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Subject[PATCH v6 15/27] compiler: Option to default to hidden symbols
Date
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@chromium.org>
---
arch/x86/boot/boot.h | 2 +-
arch/x86/include/asm/setup.h | 2 +-
arch/x86/kernel/cpu/microcode/core.c | 4 ++--
drivers/base/firmware_loader/main.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 32a09eb5c101..c4afcfecc817 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 ed8ec011a9fd..74f0a8d87986 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -71,7 +71,7 @@ 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 97f9ada9ceda..04ca89e65f79 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -149,8 +149,8 @@ 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_loader/main.c b/drivers/base/firmware_loader/main.c
index 8e9213b36e31..f04096161a52 100644
--- a/drivers/base/firmware_loader/main.c
+++ b/drivers/base/firmware_loader/main.c
@@ -94,8 +94,8 @@ 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 d79abca81a52..94f072f3a48d 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_SYMS
+#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_SYMS
+#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 87692fdae97a..5c0723604a52 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -287,6 +287,13 @@ unsigned long read_word_at_a_time(const void *addr)
__u.__val; \
})

+#ifdef CONFIG_DEFAULT_HIDDEN_SYMS
+#pragma GCC visibility push(hidden)
+#define __default_visibility __attribute__((visibility ("default")))
+#else
+#define __default_visibility
+#endif
+
#endif /* __KERNEL__ */

/*
diff --git a/init/Kconfig b/init/Kconfig
index bb383615823a..116e0de4817f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1848,6 +1848,13 @@ config CMDLINE_OVERRIDE

endif

+#
+# Default to hidden visibility for all symbols.
+# Useful for Position Independent Code to reduce global references.
+#
+config DEFAULT_HIDDEN_SYMS
+ bool
+
endmenu # General setup

source "arch/Kconfig"
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 14934afa9e68..547affc40b68 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 int 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 int kallsyms_markers[] __weak;
+extern const unsigned int 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 08900828d282..5246977a2db7 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1809,8 +1809,8 @@ 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 dbf2b457e47e..05482e65ae7d 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;
--
2.20.1.495.gaa96b0ce6b-goog
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