linux-kernel - [PATCH] uapi: propagate __struct_group() attributes to the container union

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Message-ID: <20231120110607.98956-1-dmantipov@yandex.ru>
Date:   Mon, 20 Nov 2023 14:05:08 +0300
From:   Dmitry Antipov <dmantipov@...dex.ru>
To:     Kees Cook <keescook@...omium.org>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Will Deacon <will@...nel.org>, Kalle Valo <kvalo@...nel.org>,
        linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dmitry Antipov <dmantipov@...dex.ru>,
        kernel test robot <lkp@...el.com>
Subject: [PATCH] uapi: propagate __struct_group() attributes to the container union

Recently the kernel test robot has reported an ARM-specific BUILD_BUG_ON()
in an old and unmaintained wil6210 wireless driver. The problem comes from
the structure packing rules of old ARM ABI ('-mabi=apcs-gnu'). For example,
the following structure is packed to 18 bytes instead of 16:

struct poorly_packed {
        unsigned int a;
        unsigned int b;
        unsigned short c;
        union {
                struct {
                        unsigned short d;
                        unsigned int e;
                } __attribute__((packed));
                struct {
                        unsigned short d;
                        unsigned int e;
                } __attribute__((packed)) inner;
        };
} __attribute__((packed));

To fit it into 16 bytes, it's required to add packed attribute to the
container union as well:

struct poorly_packed {
        unsigned int a;
        unsigned int b;
        unsigned short c;
        union {
                struct {
                        unsigned short d;
                        unsigned int e;
                } __attribute__((packed));
                struct {
                        unsigned short d;
                        unsigned int e;
                } __attribute__((packed)) inner;
        } __attribute__((packed));
} __attribute__((packed));

Thanks to Andrew Pinski of GCC team for sorting the things out at
https://gcc.gnu.org/pipermail/gcc/2023-November/242888.html.

Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311150821.cI4yciFE-lkp@intel.com
Signed-off-by: Dmitry Antipov <dmantipov@...dex.ru>
---
 include/uapi/linux/stddef.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h
index 5c6c4269f7ef..2ec6f35cda32 100644
--- a/include/uapi/linux/stddef.h
+++ b/include/uapi/linux/stddef.h
@@ -27,7 +27,7 @@
 	union { \
 		struct { MEMBERS } ATTRS; \
 		struct TAG { MEMBERS } ATTRS NAME; \
-	}
+	} ATTRS
 
 #ifdef __cplusplus
 /* sizeof(struct{}) is 1 in C++, not 0, can't use C version of the macro. */
-- 
2.42.0