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

Borislav Petkov
2022-10-06T11:14:08Z

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?

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.

Thx.

--
Regards/Gruss,
Boris.