linux-kernel - Re: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)

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Message-ID: <CABCJKud6+F=yhTo6xTXkHhtLWcSE99K=NcfKW_5E4swS4seKMw@mail.gmail.com>
Date:   Fri, 18 Oct 2019 10:11:16 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: Re: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)

On Fri, Oct 18, 2019 at 10:08 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
> > diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> > index 333a6695a918..9af08391f205 100644
> > --- a/include/linux/compiler-clang.h
> > +++ b/include/linux/compiler-clang.h
> > @@ -42,3 +42,5 @@
> >   * compilers, like ICC.
> >   */
> >  #define barrier() __asm__ __volatile__("" : : : "memory")
> > +
> > +#define __noscs                __attribute__((no_sanitize("shadow-call-stack")))
>
> It looks like this attribute, (and thus a requirement to use this
> feature), didn't exist until Clang 7.0: https://godbolt.org/z/p9u1we
> (as noted above)
>
> I think it's better to put __noscs behind a __has_attribute guard in
> include/linux/compiler_attributes.h.  Otherwise, what will happen when
> Clang 6.0 sees __noscs, for example? (-Wunknown-sanitizers will
> happen).

Good point, I'll fix this in v2. Thanks.

Sami