[Xen-devel] [RFC 17/22] compiler: Option to default to hidden symbols

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

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.

Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
---
 arch/x86/boot/boot.h           |  2 +-
 arch/x86/include/asm/setup.h   |  2 +-
 include/asm-generic/sections.h |  6 ++++++
 include/linux/compiler.h       |  8 ++++++++
 init/Kconfig                   |  7 +++++++
 kernel/kallsyms.c              | 16 ++++++++--------
 6 files changed, 31 insertions(+), 10 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 e4585a393965..f3ffad82bdc0 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -66,7 +66,7 @@ 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/include/asm-generic/sections.h b/include/asm-generic/sections.h
index 532372c6cf15..27c12f6dd6e2 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -28,6 +28,9 @@
  *     __entry_text_start, __entry_text_end
  *     __ctors_start, __ctors_end
  */
+#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[];
@@ -42,6 +45,9 @@ extern char __start_rodata[], __end_rodata[];
 
 /* 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
 
 extern __visible const void __nosave_begin, __nosave_end;
 
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index eca8ad75e28b..876b827fe4a7 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -78,6 +78,14 @@ extern void __chk_io_ptr(const volatile void __iomem *);
 #include <linux/compiler-clang.h>
 #endif
 
+/* Useful for Position Independent Code to reduce global references */
+#ifdef CONFIG_DEFAULT_HIDDEN
+#pragma GCC visibility push(hidden)
+#define __default_visibility  __attribute__((visibility ("default")))
+#else
+#define __default_visibility
+#endif
+
 /*
  * Generic compiler-dependent macros required for kernel
  * build go below this comment. Actual compiler/compiler version
diff --git a/init/Kconfig b/init/Kconfig
index 4fb5d6fc2c4f..a93626d40355 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1635,6 +1635,13 @@ 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 127e7cfafa55..252019c8c3a9 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -32,24 +32,24 @@
  * 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;
 
 static inline int is_kernel_inittext(unsigned long addr)
 {
-- 
2.13.2.932.g7449e964c-goog


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