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Date:   Fri, 8 Sep 2023 14:32:24 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     akpm@linux-foundation.org
Cc:     linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        keescook@chromium.org
Subject: [PATCH] uapi: partially fix __DECLARE_FLEX_ARRAY for C++
Message-ID: <930c3ee5-1282-40f4-93e0-8ff894aabf3a@p183>
MIME-Version: 1.0
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Precedence: bulk

__DECLARE_FLEX_ARRAY(T, member) macro expands to

	struct {
		struct {} __empty_member;
		T member[];
	};

which is subtly wrong in C++ because sizeof(struct{}) is 1 not 0,
changing UAPI structures layouts.

This can be fixed by expanding simply to

	T member[];

Most of the usages still be broken because of other C++ shenanigans
but this fixes simplest usage: 1 flexible member at the end.

Fix header guard while I'm at it.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 include/uapi/linux/stddef.h |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/include/uapi/linux/stddef.h
+++ b/include/uapi/linux/stddef.h
@@ -39,6 +39,10 @@
  * struct, it needs to be wrapped in an anonymous struct with at least 1
  * named member, but that member can be empty.
  */
+#ifdef __cplusplus
+#define __DECLARE_FLEX_ARRAY(T, member)		\
+	T member[]
+#else
 #define __DECLARE_FLEX_ARRAY(TYPE, NAME)	\
 	struct { \
 		struct { } __empty_ ## NAME; \
@@ -49,3 +53,5 @@
 #ifndef __counted_by
 #define __counted_by(m)
 #endif
+
+#endif