hardening fixes for v6.6-rc3

- Fix UAPI stddef.h to avoid C++-ism (Alexey Dobriyan)
 
 - Fix harmless UAPI stddef.h header guard endif (Alexey Dobriyan)
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Merge tag 'hardening-v6.6-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull hardening fixes from Kees Cook:

 - Fix UAPI stddef.h to avoid C++-ism (Alexey Dobriyan)

 - Fix harmless UAPI stddef.h header guard endif (Alexey Dobriyan)

* tag 'hardening-v6.6-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  uapi: stddef.h: Fix __DECLARE_FLEX_ARRAY for C++
  uapi: stddef.h: Fix header guard location

include/uapi/linux/stddef.h

@@ -29,6 +29,11 @@
 		struct TAG { MEMBERS } ATTRS NAME; \
 	}
 
+#ifdef __cplusplus
+/* sizeof(struct{}) is 1 in C++, not 0, can't use C version of the macro. */
+#define __DECLARE_FLEX_ARRAY(T, member)	\
+	T member[0]
+#else
 /**
  * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
  *
@@ -49,3 +54,5 @@
 #ifndef __counted_by
 #define __counted_by(m)
 #endif
+
+#endif /* _UAPI_LINUX_STDDEF_H */