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Message-ID: <20191024080418.35423b36@gandalf.local.home>
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
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