linux-kernel - Re: [PATCH] compiler: enable CONFIG_OPTIMIZE

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Date:   Wed, 2 Oct 2019 14:55:50 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Stefan Wahren <wahrenst@....net>,
        Kees Cook <keescook@...gle.com>, Arnd Bergmann <arnd@...db.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly

Hi Nick,

On Wed, Oct 2, 2019 at 6:33 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> On Tue, Oct 1, 2019 at 11:14 AM Russell King - ARM Linux admin
> <linux@...linux.org.uk> wrote:
> > On Tue, Oct 01, 2019 at 11:00:11AM -0700, Nick Desaulniers wrote:
> > > On Tue, Oct 1, 2019 at 10:55 AM Russell King - ARM Linux admin
> > > <linux@...linux.org.uk> wrote:
> > > > On Tue, Oct 01, 2019 at 10:44:43AM -0700, Nick Desaulniers wrote:
> > > > > I apologize; I don't mean to be difficult.  I would just like to avoid
> > > > > surprises when code written with the assumption that it will be
> > > > > inlined is not.  It sounds like we found one issue in arm32 and one in
> > > > > arm64 related to outlining.  If we fix those two cases, I think we're
> > > > > close to proceeding with Masahiro's cleanup, which I view as a good
> > > > > thing for the health of the Linux kernel codebase.
> > > >
> > > > Except, using the C preprocessor for this turns the arm32 code into
> > > > yuck:
> > > >
> > > > 1. We'd need to turn get_domain() and set_domain() into multi-line
> > > >    preprocessor macro definitions, using the GCC ({ }) extension
> > > >    so that get_domain() can return a value.
> > > >
> > > > 2. uaccess_save_and_enable() and uaccess_restore() also need to
> > > >    become preprocessor macro definitions too.
> > > >
> > > > So, we end up with multiple levels of nested preprocessor macros.
> > > > When something goes wrong, the compiler warning/error message is
> > > > going to be utterly _horrid_.
> > >
> > > That's why I preferred V1 of Masahiro's patch, that fixed the inline
> > > asm not to make use of caller saved registers before calling a
> > > function that might not be inlined.
> >
> > ... which I objected to based on the fact that this uaccess stuff is
> > supposed to add protection against the kernel being fooled into
> > accessing userspace when it shouldn't.  The whole intention there is
> > that [sg]et_domain(), and uaccess_*() are _always_ inlined as close
> > as possible to the call site of the accessor touching userspace.
>
> Then use the C preprocessor to force the inlining.  I'm sorry it's not
> as pretty as static inline functions.

Which makes us lose the baby^H^H^H^Htype checking performed
on function parameters, requiring to add more ugly checks.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds