[PATCH] have ifunc resolver's return type match target

[Martin Sebor <msebor@gmail.com>]

The following GCC patch has been submitted for review.  It
helps detect mismatches between the type of an ifunc or alias
declaration and the type of the resolver or alias.

   https://gcc.gnu.org/ml/gcc-patches/2017-08/msg01103.html

To let Glibc take advantage of this type checking and avoid
warnings when using the patched GCC when the change above is
committed, the patch below adjusts the Glibc __ifunc_resolver
macro to declare the ifunc resolver so that its return type
matches that of the target.  (I was going to wait to submit it
until after the GCC patch has been accepted but per Joseph's
suggestion I'm posting it here ahead of time.)

The patch has been tested on its own with the system GCC 6.3
and with the patched GCC on x86_64-linux with no regressions.

Martin

diff --git a/include/libc-symbols.h b/include/libc-symbols.h
index fe3ab81..5413e56 100644
--- a/include/libc-symbols.h
+++ b/include/libc-symbols.h
@@ -790,7 +790,8 @@ for linking")

  /* Helper / base  macros for indirect function symbols.  */
  #define __ifunc_resolver(type_name, name, expr, arg, init, classifier) \
-  classifier inhibit_stack_protector void *name##_ifunc (arg) 
              \
+  classifier inhibit_stack_protector                                   \
+  __typeof (type_name) *name##_ifunc (arg)                             \
    {                                                                    \
      init ();                                                           \
      __typeof (type_name) *res = expr;                                  \