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

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Message-ID: <CAKwvOdnsHMs0PV8uDAWktRDso--_AORNnDYdGHnp5+YYEm1kXw@mail.gmail.com>
Date:   Thu, 13 Feb 2020 14:55:47 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Fangrui Song <maskray@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] objtool: ignore .L prefixed local symbols

On Thu, Feb 13, 2020 at 2:18 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> 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.

Sure thing.  I appreciate it, and I'm on board with helping debug or
fix any compiler bugs we might have in order to re-strengthen the
warning soon.
-- 
Thanks,
~Nick Desaulniers