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

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__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: 3080ea5553 ("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>
This commit is contained in:
Alexey Dobriyan
2023-09-12 19:22:24 +03:00
committed by Kees Cook
parent 531108ec5b
commit 32a4ec211d
1 changed files with 6 additions and 0 deletions
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6
include/uapi/linux/stddef.h
View File

@@ -29,6 +29,11 @@
struct TAG { MEMBERS } ATTRS NAME; \ 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 * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
* *
@@ -44,6 +49,7 @@
struct { } __empty_ ## NAME; \ struct { } __empty_ ## NAME; \
TYPE NAME[]; \ TYPE NAME[]; \
} }
#endif
#ifndef __counted_by #ifndef __counted_by
#define __counted_by(m) #define __counted_by(m)