On 22.03.2021 17:27:38, Oliver Hartkopp wrote:
>
>
> On 22.03.21 11:28, Marc Kleine-Budde wrote:
> > 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 at least one compiler
> > (arm-linux-gnueabi-gcc (GCC) 9.3.0). 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 union as packed fixes the problem.
>
> Is this a proper answer to this issue?
With the affected compiler, yes. This patch makes the union 1 byte long,
as expected. This effectively fixes compiling.
I'm glad that the build bug triggered, which avoids having a broken
running kernel.
> Shouldn't this problem cause the developer to update the compiler?
The question is, are the other silent corruptions with a similar union
somewhere? Maybe we should escalate this problem to the linux-arm-kernel
ML.
Rong Chen, can I download the compiler you're using somewhere?
> https://lore.kernel.org/linux-can/f8075a19-10e1-abf9-6d59-1a46454b74b1@xxxxxxxxxxxx/T/#u
regards,
Marc
--
Pengutronix e.K. | Marc Kleine-Budde |
Embedded Linux | https://www.pengutronix.de |
Vertretung West/Dortmund | Phone: +49-231-2826-924 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
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