Commit 32a4ec21 authored by Alexey Dobriyan's avatar Alexey Dobriyan Committed by Kees Cook
Browse files

uapi: stddef.h: 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: default avatarAlexey Dobriyan <adobriyan@gmail.com>
Fixes: 3080ea55 ("stddef: Introduce DECLARE_FLEX_ARRAY() helper")
Link: https://lore.kernel.org/r/97242381-f1ec-4a4a-9472-1a464f575657@p183


Signed-off-by: default avatarKees Cook <keescook@chromium.org>
parent 531108ec

include/uapi/linux/stddef.h

+6 −0
Original line number Diff line number Diff line
@@ -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)