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

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Date:   Tue, 22 Oct 2019 17:49:36 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Kees Cook <keescook@...omium.org>
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>,
        Laura Abbott <labbott@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, 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 09:30:53AM -0700, Kees Cook wrote:
> On Tue, Oct 22, 2019 at 05:28:27PM +0100, Mark Rutland wrote:
> > On Fri, Oct 18, 2019 at 09:10:21AM -0700, Sami Tolvanen wrote:
> > > +ifdef CONFIG_SHADOW_CALL_STACK
> > > +KBUILD_CFLAGS	+= -fsanitize=shadow-call-stack
> > > +DISABLE_SCS	:= -fno-sanitize=shadow-call-stack
> > > +export DISABLE_SCS
> > > +endif
> > 
> > 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
>   ^^^ was this meant to be CC_FLAGS_SCS here
> 
> > KBUILD_CFLAGS += $(CFLAGS_SCS)
>                      ^^^ and here?

Whoops; yes in both cases...

> > 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.
            ^^^^^ "flags" here, too.

Mark.