Linux Virtualization - Jul 2018 - [PATCH 4.9 05/66] compiler-gcc.h: Add __attribute__((gnu_inline)) to all inline declarations

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nick Desaulniers <ndesaulniers at google.com>

commit d03db2bc26f0e4a6849ad649a09c9c73fccdc656 upstream.

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.

Suggested-by: Arnd Bergmann <arnd at arndb.de>
Suggested-by: H. Peter Anvin <hpa at zytor.com>
Suggested-by: Joe Perches <joe at perches.com>
Signed-off-by: Nick Desaulniers <ndesaulniers at google.com>
Acked-by: Juergen Gross <jgross at suse.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: acme at redhat.com
Cc: akataria at vmware.com
Cc: akpm at linux-foundation.org
Cc: andrea.parri at amarulasolutions.com
Cc: ard.biesheuvel at linaro.org
Cc: aryabinin at virtuozzo.com
Cc: astrachan at google.com
Cc: boris.ostrovsky at oracle.com
Cc: brijesh.singh at amd.com
Cc: caoj.fnst at cn.fujitsu.com
Cc: geert at linux-m68k.org
Cc: ghackmann at google.com
Cc: gregkh at linuxfoundation.org
Cc: jan.kiszka at siemens.com
Cc: jarkko.sakkinen at linux.intel.com
Cc: jpoimboe at redhat.com
Cc: keescook at google.com
Cc: kirill.shutemov at linux.intel.com
Cc: kstewart at linuxfoundation.org
Cc: linux-efi at vger.kernel.org
Cc: linux-kbuild at vger.kernel.org
Cc: manojgupta at google.com
Cc: mawilcox at microsoft.com
Cc: michal.lkml at markovi.net
Cc: mjg59 at google.com
Cc: mka at chromium.org
Cc: pombredanne at nexb.com
Cc: rientjes at google.com
Cc: rostedt at goodmis.org
Cc: sedat.dilek at gmail.com
Cc: thomas.lendacky at amd.com
Cc: tstellar at redhat.com
Cc: tweek at google.com
Cc: virtualization at lists.linux-foundation.org
Cc: will.deacon at arm.com
Cc: yamada.masahiro at socionext.com
Link: http://lkml.kernel.org/r/20180621162324.36656-2-ndesaulniers at google.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>

---
 include/linux/compiler-gcc.h |   29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -65,25 +65,40 @@
 #endif
 
 /*
+ * Feature detection for gnu_inline (gnu89 extern inline semantics). Either
+ * __GNUC_STDC_INLINE__ is defined (not using gnu89 extern inline semantics,
+ * and we opt in to the gnu89 semantics), or __GNUC_STDC_INLINE__ is not
+ * defined so the gnu89 semantics are the default.
+ */
+#ifdef __GNUC_STDC_INLINE__
+# define __gnu_inline	__attribute__((gnu_inline))
+#else
+# define __gnu_inline
+#endif
+
+/*
  * Force always-inline if the user requests it so via the .config,
  * or if gcc is too old.
  * GCC does not warn about unused static inline functions for
  * -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.
+ * A lot of inline functions can cause havoc with function tracing.
  */
 #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)) notrace __gnu_inline
 #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)) notrace __gnu_inline
 #endif
 
+#define __inline__ inline
+#define __inline inline
 #define __always_inline	inline __attribute__((always_inline))
 #define  noinline	__attribute__((noinline))