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

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Date:   Thu, 24 Oct 2019 08:04:18 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        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 Wed, 23 Oct 2019 09:59:09 -0700
Sami Tolvanen <samitolvanen@...gle.com> wrote:

> 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?
> 

You can remove a CFLAGS for a whole directory. lib, kernel/trace and
others do this. Look at kernel/trace/Makefile, we have:

ORIG_CFLAGS := $(KBUILD_CFLAGS)
KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))

Where it removes CC_FLAGS_FTRACE from CFLAGS for all objects in the
directory.

-- Steve