linux-kernel - Re: [PATCH v3] ARM: fix atags_to_fdt with stack-protector-strong

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Date:	Thu, 7 Jan 2016 11:30:29 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Kees Cook <keescook@...omium.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Simon Horman <horms+renesas@...ge.net.au>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Magnus Damm <damm+renesas@...nsource.se>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] ARM: fix atags_to_fdt with stack-protector-strong

On Wed, Jan 06, 2016 at 03:36:56PM -0800, Kees Cook wrote:
> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> index 3f9a9ebc77c3..d7d2c2981f65 100644
> --- a/arch/arm/boot/compressed/Makefile
> +++ b/arch/arm/boot/compressed/Makefile
> @@ -106,6 +106,14 @@ ORIG_CFLAGS := $(KBUILD_CFLAGS)
>  KBUILD_CFLAGS = $(subst -pg, , $(ORIG_CFLAGS))
>  endif
>  
> +# -fstack-protector-strong triggers protection checks in this code,
> +# but it is being used too early to link to meaningful stack_chk logic.
> +CFLAGS_atags_to_fdt.o := $(call cc-option, -fno-stack-protector)
> +CFLAGS_fdt.o := $(call cc-option, -fno-stack-protector)
> +CFLAGS_fdt_ro.o := $(call cc-option, -fno-stack-protector)
> +CFLAGS_fdt_rw.o := $(call cc-option, -fno-stack-protector)
> +CFLAGS_fdt_wip.o := $(call cc-option, -fno-stack-protector)

This will result in "$(call cc-option, -fno-stack-protector)" being
called five times when this Makefile is parsed, which seems very
wasteful.  I'm sure there's better solutions to that - maybe caching
the value in a variable in a higher level makefile (eg,
arch/arm/Makefile) ?

Also, I suspect that all of the decompressor should be built with
-fno-stack-protector as we don't have sufficient environment here.
Maybe it should be placed in the global CFLAGS for the decompressor?

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
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