linux-kernel - Re: [PATCH] objtool: ignore .L prefixed local symbols

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Message-ID: <20200213221758.i6pchz4gsiy2lsyc@treble>
Date:   Thu, 13 Feb 2020 16:17:58 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     peterz@...radead.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        Fangrui Song <maskray@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] objtool: ignore .L prefixed local symbols

On Thu, Feb 13, 2020 at 10:47:08AM -0800, Nick Desaulniers wrote:
> Top of tree LLVM has optimizations related to
> -fno-semantic-interposition to avoid emitting PLT relocations for
> references to symbols located in the same translation unit, where it
> will emit "local symbol" references.
> 
> Clang builds fall back on GNU as for assembling, currently. It appears a
> bug in GNU as introduced around 2.31 is keeping around local labels in
> the symbol table, despite the documentation saying:
> 
> "Local symbols are defined and used within the assembler, but they are
> normally not saved in object files."
> 
> When objtool searches for a symbol at a given offset, it's finding the
> incorrectly kept .L<symbol>$local symbol that should have been discarded
> by the assembler.
> 
> A patch for GNU as has been authored.  For now, objtool should not treat
> local symbols as the expected symbol for a given offset when iterating
> the symbol table.
> 
> commit 644592d32837 ("objtool: Fail the kernel build on fatal errors")
> exposed this issue.

Since I'm going to be dropping 644592d32837 ("objtool: Fail the kernel
build on fatal errors") anyway, I wonder if this patch is still needed?

At least the error will be downgraded to a warning.  And while the
warning could be more user friendly, it still has value because it
reveals a toolchain bug.

-- 
Josh