stddef: make __struct_group() UAPI C++-friendly

For the most part of the C++ history, it couldn't have type
declarations inside anonymous unions for different reasons. At the
same time, __struct_group() relies on the latters, so when the @tag
argument is not empty, C++ code doesn't want to build (even under
`extern "C"`):

../linux/include/uapi/linux/pkt_cls.h:25:24: error:
'struct tc_u32_sel::<unnamed union>::tc_u32_sel_hdr,' invalid;
an anonymous union may only have public non-static data members
[-fpermissive]

The safest way to fix this without trying to switch standards (which
is impossible in UAPI anyway) etc., is to disable tag declaration
for that language. This won't break anything since for now it's not
buildable at all.
Use a separate definition for __struct_group() when __cplusplus is
defined to mitigate the error, including the version from tools/.

Fixes: 50d7bd3 ("stddef: Introduce struct_group() helper macro")
Reported-by: Christopher Ferris <[email protected]>
Closes: https://lore.kernel.org/linux-hardening/[email protected]
Suggested-by: Kees Cook <[email protected]> # __struct_group_tag()
Signed-off-by: Alexander Lobakin <[email protected]>
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: alobakin

include/uapi/linux/stddef.h

@@ -8,6 +8,13 @@
 #define __always_inline inline
 #endif
 
+/* Not all C++ standards support type declarations inside an anonymous union */
+#ifndef __cplusplus
+#define __struct_group_tag(TAG)		TAG
+#else
+#define __struct_group_tag(TAG)
+#endif
+
 /**
  * __struct_group() - Create a mirrored named and anonyomous struct
  *
@@ -20,13 +27,13 @@
  * and size: one anonymous and one named. The former's members can be used
  * normally without sub-struct naming, and the latter can be used to
  * reason about the start, end, and size of the group of struct members.
- * The named struct can also be explicitly tagged for layer reuse, as well
- * as both having struct attributes appended.
+ * The named struct can also be explicitly tagged for layer reuse (C only),
+ * as well as both having struct attributes appended.
  */
 #define __struct_group(TAG, NAME, ATTRS, MEMBERS...) \
 	union { \
 		struct { MEMBERS } ATTRS; \
-		struct TAG { MEMBERS } ATTRS NAME; \
+		struct __struct_group_tag(TAG) { MEMBERS } ATTRS NAME; \
 	} ATTRS
 
 #ifdef __cplusplus

tools/include/uapi/linux/stddef.h

@@ -8,6 +8,13 @@
 #define __always_inline __inline__
 #endif
 
+/* Not all C++ standards support type declarations inside an anonymous union */
+#ifndef __cplusplus
+#define __struct_group_tag(TAG)		TAG
+#else
+#define __struct_group_tag(TAG)
+#endif
+
 /**
  * __struct_group() - Create a mirrored named and anonyomous struct
  *
@@ -20,14 +27,14 @@
  * and size: one anonymous and one named. The former's members can be used
  * normally without sub-struct naming, and the latter can be used to
  * reason about the start, end, and size of the group of struct members.
- * The named struct can also be explicitly tagged for layer reuse, as well
- * as both having struct attributes appended.
+ * The named struct can also be explicitly tagged for layer reuse (C only),
+ * as well as both having struct attributes appended.
  */
 #define __struct_group(TAG, NAME, ATTRS, MEMBERS...) \
 	union { \
 		struct { MEMBERS } ATTRS; \
-		struct TAG { MEMBERS } ATTRS NAME; \
-	}
+		struct __struct_group_tag(TAG) { MEMBERS } ATTRS NAME; \
+	} ATTRS
 
 /**
  * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union