In commit ea7800565a12 ("can: add optional DLC element to Classical
CAN frame structure") the struct can_frame::can_dlc was put into an
anonymous union with another u8 variable.
For various reasons some members in struct can_frame and canfd_frame
including the first 8 byes of data are expected to have the same
memory layout. This is enforced by a BUILD_BUG_ON check in af_can.c.
Since the above mentioned commit this check fails on some compilers
(e.g. arm-linux-gnueabi-gcc (GCC) 9.3.0) that apply an alignment on
an (anonymous) union (see Link). Rong Chen analyzed the problem and
found that the union in struct can_frame takes 4 bytes instead of
the expected 1:
| struct can_frame {
| canid_t can_id; /* 0 4 */
| union {
| __u8 len; /* 4 1 */
| __u8 can_dlc; /* 4 1 */
| }; /* 4 4 */
| __u8 __pad; /* 8 1 */
| __u8 __res0; /* 9 1 */
| __u8 len8_dlc; /* 10 1 */
|
| /* XXX 5 bytes hole, try to pack */
|
| __u8 data[8]
| __attribute__((__aligned__(8))); /* 16 8 */
|
| /* size: 24, cachelines: 1, members: 6 */
| /* sum members: 19, holes: 1, sum holes: 5 */
| /* forced alignments: 1, forced holes: 1, sum forced holes: 5 */
| /* last cacheline: 24 bytes */
| } __attribute__((__aligned__(8)));
Marking the anonymous union as __attribute__((packed)) fixes the
BUILD_BUG_ON problem on these compilers.
Fixes: ea7800565a12 ("can: add optional DLC element to Classical CAN frame structure")
Link: https://lore.kernel.org/linux-can/2c82ec23-3551-61b5-1bd8-178c3407ee83@xxxxxxxxxxxx/
Suggested-by: Rong Chen <rong.a.chen@xxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Author: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
Signed-off-by: Oliver Hartkopp <socketcan@xxxxxxxxxxxx>
---
include/uapi/linux/can.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/uapi/linux/can.h b/include/uapi/linux/can.h
index f75238ac6dce..56abe9b50561 100644
--- a/include/uapi/linux/can.h
+++ b/include/uapi/linux/can.h
@@ -111,11 +111,11 @@ struct can_frame {
* was previously named can_dlc so we need to carry that
* name for legacy support
*/
__u8 len;
__u8 can_dlc; /* deprecated */
- };
+ } __attribute__((packed)); /* disable padding by some compilers */
__u8 __pad; /* padding */
__u8 __res0; /* reserved / padding */
__u8 len8_dlc; /* optional DLC for 8 byte payload length (9 .. 15) */
__u8 data[CAN_MAX_DLEN] __attribute__((aligned(8)));
};
--
2.30.2
