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

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Date:   Sun, 11 Mar 2018 12:38:11 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.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>,
        Nicolas Pitre <nico@...aro.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: [PATCH v8 2/6] module: allow symbol exports to be disabled

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>
---
 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