[PATCH v3 15/27] compiler: Option to default to hidden symbols

Thomas Garnier
2018-05-23T19:54:09Z

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@google.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_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 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 ae13bc974416..083a6e99b884 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -68,7 +68,7 @@ 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 77e201301528..6a4f5d9d7eb6 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 0943e7065e0e..2ffd019af2d4 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 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 341b6cf8c029..81a9986cad78 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 8915a3ce5f0c..0fc3a58d9f2f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1694,6 +1694,13 @@ 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 6fb46a06c9dc..e659f452cf8c 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1746,8 +1746,8 @@ 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;