* [PATCH] can: uapi: can.h: mark union inside struct can_frame packed
@ 2021-03-22 10:28 Marc Kleine-Budde
2021-03-22 16:27 ` Oliver Hartkopp
0 siblings, 1 reply; 4+ messages in thread
From: Marc Kleine-Budde @ 2021-03-22 10:28 UTC (permalink / raw)
To: linux-can; +Cc: kernel, Marc Kleine-Budde, Rong Chen, kernel test robot
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.
Fixes: ea7800565a12 ("can: add optional DLC element to Classical CAN frame structure")
Suggested-by: Rong Chen <rong.a.chen@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
---
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..9842bb55ffd9 100644
--- a/include/uapi/linux/can.h
+++ b/include/uapi/linux/can.h
@@ -113,7 +113,7 @@ struct can_frame {
*/
__u8 len;
__u8 can_dlc; /* deprecated */
- };
+ } __attribute__((packed));
__u8 __pad; /* padding */
__u8 __res0; /* reserved / padding */
__u8 len8_dlc; /* optional DLC for 8 byte payload length (9 .. 15) */
--
2.30.2
^ permalink raw reply related [flat|nested] 4+ messages in thread* Re: [PATCH] can: uapi: can.h: mark union inside struct can_frame packed
2021-03-22 10:28 [PATCH] can: uapi: can.h: mark union inside struct can_frame packed Marc Kleine-Budde
@ 2021-03-22 16:27 ` Oliver Hartkopp
2021-03-23 8:24 ` Marc Kleine-Budde
0 siblings, 1 reply; 4+ messages in thread
From: Oliver Hartkopp @ 2021-03-22 16:27 UTC (permalink / raw)
To: Marc Kleine-Budde, linux-can; +Cc: kernel, Rong Chen, kernel test robot
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?
Shouldn't this problem cause the developer to update the compiler?
https://lore.kernel.org/linux-can/f8075a19-10e1-abf9-6d59-1a46454b74b1@hartkopp.net/T/#u
Regards,
Oliver
>
> Fixes: ea7800565a12 ("can: add optional DLC element to Classical CAN frame structure")
> Suggested-by: Rong Chen <rong.a.chen@intel.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
> ---
> 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..9842bb55ffd9 100644
> --- a/include/uapi/linux/can.h
> +++ b/include/uapi/linux/can.h
> @@ -113,7 +113,7 @@ struct can_frame {
> */
> __u8 len;
> __u8 can_dlc; /* deprecated */
> - };
> + } __attribute__((packed));
> __u8 __pad; /* padding */
> __u8 __res0; /* reserved / padding */
> __u8 len8_dlc; /* optional DLC for 8 byte payload length (9 .. 15) */
>
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [PATCH] can: uapi: can.h: mark union inside struct can_frame packed
2021-03-22 16:27 ` Oliver Hartkopp
@ 2021-03-23 8:24 ` Marc Kleine-Budde
2021-03-23 9:37 ` Rong Chen
0 siblings, 1 reply; 4+ messages in thread
From: Marc Kleine-Budde @ 2021-03-23 8:24 UTC (permalink / raw)
To: Oliver Hartkopp; +Cc: linux-can, kernel, Rong Chen, kernel test robot
[-- Attachment #1: Type: text/plain, Size: 2847 bytes --]
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@hartkopp.net/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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^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [PATCH] can: uapi: can.h: mark union inside struct can_frame packed
2021-03-23 8:24 ` Marc Kleine-Budde
@ 2021-03-23 9:37 ` Rong Chen
0 siblings, 0 replies; 4+ messages in thread
From: Rong Chen @ 2021-03-23 9:37 UTC (permalink / raw)
To: Marc Kleine-Budde, Oliver Hartkopp; +Cc: linux-can, kernel, kernel test robot
On 3/23/21 4:24 PM, Marc Kleine-Budde wrote:
> 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?
Hi Marc,
I tried the below two compilers , and detail is at
https://lore.kernel.org/linux-can/f8075a19-10e1-abf9-6d59-1a46454b74b1@hartkopp.net/T/#m1eff42a8f6c24ebf066fa74a826a0ca068457ed8
https://download.01.org/0day-ci/cross-package/gcc-9.3.0-nolibc/x86_64-gcc-9.3.0-nolibc_arm-linux-gnueabi.tar.xz
http://cdn.kernel.org/pub/tools/crosstool/files/bin/x86_64/10.1.0/x86_64-gcc-10.1.0-nolibc-arm-linux-gnueabi.tar.xz
Best Regards,
Rong Chen
>
>> https://lore.kernel.org/linux-can/f8075a19-10e1-abf9-6d59-1a46454b74b1@hartkopp.net/T/#u
> regards,
> Marc
>
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2021-03-22 10:28 [PATCH] can: uapi: can.h: mark union inside struct can_frame packed Marc Kleine-Budde
2021-03-22 16:27 ` Oliver Hartkopp
2021-03-23 8:24 ` Marc Kleine-Budde
2021-03-23 9:37 ` Rong Chen
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