linux-kernel - Re: [PATCH] arm64: fix clang integrated assembler build

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Date:   Wed, 27 May 2020 11:03:24 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Alexios Zavras <alexios.zavras@...el.com>,
        Enrico Weigelt <info@...ux.net>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Bill Wendling <morbo@...gle.com>,
        Jian Cai <jiancai@...gle.com>,
        Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH] arm64: fix clang integrated assembler build

On Wed, May 27, 2020 at 7:14 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> clang and gas seem to interpret the symbols in memmove.S and
> memset.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.

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

>
> Cc: stable@...r.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> ---
>  arch/arm64/lib/memcpy.S  | 3 +--
>  arch/arm64/lib/memmove.S | 3 +--
>  arch/arm64/lib/memset.S  | 3 +--
>  3 files changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/lib/memcpy.S b/arch/arm64/lib/memcpy.S
> index e0bf83d556f2..dc8d2a216a6e 100644
> --- a/arch/arm64/lib/memcpy.S
> +++ b/arch/arm64/lib/memcpy.S
> @@ -56,9 +56,8 @@
>         stp \reg1, \reg2, [\ptr], \val
>         .endm
>
> -       .weak memcpy
>  SYM_FUNC_START_ALIAS(__memcpy)
> -SYM_FUNC_START_PI(memcpy)
> +SYM_FUNC_START_WEAK_PI(memcpy)
>  #include "copy_template.S"
>         ret
>  SYM_FUNC_END_PI(memcpy)
> diff --git a/arch/arm64/lib/memmove.S b/arch/arm64/lib/memmove.S
> index 02cda2e33bde..1035dce4bdaf 100644
> --- a/arch/arm64/lib/memmove.S
> +++ b/arch/arm64/lib/memmove.S
> @@ -45,9 +45,8 @@ C_h   .req    x12
>  D_l    .req    x13
>  D_h    .req    x14
>
> -       .weak memmove
>  SYM_FUNC_START_ALIAS(__memmove)
> -SYM_FUNC_START_PI(memmove)
> +SYM_FUNC_START_WEAK_PI(memmove)
>         cmp     dstin, src
>         b.lo    __memcpy
>         add     tmp1, src, count
> diff --git a/arch/arm64/lib/memset.S b/arch/arm64/lib/memset.S
> index 77c3c7ba0084..a9c1c9a01ea9 100644
> --- a/arch/arm64/lib/memset.S
> +++ b/arch/arm64/lib/memset.S
> @@ -42,9 +42,8 @@ dst           .req    x8
>  tmp3w          .req    w9
>  tmp3           .req    x9
>
> -       .weak memset
>  SYM_FUNC_START_ALIAS(__memset)
> -SYM_FUNC_START_PI(memset)
> +SYM_FUNC_START_WEAK_PI(memset)
>         mov     dst, dstin      /* Preserve return value.  */
>         and     A_lw, val, #255
>         orr     A_lw, A_lw, A_lw, lsl #8
> --
> 2.26.2

-- 
Thanks,
~Nick Desaulniers