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From 8ba6fb2fa507aec278bce9fec42b38be2231588e Mon Sep 17 00:00:00 2001
From: Alexey Dobriyan <adobriyan@gmail.com>
Date: Tue, 12 Sep 2023 19:22:24 +0300
Subject: [PATCH 5/5] uapi: stddef.h: Fix __DECLARE_FLEX_ARRAY for C++
[ Upstream commit 32a4ec211d4164e667d9d0b807fadf02053cd2e9 ]
__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 to
T member[];
Now g++ doesn't like "T member[]" either, throwing errors on
the following code:
struct S {
union {
T1 member1[];
T2 member2[];
};
};
or
struct S {
T member[];
};
Use "T member[0];" which seems to work and does the right thing wrt
structure layout.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Fixes: 3080ea5553cc ("stddef: Introduce DECLARE_FLEX_ARRAY() helper")
Link: https://lore.kernel.org/r/97242381-f1ec-4a4a-9472-1a464f575657@p183
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 144ed54ed6ec4c099ff7c2b0064ed3ff89b73737)
---
include/uapi/linux/stddef.h | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h
index c027b2070d79..5c6c4269f7ef 100644
--- a/include/uapi/linux/stddef.h
+++ b/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
*
@@ -44,6 +49,7 @@
struct { } __empty_ ## NAME; \
TYPE NAME[]; \
}
+#endif
#ifndef __counted_by
#define __counted_by(m)
--
2.42.0
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