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

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Date:   Wed, 27 May 2020 11:28:43 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        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>
Subject: Re: [PATCH] arm64: fix clang integrated assembler build


On 2020-05-27, 'Nick Desaulniers' via Clang Built Linux wrote:
>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.

This may be another instance of
https://sourceware.org/pipermail/binutils/2020-March/000299.html
https://lore.kernel.org/linuxppc-dev/20200325164257.170229-1-maskray@google.com/

I haven't checked but there may be both a .globl directive and a .weak
directive

>> 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
>
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