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Date:   Wed, 27 May 2020 16:24:06 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Arnd Bergmann <arnd@arndb.de>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Fangrui Song <maskray@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>,
        clang-built-linux@googlegroups.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64: disable -fsanitize=shadow-call-stack for
 big-endian
Message-ID: <20200527152406.GD59947@C02TD0UTHF1T.local>
References: <20200527134016.753354-1-arnd@arndb.de>
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On Wed, May 27, 2020 at 03:39:46PM +0200, Arnd Bergmann wrote:
> clang-11 and earlier do not support -fsanitize=shadow-call-stack
> in combination with -mbig-endian, but the Kconfig check does not
> pass the endianess flag, so building a big-endian kernel with
> this fails at build time:
> 
> clang: error: unsupported option '-fsanitize=shadow-call-stack' for target 'aarch64_be-unknown-linux'
> 
> Change the Kconfig check to let Kconfig figure this out earlier
> and prevent the broken configuration. I assume this is a bug
> in clang that needs to be fixed, but we also have to work
> around existing releases.
> 
> Fixes: 5287569a790d ("arm64: Implement Shadow Call Stack")
> Link: https://bugs.llvm.org/show_bug.cgi?id=46076
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

I suspect this is similar to the patchable-function-entry issue, and
this is an oversight that we'd rather fix toolchain side.

Nick, Fangrui, thoughts?

Mark.

> ---
>  arch/arm64/Kconfig | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index a82441d6dc36..692e1575a6c8 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1031,7 +1031,9 @@ config ARCH_ENABLE_SPLIT_PMD_PTLOCK
>  
>  # Supported by clang >= 7.0
>  config CC_HAVE_SHADOW_CALL_STACK
> -	def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
> +	bool
> +	default $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18 -mbig-endian) if CPU_BIG_ENDIAN
> +	default $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18 -mlittle-endian) if !CPU_BIG_ENDIAN
>  
>  config SECCOMP
>  	bool "Enable seccomp to safely compute untrusted bytecode"
> -- 
> 2.26.2
> 
> 
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