[PATCH] stddef: make __struct_group() UAPI C++-friendly

[Alexander Lobakin <aleksander.lobakin@intel.com>]

For the most part of 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: 50d7bd38c3aa ("stddef: Introduce struct_group() helper macro")
Reported-by: Christopher Ferris <cferris@google.com>
Closes: https://lore.kernel.org/linux-hardening/Z1HZpe3WE5As8UAz@google.com
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
---
 include/uapi/linux/stddef.h       | 12 ++++++++++--
 tools/include/uapi/linux/stddef.h | 14 +++++++++++---
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h
index 58154117d9b0..ad51b1362bbb 100644
--- a/include/uapi/linux/stddef.h
+++ b/include/uapi/linux/stddef.h
@@ -20,14 +20,22 @@
  * 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.
  */
+#ifndef __cplusplus
 #define __struct_group(TAG, NAME, ATTRS, MEMBERS...) \
 	union { \
 		struct { MEMBERS } ATTRS; \
 		struct TAG { MEMBERS } ATTRS NAME; \
 	} ATTRS
+#else
+#define __struct_group(__IGNORED, NAME, ATTRS, MEMBERS...) \
+	union { \
+		struct { MEMBERS } ATTRS; \
+		struct { MEMBERS } ATTRS NAME; \
+	} ATTRS
+#endif
 
 #ifdef __cplusplus
 /* sizeof(struct{}) is 1 in C++, not 0, can't use C version of the macro. */
diff --git a/tools/include/uapi/linux/stddef.h b/tools/include/uapi/linux/stddef.h
index bb6ea517efb5..12b854ecc215 100644
--- a/tools/include/uapi/linux/stddef.h
+++ b/tools/include/uapi/linux/stddef.h
@@ -20,14 +20,22 @@
  * 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.
  */
+#ifndef __cplusplus
 #define __struct_group(TAG, NAME, ATTRS, MEMBERS...) \
 	union { \
 		struct { MEMBERS } ATTRS; \
 		struct TAG { MEMBERS } ATTRS NAME; \
-	}
+	} ATTRS
+#else
+#define __struct_group(__IGNORED, NAME, ATTRS, MEMBERS...) \
+	union { \
+		struct { MEMBERS } ATTRS; \
+		struct { MEMBERS } ATTRS NAME; \
+	} ATTRS
+#endif
 
 /**
  * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
-- 
2.47.1