Messages in this thread | | | Date | Thu, 6 Oct 2022 13:13:44 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v2 02/16] x86/compressed: efi-mixed: move 32-bit entrypoint code into .text section |
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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.
https://people.kernel.org/tglx/notes-about-netiquette
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