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@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311150821.cI4yciFE-lkp@intel.com
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Link: https://lore.kernel.org/r/20231120110607.98956-1-dmantipov@yandex.ru
Fixes: 50d7bd38c3aa ("stddef: Introduce struct_group() helper macro")
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h
index 5c6c4269f7efe4d649366907191adfbcffef41e4..2ec6f35cda32e9e61afbbe8708f4bbb5bf1e39f7 100644 (file)
--- 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. */