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

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Message-ID: <20220406102751.GA2731@worktop.programming.kicks-ass.net>
Date:   Wed, 6 Apr 2022 12:27:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Bernardo Meurer Costa <beme@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/extable: prefer local labels in .set directives

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@...gle.com>
> Debugged-by: Nathan Chancellor <nathan@...nel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

Thanks!