linux-kernel - Re: [PATCH] x86/Kconfig: Fix CONFIG_CC_HAS_SANE_STACKPROTECTOR when cross compiling with clang

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Date:   Fri, 1 Jul 2022 15:17:21 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     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
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Daniel Kolesa <daniel@...aforge.org>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        llvm@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH] x86/Kconfig: Fix CONFIG_CC_HAS_SANE_STACKPROTECTOR when
 cross compiling with clang

Gentle ping for review.

On Fri, Jun 17, 2022 at 11:08:46AM -0700, Nathan Chancellor wrote:
> Chimera Linux notes that CONFIG_CC_HAS_SANE_STACKPROTECTOR cannot be
> enabled when cross compiling an x86_64 kernel with clang, even though it
> does work when natively compiling.
> 
> When building on aarch64:
> 
>   $ make -sj"$(nproc)" ARCH=x86_64 LLVM=1 defconfig
> 
>   $ grep STACKPROTECTOR .config
> 
> When building on x86_64:
> 
>   $ make -sj"$(nproc)" ARCH=x86_64 LLVM=1 defconfig
> 
>   $ grep STACKPROTECTOR .config
>   CONFIG_CC_HAS_SANE_STACKPROTECTOR=y
>   CONFIG_HAVE_STACKPROTECTOR=y
>   CONFIG_STACKPROTECTOR=y
>   CONFIG_STACKPROTECTOR_STRONG=y
> 
> When clang is invoked without a '--target' flag, code is generated for
> the default target, which is usually the host (it is configurable via
> cmake). As a result, the has-stack-protector scripts will generate code
> for the default target but check for x86 specific segment registers,
> which cannot succeed if the default target is not x86.
> 
> $(CLANG_FLAGS) contains an explicit '--target' flag so pass that
> variable along to the has-stack-protector scripts so that the stack
> protector can be enabled when cross compiling with clang. The 32-bit
> stack protector cannot currently be enabled with clang, as it does not
> support '-mstack-protector-guard-symbol', so this results in no
> functional change for ARCH=i386 when cross compiling.
> 
> Link: https://github.com/chimera-linux/cports/commit/0fb7e506d5f83fdf2104feb22cdac34934561226
> Link: https://github.com/llvm/llvm-project/issues/48553
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> ---
> 
> Fixes: 2a61f4747eea ("stack-protector: test compiler capability in Kconfig and drop AUTO mode")
> 
> might be appropriate; I am conflicted on fixes tags for problems that
> that arise due to use cases that were not considered at the time of a
> change, as it feels wrong to blame the commit for not looking far enough
> into the future where it might be common for people to have workstations
> running another architecture other than x86_64.
> 
> Chimera appears to use a 5.15 kernel so a
> 
> Cc: stable@...r.kernel.org
> 
> might be nice but some maintainers are picky about that so I leave it up
> to you all.
> 
>  arch/x86/Kconfig | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index be0b95e51df6..076adde7ead9 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -391,8 +391,8 @@ config PGTABLE_LEVELS
>  
>  config CC_HAS_SANE_STACKPROTECTOR
>  	bool
> -	default $(success,$(srctree)/scripts/gcc-x86_64-has-stack-protector.sh $(CC)) if 64BIT
> -	default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
> +	default $(success,$(srctree)/scripts/gcc-x86_64-has-stack-protector.sh $(CC) $(CLANG_FLAGS)) if 64BIT
> +	default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC) $(CLANG_FLAGS))
>  	help
>  	  We have to make sure stack protector is unconditionally disabled if
>  	  the compiler produces broken code or if it does not let us control
> 
> base-commit: b13baccc3850ca8b8cccbf8ed9912dbaa0fdf7f3
> -- 
> 2.36.1
> 
>