linux-kernel - Re: [kbuild-all] Re: include/linux/compiler_types.h:315:38: error: call to '__compiletime_assert_536' declared with attribute error: BUILD_BUG_ON failed: offsetof(struct can_frame, len) != offsetof(struct canfd_frame, len) || offsetof(struct can

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Date:   Tue, 23 Mar 2021 09:32:10 +0100
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Marc Kleine-Budde <mkl@...gutronix.de>,
        Rong Chen <rong.a.chen@...el.com>,
        Patrick Menschel <menschel.p@...teo.de>
Cc:     kernel test robot <lkp@...el.com>, kbuild-all@...ts.01.org,
        linux-kernel@...r.kernel.org, linux-can <linux-can@...r.kernel.org>
Subject: Re: [kbuild-all] Re: include/linux/compiler_types.h:315:38: error:
 call to '__compiletime_assert_536' declared with attribute error:
 BUILD_BUG_ON failed: offsetof(struct can_frame, len) != offsetof(struct
 canfd_frame, len) || offsetof(struct can_frame, data) != offsetof(struc...

Answering myself ...

On 23.03.21 08:45, Oliver Hartkopp wrote:
> On 23.03.21 08:34, Marc Kleine-Budde wrote:
>> On 23.03.2021 10:54:40, Rong Chen wrote:
>>> I tried arm-linux-gnueabi (gcc version 10.2.0) and the problem still
>>> exists, btw we prefer to not use the latest gcc compiler to avoid
>>> false positives.
>>
>> FWIW:
>>
>> I'm using latest debian arm compiler and the BUILD_BUG never triggered.
>> gcc version 10.2.1 20210110 (Debian 10.2.1-6)
>>

@Rong / Marc:

I wonder if the compiler configurations (gcc -v) or the options used at 
kernel build time are identical.

Maybe there is a different optimization option selected which causes the 
compiler to extend the u8 union to a 32 bit space?!?

And maybe Debian is a bit more conservative in selecting their 
optimizations than the setup that Rong was using for the build ...

Best,
Oliver

> 
> Thanks Marc!
> 
> IMO we facing a compiler problem here - and we should be very happy that 
> the BUILD_BUG_ON() triggered an issue after years of silence.
> 
> I do not have a good feeling about what kind of strange effects this 
> compiler issue might have in other code of other projects.
> 
> So I would explicitly suggest NOT to change the af_can.c code to work 
> around this compiler issue.
> 
> Let the gcc people fix their product and let them thank all of us for 
> detecting it.
> 
> Regards,
> Oliver