[PATCH v2 1/2] compiler-gcc.h: add gnu_inline to all inline declarations

[Nick Desaulniers]

Functions marked extern inline do not emit an externally visible
function when the gnu89 C standard is used. Some KBUILD Makefiles
overwrite KBUILD_CFLAGS. This is an issue for GCC 5.1+ users as without
an explicit C standard specified, the default is gnu11. Since c99, the
semantics of extern inline have changed such that an externally visible
function is always emitted. This can lead to multiple definition errors
of extern inline functions at link time of compilation units whose build
files have removed an explicit C standard compiler flag for users of GCC
5.1+ or Clang.

Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Suggested-by: H. Peter Anvin <hpa@xxxxxxxxx>
Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
---
 include/linux/compiler-gcc.h | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index b4bf73f5e38f..7827bdf0e5e9 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -72,17 +72,24 @@
  * -Wunused-function.  This turns out to avoid the need for complex #ifdef
  * directives.  Suppress the warning in clang as well by using "unused"
  * function attribute, which is redundant but not harmful for gcc.
+ * Prefer gnu_inline, so that extern inline functions do not emit an
+ * externally visible function. This makes extern inline behave as per gnu89
+ * semantics rather than c99. This prevents multiple symbol definition errors
+ * of extern inline functions at link time.
  */
 #if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) ||		\
     !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4)
-#define inline inline		__attribute__((always_inline,unused)) notrace
-#define __inline__ __inline__	__attribute__((always_inline,unused)) notrace
-#define __inline __inline	__attribute__((always_inline,unused)) notrace
+#define inline \
+	inline __attribute__((always_inline, unused, gnu_inline)) notrace
+#define __inline__ \
+	__inline__ __attribute__((always_inline, unused, gnu_inline)) notrace
+#define __inline \
+	__inline __attribute__((always_inline, unused, gnu_inline)) notrace
 #else
 /* A lot of inline functions can cause havoc with function tracing */
-#define inline inline		__attribute__((unused)) notrace
-#define __inline__ __inline__	__attribute__((unused)) notrace
-#define __inline __inline	__attribute__((unused)) notrace
+#define inline inline		__attribute__((unused, gnu_inline)) notrace
+#define __inline__ __inline__	__attribute__((unused, gnu_inline)) notrace
+#define __inline __inline	__attribute__((unused, gnu_inline)) notrace
 #endif
 
 #define __always_inline	inline __attribute__((always_inline))
-- 
2.17.1.1185.g55be947832-goog