linux-kernel - Re: [PATCH v2 4/5] x86/boot: use -fno-builtin-bcmp

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Date:   Wed, 19 Aug 2020 13:11:17 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Tony Luck <tony.luck@...el.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Joe Perches <joe@...ches.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Daniel Axtens <dja@...ens.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Yury Norov <yury.norov@...il.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Bruce Ashfield <bruce.ashfield@...il.com>,
        Marco Elver <elver@...gle.com>,
        Vamshi K Sthambamkadi <vamshi.k.sthambamkadi@...il.com>,
        Andi Kleen <ak@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>
Subject: Re: [PATCH v2 4/5] x86/boot: use -fno-builtin-bcmp

On Wed, Aug 19, 2020 at 12:29 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> On Wed, Aug 19, 2020 at 12:16:53PM -0700, Nick Desaulniers wrote:
> > We're reverting
> > commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
> > in favor of -fno-builtin-bcmp. Remove the additional definition here,
> > too.
> >
> > arch/x86/purgatory/Makefile uses -ffreestanding, so there's no risk of
> > this libcall optimization occurring for arch/x86/boot/purgatory.ro.
> >
> > arch/x86/boot/Makefile resets KBUILD_CFLAGS, so make sure to reset this
> > flag that was set for the top level Makefile.
> >
> > Fixes: 4ce97317f41d ("x86/purgatory: Do not use __builtin_memcpy and __builtin_memset")
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > ---
> >  arch/x86/boot/Makefile | 1 +
> >  arch/x86/boot/string.c | 8 --------
> >  2 files changed, 1 insertion(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> > index fe605205b4ce..ef7f15bfceab 100644
> > --- a/arch/x86/boot/Makefile
> > +++ b/arch/x86/boot/Makefile
> > @@ -70,6 +70,7 @@ KBUILD_CFLAGS       := $(REALMODE_CFLAGS) -D_SETUP
> >  KBUILD_AFLAGS        := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> >  KBUILD_CFLAGS        += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> >  KBUILD_CFLAGS        += -fno-asynchronous-unwind-tables
> > +KBUILD_CFLAGS        += -fno-builtin-bcmp
> >  GCOV_PROFILE := n
> >  UBSAN_SANITIZE := n
> >
>
> This should be unnecessary: KBUILD_CFLAGS in arch/x86/boot/Makefile is
> set to REALMODE_CFLAGS (defined in arch/x86/Makefile), which includes
> -ffreestanding.

I triple checked by grepping the disassemblies of
./arch/x86/purgatory/purgatory.ro, arch/x86/purgatory/*.o, and
arch/x86/boot/*.o for bcmp; should be fine to drop that hunk.  Will
wait a bit to see if there's other feedback, and whether folks are on
board with the plan to merge the series proposed in the cover letter
or not.

--
Thanks,
~Nick Desaulniers