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

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Message-ID: <CAK7LNAQWjq0DoTD6LkQfRSMz6sS6_SFapd5YzKjz2U1ZmFEm9w@mail.gmail.com>
Date:   Thu, 24 Oct 2019 23:38:07 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Mark Rutland <mark.rutland@....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>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <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 Thu, Oct 24, 2019 at 10:28 PM Mark Rutland <mark.rutland@....com> wrote:
>
> On Tue, Oct 22, 2019 at 12:26:02PM -0700, Sami Tolvanen wrote:
> > On Tue, Oct 22, 2019 at 9:28 AM Mark Rutland <mark.rutland@....com> wrote:
>
> > > > +config SHADOW_CALL_STACK
> > > > +     bool "Clang Shadow Call Stack"
> > > > +     depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
> > > > +     depends on CC_IS_CLANG && CLANG_VERSION >= 70000
> > >
> > > Is there a reason for an explicit version check rather than a
> > > CC_HAS_<feature> check? e.g. was this available but broken in prior
> > > versions of clang?
> >
> > No, this feature was added in Clang 7. However,
> > -fsanitize=shadow-call-stack might require architecture-specific
> > flags, so a simple $(cc-option, -fsanitize=shadow-call-stack) in
> > arch/Kconfig is not going to work. I could add something like this to
> > arch/arm64/Kconfig though:
> >
> > select ARCH_SUPPORTS_SHADOW_CALL_STACK if CC_HAVE_SHADOW_CALL_STACK
> > ...
> > config CC_HAVE_SHADOW_CALL_STACK
> >        def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
> >
> > And then drop CC_IS_CLANG and version check entirely. Thoughts?
>
> That sounds good to me, yes!
>
> Thanks,
> Mark.


If you use cc-option, please add a comment like

    # supported by Clang 7 or later.


I do not know the minimal supported clang version.
When we bump the minimal version to clang 7,
we can drop the cc-option test entirely.





--
Best Regards
Masahiro Yamada