Merge #17033: Disable _FORTIFY_SOURCE when enable-debug

44f7a8d7a7 Disable _FORTIFY_SOURCE when enable-debug (Andrew Chow)

Pull request description:

  The `_FORTIFY_SOURCE` macro is enabled by default when hardening is enabled, but it requires optimization in order to be used. Since we disable all optimization with `--enable-debug`, this macro doesn't actually do anything and instead just causes a lot of warnings to be printed. This PR explicitly disables `_FORTIFY_SOURCE` so that these useless warnings aren't printed.

ACKs for top commit:
  laanwj:
    Thanks. ACK 44f7a8d7a7

Tree-SHA512: e9302aef794dfd9ca9d0d032179ecc51d3212a9a0204454419f410011343b27c32e6be05f385051b5b594c607b91b8e0e588f644584d6684429a649a413077d9

configure.ac

@@ -753,12 +753,17 @@ if test x$use_hardening != xno; then
   AX_CHECK_COMPILE_FLAG([-Wstack-protector],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -Wstack-protector"])
   AX_CHECK_COMPILE_FLAG([-fstack-protector-all],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-protector-all"])
 
-  AX_CHECK_PREPROC_FLAG([-D_FORTIFY_SOURCE=2],[
-    AX_CHECK_PREPROC_FLAG([-U_FORTIFY_SOURCE],[
-      HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -U_FORTIFY_SOURCE"
+  # When enable_debug is yes, all optimizations are disabled.
+  # However, FORTIFY_SOURCE requires that there is some level of optimization, otherwise it does nothing and just creates a compiler warning.
+  # Since FORTIFY_SOURCE is a no-op without optimizations, do not enable it when enable_debug is yes.
+  if test x$enable_debug != xyes; then
+    AX_CHECK_PREPROC_FLAG([-D_FORTIFY_SOURCE=2],[
+      AX_CHECK_PREPROC_FLAG([-U_FORTIFY_SOURCE],[
+        HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -U_FORTIFY_SOURCE"
+      ])
+      HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -D_FORTIFY_SOURCE=2"
     ])
-    HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -D_FORTIFY_SOURCE=2"
-  ])
+  fi
 
   AX_CHECK_LINK_FLAG([[-Wl,--dynamicbase]], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,--dynamicbase"])
   AX_CHECK_LINK_FLAG([[-Wl,--nxcompat]], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,--nxcompat"])