linux-kernel - Re: [PATCH v4] x86/entry: emit a symbol for register restoring thunk

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Date:   Wed, 13 Jan 2021 09:56:04 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Brown <broonie@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Fangrui Song <maskray@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Joe Perches <joe@...ches.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v4] x86/entry: emit a symbol for register restoring thunk

On Wed, Jan 13, 2021 at 8:59 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Tue, Jan 12, 2021 at 09:01:54PM +0000, Mark Brown wrote:
> > On Tue, Jan 12, 2021 at 11:46:24AM -0800, Nick Desaulniers wrote:
> >
> > This:
> >
> > > when building with LLVM_IAS=1 (Clang's integrated assembler). Josh
> > > notes:
> >
> > >   So basically, you can use an .L symbol *inside* a function or a code
> > >   segment, you just can't use the .L symbol to contain the code using a
> > >   SYM_*_START/END annotation pair.
> >
> > is a stronger statement than this:
> >
> > > +  Developers should avoid using local symbol names that are prefixed with
> > > +  ``.L``, as this has special meaning for the assembler; a symbol entry will
> > > +  not be emitted into the symbol table. This can prevent ``objtool`` from
> > > +  generating correct unwind info. Symbols with STB_LOCAL binding may still be
> > > +  used, and ``.L`` prefixed local symbol names are still generally useable
> > > +  within a function, but ``.L`` prefixed local symbol names should not be used
> > > +  to denote the beginning or end of code regions via
> > > +  ``SYM_CODE_START_LOCAL``/``SYM_CODE_END``.
> >
> > and seems more what I'd expect - SYM_FUNC* is also affected for example.
> > Even though other usages are probably not very likely it seems better to
> > keep the stronger statement in case someone comes up with one, and to
> > stop anyone spending time wondering why only SYM_CODE_START_LOCAL is
> > affected.
>
> Agreed, I think the comment is misleading/wrong/unclear in multiple
> ways.  In most cases the use of .L symbols is still fine.  What's no
> longer fine is when they're used to contain code in any kind of
> START/END pair.

Apologies, that was not my intention.  I've sent a follow up in
https://lore.kernel.org/lkml/20210113174620.958429-1-ndesaulniers@google.com/T/#u
since BP picked up v3 in tip x86/entry:
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=x86/entry&id=bde718b7e154afc99e1956b18a848401ce8e1f8e

-- 
Thanks,
~Nick Desaulniers