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Date: Tue, 13 Mar 2018 13:59:33 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Herbert Xu <herbert@...dor.apana.org.au>,
	"David S . Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kate Stewart <kstewart@...uxfoundation.org>,
	Thomas Garnier <thgarnie@...gle.com>,
	Arnd Bergmann <arnd@...db.de>,
	Philippe Ombredanne <pombredanne@...b.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Matthias Kaehlcke <mka@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	Tom Lendacky <thomas.lendacky@....com>,
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Dominik Brodowski <linux@...inikbrodowski.net>,
	Borislav Petkov <bp@...en8.de>,
	Borislav Petkov <bp@...e.de>,
	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>,
	Pavel Machek <pavel@....cz>,
	Juergen Gross <jgross@...e.com>,
	Alok Kataria <akataria@...are.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux.com>,
	Dennis Zhou <dennisszhou@...il.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Luis R . Rodriguez" <mcgrof@...nel.org>,
	Christopher Li <sparse@...isli.org>,
	Jason Baron <jbaron@...mai.com>,
	Ashish Kalra <ashish@...estacks.com>,
	Kyle McMartin <kyle@...hat.com>,
	Dou Liyang <douly.fnst@...fujitsu.com>,
	Lukas Wunner <lukas@...ner.de>,
	Petr Mladek <pmladek@...e.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	Ingo Molnar <mingo@...nel.org>,
	Nicholas Piggin <npiggin@...il.com>,
	Cao jin <caoj.fnst@...fujitsu.com>,
	"H . J . Lu" <hjl.tools@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Joerg Roedel <joro@...tes.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Jia Zhang <qianyue.zj@...baba-inc.com>,
	Jiri Slaby <jslaby@...e.cz>,
	Kyle Huey <me@...ehuey.com>,
	Jonathan Corbet <corbet@....net>,
	Matthew Wilcox <mawilcox@...rosoft.com>,
	Michal Hocko <mhocko@...e.com>,
	Rob Landley <rob@...dley.net>,
	Baoquan He <bhe@...hat.com>,
	Daniel Micay <danielmicay@...il.com>,
	Jan H . Schönherr <jschoenh@...zon.de>
Cc: x86@...nel.org,
	linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org,
	linux-arch@...r.kernel.org,
	linux-sparse@...r.kernel.org,
	kvm@...r.kernel.org,
	linux-doc@...r.kernel.org,
	kernel-hardening@...ts.openwall.com
Subject: [PATCH v2 15/27] compiler: Option to default to hidden symbols

Provide an option to default visibility to hidden except for key
symbols. This option is disabled by default and will be used by x86_64
PIE support to remove errors between compilation units.

The default visibility is also enabled for external symbols that are
compared as they maybe equals (start/end of sections). In this case,
older versions of GCC will remove the comparison if the symbols are
hidden. This issue exists at least on gcc 4.9 and before.

Signed-off-by: Thomas Garnier <thgarnie@...gle.com>
---
 arch/x86/boot/boot.h                 |  2 +-
 arch/x86/include/asm/setup.h         |  2 +-
 arch/x86/kernel/cpu/microcode/core.c |  4 ++--
 drivers/base/firmware_class.c        |  4 ++--
 include/asm-generic/sections.h       |  6 ++++++
 include/linux/compiler.h             |  7 +++++++
 init/Kconfig                         |  7 +++++++
 kernel/kallsyms.c                    | 16 ++++++++--------
 kernel/trace/trace.h                 |  4 ++--
 lib/dynamic_debug.c                  |  4 ++--
 10 files changed, 38 insertions(+), 18 deletions(-)

diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
index ef5a9cc66fb8..d726c35bdd96 100644
--- a/arch/x86/boot/boot.h
+++ b/arch/x86/boot/boot.h
@@ -193,7 +193,7 @@ static inline bool memcmp_gs(const void *s1, addr_t s2, size_t len)
 }
 
 /* Heap -- available for dynamic lists. */
-extern char _end[];
+extern char _end[] __default_visibility;
 extern char *HEAP;
 extern char *heap_end;
 #define RESET_HEAP() ((void *)( HEAP = _end ))
diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index 3108e297d87d..dfba64fe1c7e 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -70,7 +70,7 @@ static inline void x86_ce4100_early_setup(void) { }
  * This is set up by the setup-routine at boot-time
  */
 extern struct boot_params boot_params;
-extern char _text[];
+extern char _text[] __default_visibility;
 
 static inline bool kaslr_enabled(void)
 {
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index aa1b9a422f2b..ed5675db6e82 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -141,8 +141,8 @@ static bool __init check_loader_disabled_bsp(void)
 	return *res;
 }
 
-extern struct builtin_fw __start_builtin_fw[];
-extern struct builtin_fw __end_builtin_fw[];
+extern struct builtin_fw __start_builtin_fw[] __default_visibility;
+extern struct builtin_fw __end_builtin_fw[] __default_visibility;
 
 bool get_builtin_firmware(struct cpio_data *cd, const char *name)
 {
diff --git a/drivers/base/firmware_class.c b/drivers/base/firmware_class.c
index 7dd36ace6152..939a1952d0ab 100644
--- a/drivers/base/firmware_class.c
+++ b/drivers/base/firmware_class.c
@@ -136,8 +136,8 @@ static struct firmware_cache fw_cache;
 
