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

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Message-ID: <CAKwvOdkqfbXVQ8dwoT5RVza6bZw3cBQUEGcuRHu0-LhObkg--w@mail.gmail.com>
Date:   Fri, 18 Oct 2019 13:33:27 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        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>
Subject: Re: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)

On Fri, Oct 18, 2019 at 11:33 AM Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> On Fri, Oct 18, 2019 at 7:11 PM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > 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.
>
> +1, please CC whenever you send it!

Sami pointed out to me off thread that __has_attribute would only
check `no_sanitize`, not `shadow-call-stack`.  So maybe best to keep
the definition here (include/linux/compiler-clang.h), but wrapped in a
`__has_feature` check so that Clang 6.0 doesn't start complaining.
-- 
Thanks,
~Nick Desaulniers