linux-kernel - Re: [PATCH] x86: fix clang integrated assembler build

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Date:   Wed, 27 May 2020 11:04:22 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "# 3.4.x" <stable@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Jiri Slaby <jslaby@...e.cz>,
        Juergen Gross <jgross@...e.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Tony Luck <tony.luck@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Fangrui Song <maskray@...gle.com>,
        Bill Wendling <morbo@...gle.com>, Jian Cai <jiancai@...gle.com>
Subject: Re: [PATCH] x86: fix clang integrated assembler build

On Wed, May 27, 2020 at 7:16 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> clang and gas seem to interpret the symbols in memmove_64.S and
> memset_64.S differently, such that clang does not make them
> 'weak' as expected, which leads to a linker error, with both
> ld.bfd and ld.lld:
>
> ld.lld: error: duplicate symbol: memmove
> >>> defined at common.c
> >>>            kasan/common.o:(memmove) in archive mm/built-in.a
> >>> defined at memmove.o:(__memmove) in archive arch/arm64/lib/lib.a
>
> ld.lld: error: duplicate symbol: memset
> >>> defined at common.c
> >>>            kasan/common.o:(memset) in archive mm/built-in.a
> >>> defined at memset.o:(__memset) in archive arch/arm64/lib/lib.a
>
> Copy the exact way these are written in memcpy_64.S, which does
> not have the same problem.
>
> I don't know why this makes a difference, and it would be good
> to have someone with a better understanding of assembler internals
> review it.
>
> It might be either a bug in the kernel or a bug in the assembler,
> no idea which one. My patch makes it work with all versions of
> clang and gcc, which is probably helpful even if it's a workaround
> for a clang bug.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

+ Bill, Fangrui, Jian
I think we saw this bug or a very similar bug internally around the
ordering of .weak to .global.

> ---
>  arch/x86/lib/memmove_64.S | 4 ++--
>  arch/x86/lib/memset_64.S  | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/lib/memmove_64.S b/arch/x86/lib/memmove_64.S
> index 7ff00ea64e4f..dcca01434be8 100644
> --- a/arch/x86/lib/memmove_64.S
> +++ b/arch/x86/lib/memmove_64.S
> @@ -26,8 +26,8 @@
>   */
>  .weak memmove
>
> -SYM_FUNC_START_ALIAS(memmove)
> -SYM_FUNC_START(__memmove)
> +SYM_FUNC_START_ALIAS(__memmove)
> +SYM_FUNC_START_LOCAL(memmove)
>
>         mov %rdi, %rax
>
> diff --git a/arch/x86/lib/memset_64.S b/arch/x86/lib/memset_64.S
> index 9ff15ee404a4..a97f2ea4e0b2 100644
> --- a/arch/x86/lib/memset_64.S
> +++ b/arch/x86/lib/memset_64.S
> @@ -19,8 +19,8 @@
>   *
>   * rax   original destination
>   */
> -SYM_FUNC_START_ALIAS(memset)
> -SYM_FUNC_START(__memset)
> +SYM_FUNC_START_ALIAS(__memset)
> +SYM_FUNC_START_LOCAL(memset)
>         /*
>          * Some CPUs support enhanced REP MOVSB/STOSB feature. It is recommended
>          * to use it when possible. If not available, use fast string instructions.
> --
> 2.26.2
>
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-- 
Thanks,
~Nick Desaulniers