linux-kernel - [PATCH] module: allow symbol exports to be disabled

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Date:   Thu, 28 Dec 2017 13:06:58 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     hpa@...or.com, tglx@...utronix.de, mingo@...hat.com, x86@...nel.org
Cc:     matt@...eblueprint.co.uk, linux-efi@...r.kernel.org,
        keescook@...omium.org, jeyu@...nel.org,
        linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: [PATCH] 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                | 9 +++++++++
 3 files changed, 12 insertions(+), 5 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 adaa4a964f0c..312bd0b64a61 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 5112d0c41512..0a46d2176555 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -107,6 +107,15 @@ struct kernel_symbol {
  */
 #define __EXPORT_SYMBOL(sym, sec)	=== __KSYM_##sym ===
 
+#elif defined(__DISABLE_EXPORTS)
+
+/*
+ * Allow symbol exports to be disabled completely so that C code that may
+ * be reused in other execution contexts such as the UEFI stub or the
+ * decompressor.
+ */
+#define __EXPORT_SYMBOL(sym, sec)
+
 #elif defined(CONFIG_TRIM_UNUSED_KSYMS)
 
 #include <generated/autoksyms.h>
-- 
2.11.0