 #ifdef CONFIG_FW_LOADER
 
-extern struct builtin_fw __start_builtin_fw[];
-extern struct builtin_fw __end_builtin_fw[];
+extern struct builtin_fw __start_builtin_fw[] __default_visibility;
+extern struct builtin_fw __end_builtin_fw[] __default_visibility;
 
 static void fw_copy_to_prealloc_buf(struct firmware *fw,
 				    void *buf, size_t size)
diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index 849cd8eb5ca0..0a0e23405ddd 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -32,6 +32,9 @@
  *	__softirqentry_text_start, __softirqentry_text_end
  *	__start_opd, __end_opd
  */
+#ifdef CONFIG_DEFAULT_HIDDEN
+#pragma GCC visibility push(default)
+#endif
 extern char _text[], _stext[], _etext[];
 extern char _data[], _sdata[], _edata[];
 extern char __bss_start[], __bss_stop[];
@@ -49,6 +52,9 @@ extern char __start_once[], __end_once[];
 
 /* Start and end of .ctors section - used for constructor calls. */
 extern char __ctors_start[], __ctors_end[];
+#ifdef CONFIG_DEFAULT_HIDDEN
+#pragma GCC visibility pop
+#endif
 
 /* Start and end of .opd section - used for function descriptors. */
 extern char __start_opd[], __end_opd[];
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index ab4711c63601..a9ac84e37af9 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -278,6 +278,13 @@ unsigned long read_word_at_a_time(const void *addr)
 	__u.__val;					\
 })
 
+#ifdef CONFIG_DEFAULT_HIDDEN
+#pragma GCC visibility push(hidden)
+#define __default_visibility  __attribute__((visibility ("default")))
+#else
+#define __default_visibility
+#endif
+
 #endif /* __KERNEL__ */
 
 #endif /* __ASSEMBLY__ */
diff --git a/init/Kconfig b/init/Kconfig
index acc9087546ac..c924babc6d47 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1667,6 +1667,13 @@ config PROFILING
 config TRACEPOINTS
 	bool
 
+#
+# Default to hidden visibility for all symbols.
+# Useful for Position Independent Code to reduce global references.
+#
+config DEFAULT_HIDDEN
+	bool
+
 source "arch/Kconfig"
 
 endmenu		# General setup
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index a23e21ada81b..f4e58b7a6daf 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -29,24 +29,24 @@
  * These will be re-linked against their real values
  * during the second link stage.
  */
-extern const unsigned long kallsyms_addresses[] __weak;
-extern const int kallsyms_offsets[] __weak;
-extern const u8 kallsyms_names[] __weak;
+extern const unsigned long kallsyms_addresses[] __weak __default_visibility;
+extern const int kallsyms_offsets[] __weak __default_visibility;
+extern const u8 kallsyms_names[] __weak __default_visibility;
 
 /*
  * Tell the compiler that the count isn't in the small data section if the arch
  * has one (eg: FRV).
  */
 extern const unsigned long kallsyms_num_syms
-__attribute__((weak, section(".rodata")));
+__attribute__((weak, section(".rodata"))) __default_visibility;
 
 extern const unsigned long kallsyms_relative_base
-__attribute__((weak, section(".rodata")));
+__attribute__((weak, section(".rodata"))) __default_visibility;
 
-extern const u8 kallsyms_token_table[] __weak;
-extern const u16 kallsyms_token_index[] __weak;
+extern const u8 kallsyms_token_table[] __weak __default_visibility;
+extern const u16 kallsyms_token_index[] __weak __default_visibility;
 
-extern const unsigned long kallsyms_markers[] __weak;
+extern const unsigned long kallsyms_markers[] __weak __default_visibility;
 
 /*
  * Expand a compressed symbol data into the resulting uncompressed string,
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 2a6d0325a761..5aebb0dcecba 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1741,8 +1741,8 @@ extern int trace_event_enable_disable(struct trace_event_file *file,
 				      int enable, int soft_disable);
 extern int tracing_alloc_snapshot(void);
 
-extern const char *__start___trace_bprintk_fmt[];
-extern const char *__stop___trace_bprintk_fmt[];
+extern const char *__start___trace_bprintk_fmt[] __default_visibility;
+extern const char *__stop___trace_bprintk_fmt[] __default_visibility;
 
 extern const char *__start___tracepoint_str[];
 extern const char *__stop___tracepoint_str[];
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index c7c96bc7654a..40b752b53627 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -37,8 +37,8 @@
 #include <linux/device.h>
 #include <linux/netdevice.h>
 
-extern struct _ddebug __start___verbose[];
-extern struct _ddebug __stop___verbose[];
+extern struct _ddebug __start___verbose[] __default_visibility;
+extern struct _ddebug __stop___verbose[] __default_visibility;
 
 struct ddebug_table {
 	struct list_head link;
-- 
2.16.2.660.g709887971b-goog

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