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uapi: stddef.h: Fix __DECLARE_FLEX_ARRAY for C++

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[ 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>
This commit is contained in:
Alexey Dobriyan 2023-09-12 19:22:24 +03:00 • committed by Ksawlii
parent bfd4f20076
commit 27b581d460
1 changed files with 7 additions and 0 deletions
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7
include/uapi/linux/stddef.h
View file

@ -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
*
@ -45,3 +50,5 @@
TYPE NAME[]; \
}
#endif
#endif