From 27b581d4601a63173b3770ad24d382c8284d5f0b Mon Sep 17 00:00:00 2001 From: Alexey Dobriyan Date: Tue, 12 Sep 2023 19:22:24 +0300 Subject: [PATCH] uapi: stddef.h: Fix __DECLARE_FLEX_ARRAY for C++ [ 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 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 Signed-off-by: Sasha Levin --- include/uapi/linux/stddef.h | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h index 7837ba4fe..46c7bab50 100755 --- 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 * @@ -45,3 +50,5 @@ TYPE NAME[]; \ } #endif + +#endif