linux-kernel - Re: [PATCH v3] x86/entry: emit a symbol for register restoring thunk

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Date:   Mon, 11 Jan 2021 17:13:16 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Fāng-ruì Sòng <maskray@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v3] x86/entry: emit a symbol for register restoring thunk

On Mon, Jan 11, 2021 at 5:00 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Mon, Jan 11, 2021 at 04:41:52PM -0800, Fāng-ruì Sòng wrote:
> > To be fair: we cannot use
>
> Who's "we"?
>
> > .L-prefixed local because of the objtool limitation.
>
> What objtool limitation? I thought clang's assembler removes .text which
> objtool uses. It worked fine with GNU as so far.

I don't think we need to completely stop using .L prefixes in the
kernel, just this one location since tracking the control flow seems a
little tricky for objtool. Maybe Josh can clarify more if needed?

>
> > The LLVM integrated assembler behavior is a good one
>
> Please explain what "good one" means in that particular context.
>
> > and binutils global maintainers have agreed so H.J. went ahead and
> > implemented it for GNU as x86.
>
> But they don't break old behavior, do they? Or are they removing .text
> unconditionally now too?

Unconditionally. See
https://sourceware.org/pipermail/binutils/2021-January/114700.html
where that flag was rejected and the optimization was adopted as the
optimization was obvious to GNU binutils developers. So I suspect this
will become a problem for GNU binutils users as well after the latest
release that contains
https://sourceware.org/pipermail/binutils/attachments/20210105/75dd4a9d/attachment-0001.bin.

> > --generate-unused-section-symbols=[yes|no] as an assembler option has
> > been rejected.
>
> Meaning what exactly? There's no way for clang's integrated assembler to
> even get a cmdline option to not strip .text?

I can clean that up in v5; The section symbols were not generated then
stripped; they were simply never generated.
-- 
Thanks,
~Nick Desaulniers