2023-09-13 01:11:27

by Alexey Dobriyan

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 <[email protected]>
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)

2023-09-16 14:07:59

by Kees Cook

Subject: Re: [PATCH v3 1/2] uapi: fix __DECLARE_FLEX_ARRAY for C++

On Tue, Sep 12, 2023 at 07:22:24PM +0300, Alexey Dobriyan wrote:
> __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.

Looking at this again just now, what about using a 0-length array
instead of an anonymous struct?

https://godbolt.org/z/rGaxPWjef

Then we don't need an #ifdef at all...

struct {
int __empty_member[0];
T member[];
};

-Kees

--
Kees Cook