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

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Message-ID: <CABCJKudsD6jghk4i8Tp4aJg0d7skt6sU=gQ3JXqW8sjkUuX7vA@mail.gmail.com>
Date:   Wed, 23 Oct 2019 09:59:09 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.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>,
        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 Tue, Oct 22, 2019 at 9:28 AM Mark Rutland <mark.rutland@....com> wrote:
> I think it would be preferable to follow the example of CC_FLAGS_FTRACE
> so that this can be filtered out, e.g.
>
> ifdef CONFIG_SHADOW_CALL_STACK
> CFLAGS_SCS := -fsanitize=shadow-call-stack
> KBUILD_CFLAGS += $(CFLAGS_SCS)
> export CC_FLAGS_SCS
> endif
>
> ... with removal being:
>
> CFLAGS_REMOVE := $(CC_FLAGS_SCS)
>
> ... or:
>
> CFLAGS_REMOVE_obj.o := $(CC_FLAGS_SCS)
>
> That way you only need to define the flags once, so the enable and
> disable falgs remain in sync by construction.

CFLAGS_REMOVE appears to be only implemented for objects, which means
there's no convenient way to filter out flags for everything in
arch/arm64/kvm/hyp, for example. I could add a CFLAGS_REMOVE
separately for each object file, or we could add something like
ccflags-remove-y to complement ccflags-y, which should be relatively
simple. Masahiro, do you have any suggestions?

Sami