Messages in this thread | | | From | Ard Biesheuvel <> | Date | Thu, 6 Oct 2022 13:19:30 +0200 | Subject | Re: [PATCH v2 02/16] x86/compressed: efi-mixed: move 32-bit entrypoint code into .text section |
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On Thu, 6 Oct 2022 at 13:13, Borislav Petkov <bp@alien8.de> 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.
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