linux-kernel - Re: [PATCH v8 2/6] module: allow symbol exports to be disabled

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Date:   Sun, 11 Mar 2018 17:31:29 -0400 (EDT)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
cc:     linux-arm-kernel@...ts.infradead.org,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Will Deacon <will.deacon@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Garnier <thgarnie@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Russell King <linux@...linux.org.uk>,
        Paul Mackerras <paulus@...ba.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Petr Mladek <pmladek@...e.com>, Ingo Molnar <mingo@...hat.com>,
        James Morris <james.l.morris@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v8 2/6] module: allow symbol exports to be disabled

On Sun, 11 Mar 2018, Ard Biesheuvel wrote:

> To allow existing C code to be incorporated into the decompressor or
> the UEFI stub, introduce a CPP macro that turns all EXPORT_SYMBOL_xxx
> declarations into nops, and #define it in places where such exports
> are undesirable. Note that this gets rid of a rather dodgy redefine
> of linux/export.h's header guard.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>

Acked-by: Nicolas Pitre <nico@...aro.org>

> ---
>  arch/x86/boot/compressed/kaslr.c      |  5 +----
>  drivers/firmware/efi/libstub/Makefile |  3 ++-
>  include/linux/export.h                | 11 ++++++++++-
>  3 files changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 8199a6187251..3a2a6d7049e4 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -23,11 +23,8 @@
>   * _ctype[] in lib/ctype.c is needed by isspace() of linux/ctype.h.
>   * While both lib/ctype.c and lib/cmdline.c will bring EXPORT_SYMBOL
>   * which is meaningless and will cause compiling error in some cases.
> - * So do not include linux/export.h and define EXPORT_SYMBOL(sym)
> - * as empty.
>   */
> -#define _LINUX_EXPORT_H
> -#define EXPORT_SYMBOL(sym)
> +#define __DISABLE_EXPORTS
>  
>  #include "misc.h"
>  #include "error.h"
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index 7b3ba40f0745..896a882c89f4 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -20,7 +20,8 @@ cflags-$(CONFIG_EFI_ARMSTUB)	+= -I$(srctree)/scripts/dtc/libfdt
>  KBUILD_CFLAGS			:= $(cflags-y) -DDISABLE_BRANCH_PROFILING \
>  				   -D__NO_FORTIFY \
>  				   $(call cc-option,-ffreestanding) \
> -				   $(call cc-option,-fno-stack-protector)
> +				   $(call cc-option,-fno-stack-protector) \
> +				   -D__DISABLE_EXPORTS
>  
>  GCOV_PROFILE			:= n
>  KASAN_SANITIZE			:= n
> diff --git a/include/linux/export.h b/include/linux/export.h
> index 1a1dfdb2a5c6..25005b55b079 100644
> --- a/include/linux/export.h
> +++ b/include/linux/export.h
> @@ -72,7 +72,16 @@ extern struct module __this_module;
>  	__attribute__((section("___ksymtab" sec "+" #sym), used))	\
>  	= { (unsigned long)&sym, __kstrtab_##sym }
>  
> -#if defined(__KSYM_DEPS__)
> +#if defined(__DISABLE_EXPORTS)
> +
> +/*
> + * Allow symbol exports to be disabled completely so that C code may
> + * be reused in other execution contexts such as the UEFI stub or the
> + * decompressor.
> + */
> +#define __EXPORT_SYMBOL(sym, sec)
> +
> +#elif defined(__KSYM_DEPS__)
>  
>  /*
>   * For fine grained build dependencies, we want to tell the build system
> -- 
> 2.15.1
> 
>