Re: [PATCH] x86/extable: prefer local labels in .set directives

[Nathan Chancellor <nathan@kernel.org>]

On Tue, Mar 29, 2022 at 01:21:45PM -0700, Nick Desaulniers wrote:
> Bernardo reported an error that Nathan bisected down to
> (x86_64) defconfig+LTO_CLANG_FULL+X86_PMEM_LEGACY.
> 
>     LTO     vmlinux.o
>   ld.lld: error: <instantiation>:1:13: redefinition of 'found'
>   .set found, 0
>               ^
> 
>   <inline asm>:29:1: while in macro instantiation
>   extable_type_reg reg=%eax, type=(17 | ((0) << 16))
>   ^
> 
> This appears to be another LTO specific issue similar to what was folded
> into commit 4b5305decc84 ("x86/extable: Extend extable functionality"),
> where the `.set found, 0` in DEFINE_EXTABLE_TYPE_REG in
> arch/x86/include/asm/asm.h conflicts with the symbol for the static
> function `found` in arch/x86/kernel/pmem.c.
> 
> Assembler .set directive declare symbols with global visibility, so the
> assembler may not rename such symbols in the event of a conflict. LTO
> could rename static functions if there was a conflict in C sources, but
> it cannot see into symbols defined in inline asm.
> 
> The symbols are also retained in the symbol table, regardless of LTO.
> 
> Give the symbols .L prefixes making them locally visible, so that they
> may be renamed for LTO to avoid conflicts, and to drop them from the
> symbol table regardless of LTO.
> 
> Fixes: 4b5305decc84 ("x86/extable: Extend extable functionality")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1612
> Link: https://sourceware.org/binutils/docs/as/Symbol-Names.html#Local-Symbol-Names
> Reported-by: Bernardo Meurer Costa <beme@google.com>
> Debugged-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

This resolves the build error for me and it passes a smoke test boot in
QEMU.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

Should this have an explicit "Cc: stable@vger.kernel.org"? The build
error was reported against Linux 5.17.

> ---
>  arch/x86/include/asm/asm.h | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
> index c878fed3056f..fbcfec4dc4cc 100644
> --- a/arch/x86/include/asm/asm.h
> +++ b/arch/x86/include/asm/asm.h
> @@ -154,24 +154,24 @@
>  
>  # define DEFINE_EXTABLE_TYPE_REG \
>  	".macro extable_type_reg type:req reg:req\n"						\
> -	".set found, 0\n"									\
> -	".set regnr, 0\n"									\
> +	".set .Lfound, 0\n"									\
> +	".set .Lregnr, 0\n"									\
>  	".irp rs,rax,rcx,rdx,rbx,rsp,rbp,rsi,rdi,r8,r9,r10,r11,r12,r13,r14,r15\n"		\
>  	".ifc \\reg, %%\\rs\n"									\
> -	".set found, found+1\n"									\
> -	".long \\type + (regnr << 8)\n"								\
> +	".set .Lfound, .Lfound+1\n"								\
> +	".long \\type + (.Lregnr << 8)\n"							\
>  	".endif\n"										\
> -	".set regnr, regnr+1\n"									\
> +	".set .Lregnr, .Lregnr+1\n"								\
>  	".endr\n"										\
> -	".set regnr, 0\n"									\
> +	".set .Lregnr, 0\n"									\
>  	".irp rs,eax,ecx,edx,ebx,esp,ebp,esi,edi,r8d,r9d,r10d,r11d,r12d,r13d,r14d,r15d\n"	\
>  	".ifc \\reg, %%\\rs\n"									\
> -	".set found, found+1\n"									\
> -	".long \\type + (regnr << 8)\n"								\
> +	".set .Lfound, .Lfound+1\n"								\
> +	".long \\type + (.Lregnr << 8)\n"							\
>  	".endif\n"										\
> -	".set regnr, regnr+1\n"									\
> +	".set .Lregnr, .Lregnr+1\n"								\
>  	".endr\n"										\
> -	".if (found != 1)\n"									\
> +	".if (.Lfound != 1)\n"									\
>  	".error \"extable_type_reg: bad register argument\"\n"					\
>  	".endif\n"										\
>  	".endm\n"
> -- 
> 2.35.1.1021.g381101b075-goog
>