linux-kernel - Re: [PATCH] arm64: disable patchable function entry on big-endian clang builds

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Date:   Tue, 5 May 2020 15:25:56 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Torsten Duwe <duwe@....de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Amit Daniel Kachhap <amit.kachhap@....com>,
        Torsten Duwe <duwe@...e.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Julien Thierry <jthierry@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marc Zyngier <maz@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Alexandre Ghiti <alex@...ti.fr>,
        Kristina Martsenko <kristina.martsenko@....com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Steve Capper <steve.capper@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] arm64: disable patchable function entry on big-endian
 clang builds

On Tue, May 05, 2020 at 04:12:36PM +0200, Arnd Bergmann wrote:
> Clang only supports the patchable_function_entry attribute on
> little-endian arm64 builds, but not on big-endian:
> 
> include/linux/kasan-checks.h:16:8: error: unknown attribute 'patchable_function_entry' ignored [-Werror,-Wunknown-attributes]
> 
> Disable that configuration with another dependency. Unfortunately
> the existing check is not enough, as $(cc-option) at this point does
> not pass the -mbig-endian flag.

Well that's unfortunate. :(

Do we know if this is deliberate and/or likely to change in future? This
practically rules out a BE distro kernel with things like PAC, which
isn't ideal.

> Fixes: 3b23e4991fb6 ("arm64: implement ftrace with regs")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

This looks fine for now, and we can add a version check in future, so:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.

> ---
>  arch/arm64/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 4b256fa6db7a..a33d6402b934 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -151,7 +151,7 @@ config ARM64
>  	select HAVE_DMA_CONTIGUOUS
>  	select HAVE_DYNAMIC_FTRACE
>  	select HAVE_DYNAMIC_FTRACE_WITH_REGS \
> -		if $(cc-option,-fpatchable-function-entry=2)
> +		if $(cc-option,-fpatchable-function-entry=2) && !(CC_IS_CLANG && CPU_BIG_ENDIAN)
>  	select HAVE_EFFICIENT_UNALIGNED_ACCESS
>  	select HAVE_FAST_GUP
>  	select HAVE_FTRACE_MCOUNT_RECORD
> -- 
> 2.26.0
>