linux-kernel - Re: [PATCH] x86/Kconfig: Drop check for '-mabi=ms' for CONFIG_EFI

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Date:   Mon, 17 Oct 2022 17:45:56 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, patches@...ts.linux.dev,
        stable@...r.kernel.org
Subject: Re: [PATCH] x86/Kconfig: Drop check for '-mabi=ms' for CONFIG_EFI_STUB

On Thu, 29 Sept 2022 at 17:20, Nathan Chancellor <nathan@...nel.org> wrote:
>
> A recent change in LLVM made CONFIG_EFI_STUB unselectable because it no
> longer pretends to support '-mabi=ms', breaking the dependency in
> Kconfig. Lack of CONFIG_EFI_STUB can prevent kernels from booting via
> EFI in certain circumstances.
>
> This check was added by commit 8f24f8c2fc82 ("efi/libstub: Annotate
> firmware routines as __efiapi") to ensure that '__attribute__((ms_abi))'
> was available, as '-mabi=ms' is not actually used in any cflags.
> According to the GCC documentation, this attribute has been supported
> since GCC 4.4.7. The kernel currently requires GCC 5.1 so this check is
> not necessary; even when that change landed in 5.6, the kernel required
> GCC 4.9 so it was unnecessary then as well.  Clang supports
> '__attribute__((ms_abi))' for all versions that are supported for
> building the kernel so no additional check is needed. Remove the
> 'depends on' line altogether to allow CONFIG_EFI_STUB to be selected
> when CONFIG_EFI is enabled, regardless of compiler.
>
> Cc: stable@...r.kernel.org
> Fixes: 8f24f8c2fc82 ("efi/libstub: Annotate firmware routines as __efiapi")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1725
> Link: https://gcc.gnu.org/onlinedocs/gcc-4.4.7/gcc/Function-Attributes.html
> Link: https://github.com/llvm/llvm-project/commit/d1ad006a8f64bdc17f618deffa9e7c91d82c444d
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

Acked-by: Ard Biesheuvel <ardb@...nel.org>

I can take this as a EFI fix as well.

> ---
>  arch/x86/Kconfig | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index f9920f1341c8..81012154d9ed 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1956,7 +1956,6 @@ config EFI
>  config EFI_STUB
>         bool "EFI stub support"
>         depends on EFI
> -       depends on $(cc-option,-mabi=ms) || X86_32
>         select RELOCATABLE
>         help
>           This kernel feature allows a bzImage to be loaded directly
>
> base-commit: f76349cf41451c5c42a99f18a9163377e4b364ff
> --
> 2.37.3
>