On Thu, 6 Oct 2022 at 12:42, Borislav Petkov <bp@xxxxxxxxx> wrote: > > On Wed, Sep 21, 2022 at 04:54:08PM +0200, Ard Biesheuvel wrote: > > Move the code that stores the arguments passed to the EFI entrypoint > > into the .text section, so that it can be moved into a separate > > compilation unit in a subsequent patch. > > > > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > > --- > > arch/x86/boot/compressed/head_64.S | 34 ++++++++++++-------- > > 1 file changed, 20 insertions(+), 14 deletions(-) > > > > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S > > index d33f060900d2..1ba2fc2357e6 100644 > > --- a/arch/x86/boot/compressed/head_64.S > > +++ b/arch/x86/boot/compressed/head_64.S > > @@ -303,24 +303,28 @@ SYM_FUNC_START(efi32_stub_entry) > > popl %ecx > > popl %edx > > popl %esi > > + jmp efi32_entry > > +SYM_FUNC_END(efi32_stub_entry) > > > > + .text > > +SYM_FUNC_START_LOCAL(efi32_entry) > > call 1f > > -1: pop %ebp > > - subl $ rva(1b), %ebp > > - > > - movl %esi, rva(efi32_boot_args+8)(%ebp) > > -SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL) > > - movl %ecx, rva(efi32_boot_args)(%ebp) > > - movl %edx, rva(efi32_boot_args+4)(%ebp) > > - movb $0, rva(efi_is64)(%ebp) > > +1: pop %ebx > > I'm guessing according to the EFI mixed mode calling convention, %ebx is > not a live register which gets overwritten here...? > > Looking at efi32_pe_entry() from where this is called, %ebx looks live. > > What am I missing? > 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. 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.
