[PATCH] arm64: Change .weak to SYM_FUNC_START_WEAK_PI for arch/arm64/lib/mem*.S

[Nick Desaulniers]

On Thu, Oct 29, 2020 at 11:20 AM 'Fangrui Song' via Clang Built Linux
<clang-built-linux at googlegroups.com> wrote:
>
> Commit 39d114ddc682 ("arm64: add KASAN support") added .weak directives to
> arch/arm64/lib/mem*.S instead of changing the existing SYM_FUNC_START_PI
> macros. This can lead to the assembly snippet `.weak memcpy ... .globl
> memcpy` which will produce a STB_WEAK memcpy with GNU as but STB_GLOBAL
> memcpy with LLVM's integrated assembler before LLVM 12. LLVM 12 (since
> https://reviews.llvm.org/D90108) will error on such an overridden symbol
> binding.
>
> Use the appropriate SYM_FUNC_START_WEAK_PI instead.
>
> Fixes: 39d114ddc682 ("arm64: add KASAN support")
> Reported-by: Sami Tolvanen <samitolvanen at google.com>
> Signed-off-by: Fangrui Song <maskray at google.com>
> Cc: <stable at vger.kernel.org>

Before your patch, with GNU `as`:
$ llvm-readelf -s arch/arm64/lib/memcpy.o | grep memcpy
    19: 0000000000000000   340 FUNC    WEAK   DEFAULT     1 memcpy
    20: 0000000000000000   340 FUNC    GLOBAL DEFAULT     1 __memcpy
    21: 0000000000000000   340 FUNC    GLOBAL DEFAULT     1 __pi_memcpy
$ llvm-readelf -s arch/arm64/lib/memmove.o | grep memmove
    19: 0000000000000000   340 FUNC    WEAK   DEFAULT     1 memmove
    20: 0000000000000000   340 FUNC    GLOBAL DEFAULT     1 __memmove
    21: 0000000000000000   340 FUNC    GLOBAL DEFAULT     1 __pi_memmove
$  llvm-readelf -s arch/arm64/lib/memset.o | grep memset
    19: 0000000000000000   392 FUNC    WEAK   DEFAULT     1 memset
    20: 0000000000000000   392 FUNC    GLOBAL DEFAULT     1 __memset
    21: 0000000000000000   392 FUNC    GLOBAL DEFAULT     1 __pi_memset
After your patch, with GNU `as`:
$ llvm-readelf -s arch/arm64/lib/memcpy.o | grep memcpy
    19: 0000000000000000   340 FUNC    GLOBAL DEFAULT     1 __memcpy
    20: 0000000000000000   340 FUNC    GLOBAL DEFAULT     1 __pi_memcpy
    21: 0000000000000000   340 FUNC    WEAK   DEFAULT     1 memcpy
$ llvm-readelf -s arch/arm64/lib/memmove.o | grep memmove
    19: 0000000000000000   340 FUNC    GLOBAL DEFAULT     1 __memmove
    20: 0000000000000000   340 FUNC    GLOBAL DEFAULT     1 __pi_memmove
    21: 0000000000000000   340 FUNC    WEAK   DEFAULT     1 memmove
$ llvm-readelf -s arch/arm64/lib/memset.o | grep memset
    19: 0000000000000000   392 FUNC    GLOBAL DEFAULT     1 __memset
    20: 0000000000000000   392 FUNC    GLOBAL DEFAULT     1 __pi_memset
    21: 0000000000000000   392 FUNC    WEAK   DEFAULT     1 memset

So it didn't change any bindings (good), but it did reorder symbols.
I suspect if you flip the ordering of SYM_FUNC_START_WEAK_PI relative
to SYM_FUNC_START_ALIAS you can get the previous ordering back, but I
also suspect that the ordering of these symbols is not important.

Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>
Tested-by: Nick Desaulniers <ndesaulniers at google.com>

> ---
>  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
> --
> 2.29.1.341.ge80a0c044ae-goog
>
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-- 
Thanks,
~Nick Desaulniers