On Thu, Oct 29, 2020 at 11:05 AM 'Fangrui Song' via Clang Built Linux clang-built-linux@googlegroups.com wrote:

Commit 393f203f5fd5 ("x86_64: kasan: add interceptors for memset/memmove/memcpy functions") added .weak directives to arch/x86/lib/mem*_64.S instead of changing the existing SYM_FUNC_START_* 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 instead.

Fixes: 393f203f5fd5 ("x86_64: kasan: add interceptors for memset/memmove/memcpy functions") Reported-by: Sami Tolvanen samitolvanen@google.com Signed-off-by: Fangrui Song maskray@google.com Cc: stable@vger.kernel.org

---

arch/x86/lib/memcpy_64.S | 4 +--- arch/x86/lib/memmove_64.S | 4 +--- arch/x86/lib/memset_64.S | 4 +--- 3 files changed, 3 insertions(+), 9 deletions(-)

diff --git a/arch/x86/lib/memcpy_64.S b/arch/x86/lib/memcpy_64.S index 037faac46b0c..1e299ac73c86 100644 --- a/arch/x86/lib/memcpy_64.S +++ b/arch/x86/lib/memcpy_64.S @@ -16,8 +16,6 @@

• to a jmp to memcpy_erms which does the REP; MOVSB mem copy.

*/

-.weak memcpy

/*

• memcpy - Copy a memory block.

@@ -30,7 +28,7 @@

• rax original destination

*/ SYM_FUNC_START_ALIAS(__memcpy) -SYM_FUNC_START_LOCAL(memcpy) +SYM_FUNC_START_WEAK(memcpy) ALTERNATIVE_2 "jmp memcpy_orig", "", X86_FEATURE_REP_GOOD, \ "jmp memcpy_erms", X86_FEATURE_ERMS

Thanks for the patch. This exposes a lack of symmetry in the assembler subroutine for memcpy; memmove and memset use SYM_FUNC_START for their double underscore prefixed symbols, and SYM_FUNC_START_WEAK for the non prefixed symbols, and no SYM_FUNC_START_ALIAS after your patch. It's also curious to me why you removed SYM_FUNC_START_ALIAS for memmove and memset, but not memcpy? Can we sort that out so that they all follow the same convention?

Before your patch, with GNU `as` I see: $ llvm-readelf -s arch/x86/lib/memcpy_64.o | grep -e memcpy -e __memcpy 16: 0000000000000000 26 FUNC GLOBAL DEFAULT 4 __memcpy 17: 0000000000000000 26 FUNC WEAK DEFAULT 4 memcpy $ llvm-readelf -s arch/x86/lib/memmove_64.o| grep -e memmove -e __memmove 13: 0000000000000000 409 FUNC WEAK DEFAULT 1 memmove 14: 0000000000000000 409 FUNC GLOBAL DEFAULT 1 __memmove $ llvm-readelf -s arch/x86/lib/memset_64.o| grep -e memset -e __memset 15: 0000000000000000 47 FUNC WEAK DEFAULT 1 memset 16: 0000000000000000 47 FUNC GLOBAL DEFAULT 1 __memset

After your patch, with GNU `as` I see: $ llvm-readelf -s arch/x86/lib/memcpy_64.o | grep -e memcpy -e __memcpy 16: 0000000000000000 26 FUNC GLOBAL DEFAULT 4 __memcpy 17: 0000000000000000 26 FUNC WEAK DEFAULT 4 memcpy $ llvm-readelf -s arch/x86/lib/memmove_64.o| grep -e memmove -e __memmove 13: 0000000000000000 409 FUNC WEAK DEFAULT 1 memmove 14: 0000000000000000 409 FUNC GLOBAL DEFAULT 1 __memmove $ llvm-readelf -s arch/x86/lib/memset_64.o| grep -e memset -e __memset 15: 0000000000000000 47 FUNC WEAK DEFAULT 1 memset 16: 0000000000000000 47 FUNC GLOBAL DEFAULT 1 __memset

So in that sense, your patch is no functional change, and simply resolves ambiguities in repeatedly defining a symbol with different bindings. I guess we can save uncovering why memcpy doesn't follow the same convention for another day. In that sense:

Reviewed-by: Nick Desaulniers ndesaulniers@google.com Tested-by: Nick Desaulniers ndesaulniers@google.com

And thanks for the patch!

diff --git a/arch/x86/lib/memmove_64.S b/arch/x86/lib/memmove_64.S index 7ff00ea64e4f..41902fe8b859 100644 --- a/arch/x86/lib/memmove_64.S +++ b/arch/x86/lib/memmove_64.S @@ -24,9 +24,7 @@

• Output:
• rax: dest

*/ -.weak memmove

-SYM_FUNC_START_ALIAS(memmove) +SYM_FUNC_START_WEAK(memmove) SYM_FUNC_START(__memmove)

    mov %rdi, %rax

diff --git a/arch/x86/lib/memset_64.S b/arch/x86/lib/memset_64.S index 9ff15ee404a4..0bfd26e4ca9e 100644 --- a/arch/x86/lib/memset_64.S +++ b/arch/x86/lib/memset_64.S @@ -6,8 +6,6 @@ #include <asm/alternative-asm.h> #include <asm/export.h>

-.weak memset

/*

• ISO C memset - set a memory block to a byte value. This function uses fast
• string to get better performance than the original function. The code is

@@ -19,7 +17,7 @@

• rax original destination

*/ -SYM_FUNC_START_ALIAS(memset) +SYM_FUNC_START_WEAK(memset) SYM_FUNC_START(__memset) /* * Some CPUs support enhanced REP MOVSB/STOSB feature. It is recommended -- 2.29.1.341.ge80a0c044ae-goog

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