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

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Date:   Thu, 6 Oct 2022 12:42:13 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michael Roth <michael.roth@....com>
Subject: Re: [PATCH v2 02/16] x86/compressed: efi-mixed: move 32-bit
 entrypoint code into .text section

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@...nel.org>
> ---
>  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?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette