linux-kernel - Re: [PATCH v3 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp

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Message-ID: <CAKwvOdnh+YoACaX4Oxk7ZiEQAQ2VgA6W=Dtbk7gzK5yJduFvGQ@mail.gmail.com>
Date:   Wed, 11 Sep 2019 14:01:59 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v3 3/3] powerpc/prom_init: Use -ffreestanding to avoid a
 reference to bcmp

On Wed, Sep 11, 2019 at 11:21 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> r370454 gives LLVM the ability to convert certain loops into a reference
> to bcmp as an optimization; this breaks prom_init_check.sh:
>
>   CALL    arch/powerpc/kernel/prom_init_check.sh
> Error: External symbol 'bcmp' referenced from prom_init.c
> make[2]: *** [arch/powerpc/kernel/Makefile:196: prom_init_check] Error 1
>
> bcmp is defined in lib/string.c as a wrapper for memcmp so this could be
> added to the whitelist. However, commit 450e7dd4001f ("powerpc/prom_init:
> don't use string functions from lib/") copied memcmp as prom_memcmp to
> avoid KASAN instrumentation so having bcmp be resolved to regular memcmp
> would break that assumption. Furthermore, because the compiler is the
> one that inserted bcmp, we cannot provide something like prom_bcmp.
>
> To prevent LLVM from being clever with optimizations like this, use
> -ffreestanding to tell LLVM we are not hosted so it is not free to make
> transformations like this.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/647
> Link: https://github.com/llvm/llvm-project/commit/5c9f3cfec78f9e9ae013de9a0d092a68e3e79e002

The above link doesn't work for me (HTTP 404).  PEBKAC?
https://github.com/llvm/llvm-project/commit/5c9f3cfec78f9e9ae013de9a0d092a68e3e79e002

> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>
> New patch in the series so no previous version.
>
>  arch/powerpc/kernel/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
> index 19f19c8c874b..aa78b3f6271e 100644
> --- a/arch/powerpc/kernel/Makefile
> +++ b/arch/powerpc/kernel/Makefile
> @@ -21,7 +21,7 @@ CFLAGS_prom_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
>  CFLAGS_btext.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
>  CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
>
> -CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector)
> +CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector) -ffreestanding
>
>  ifdef CONFIG_FUNCTION_TRACER
>  # Do not trace early boot code
> --
> 2.23.0
>


-- 
Thanks,
~Nick Desaulniers