded
> > from memory at a later point.
>
> Hmm, might be prudent to have a comment there because it is using %ebx a
> couple of insns before the JMP:
>
>         subl    %esi, %ebx
>                       ^^^^
>         movl    %ebx, rva(image_offset)(%ebp)   // save image_offset
>
> <--- I think you mean that after this, %ebx is not needed anymore?
>

Exactly.

>         xorl    %esi, %esi
>         jmp     efi32_entry
>
> 2:      popl    %edi                            // restore callee-save registers
>         popl    %ebx
>
> and this restores its original value ofc.
>
> > efi32_stub_entry() is the 'EFI handover protocol' entry point, which
> > cannot return to the firmware (and we discard the return address
> > already) so %ebx can be clobbered.
>
> That info would be good to have in a comment above it.
>

Fair enough.