linux-kernel - Re: [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug

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Date:   Mon, 29 Jun 2020 07:43:36 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Will Deacon <will@...nel.org>, Dave Martin <Dave.Martin@....com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 05/17] ctype: Work around Clang
 -mbranch-protection=none bug

On Mon, Jun 29, 2020 at 10:15:47AM +0200, Ard Biesheuvel wrote:
> On Mon, 29 Jun 2020 at 08:18, Kees Cook <keescook@...omium.org> wrote:
> >
> > In preparation for building efi/libstub with -mbranch-protection=none
> > (EFI does not support branch protection features[1]), add no-op code
> > to work around a Clang bug that emits an unwanted .note.gnu.property
> > section for object files without code[2].
> >
> > [1] https://lore.kernel.org/lkml/CAMj1kXHck12juGi=E=P4hWP_8vQhQ+-x3vBMc3TGeRWdQ-XkxQ@mail.gmail.com
> > [2] https://bugs.llvm.org/show_bug.cgi?id=46480
> >
> > Cc: Ard Biesheuvel <ardb@...nel.org>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Dave Martin <Dave.Martin@....com>
> > Cc: clang-built-linux@...glegroups.com
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  lib/ctype.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/lib/ctype.c b/lib/ctype.c
> > index c819fe269eb2..21245ed57d90 100644
> > --- a/lib/ctype.c
> > +++ b/lib/ctype.c
> > @@ -36,3 +36,13 @@ _L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,     /* 224-239 */
> >  _L,_L,_L,_L,_L,_L,_L,_P,_L,_L,_L,_L,_L,_L,_L,_L};      /* 240-255 */
> >
> >  EXPORT_SYMBOL(_ctype);
> > +
> > +/*
> > + * Clang will generate .note.gnu.property sections for object files
> > + * without code, even in the presence of -mbranch-protection=none.
> > + * To work around this, define an unused static function.
> > + * https://bugs.llvm.org/show_bug.cgi?id=46480
> > + */
> > +#ifdef CONFIG_CC_IS_CLANG
> > +void __maybe_unused __clang_needs_code_here(void) { }
> > +#endif
> > --
> > 2.25.1
> >
> 
> I take it we don't need this horrible hack if we build the EFI stub
> with branch protections and filter out the .note.gnu.property section
> explicitly?

Correct.

> Sorry to backpedal, but that is probably a better approach after all,
> given that the instructions don't hurt, and we will hopefully be able
> to arm them once UEFI (as well as PE/COFF) gets around to describing
> this in a way that both the firmware and the OS can consume.

Okay, will revert to the v3 solution.

-- 
Kees Cook