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Message-ID: <97242381-f1ec-4a4a-9472-1a464f575657@p183>
Date: Tue, 12 Sep 2023 19:22:24 +0300
From: Alexey Dobriyan <adobriyan@...il.com>
To: akpm@...ux-foundation.org, keescook@...omium.org
Cc: linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
linux-hardening@...r.kernel.org, David.Laight@...lab.com
Subject: [PATCH v3 1/2] uapi: fix __DECLARE_FLEX_ARRAY for C++
__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@...il.com>
Fixes: 3080ea5553cc ("stddef: Introduce DECLARE_FLEX_ARRAY() helper")
---
include/uapi/linux/stddef.h | 6 ++++++
1 file changed, 6 insertions(+)
--- 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
*
@@ -45,6 +50,7 @@
TYPE NAME[]; \
}
#endif
+#endif
#ifndef __counted_by
#define __counted_by(m)
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