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H A Dstddef.h32a4ec21 Tue Sep 12 11:22:24 CDT 2023 Alexey Dobriyan <adobriyan@gmail.com> 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: 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>