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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...
To:     Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Cc:     Oliver Hartkopp <socketcan@hartkopp.net>,
        kernel test robot <lkp@intel.com>, kbuild-all@lists.01.org,
        open list <linux-kernel@vger.kernel.org>,
        Marc Kleine-Budde <mkl@pengutronix.de>,
        linux-can <linux-can@vger.kernel.org>
References: <202103210435.I0fiBGAC-lkp@intel.com>
 <dad98ebd-77a4-3305-e681-278cabe38793@hartkopp.net>
 <7f4f7e1c-194b-a903-d474-e3b742556a55@intel.com>
 <f8075a19-10e1-abf9-6d59-1a46454b74b1@hartkopp.net>
 <b10903ca-c424-b305-d981-fe0004500190@intel.com>
 <CAMZ6RqK0-7DaoscTgKD+APDxtPw1q0Dz0Kef_doa0PZOnBav=w@mail.gmail.com>
From:   Rong Chen <rong.a.chen@intel.com>
Message-ID: <52e57719-7b55-b21a-5e30-4be2fb4e776c@intel.com>
Date:   Tue, 23 Mar 2021 14:06:21 +0800
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Hi Vincent,

On 3/23/21 1:46 PM, Vincent MAILHOL wrote:
> Hi Oliver and Rong,
>
> This is an interesting and quite surprising issue!
>
> On Tue. 23 mars 2021 at 11:54, Rong Chen <rong.a.chen@intel.com> wrote:
>> On 3/23/21 12:24 AM, Oliver Hartkopp wrote:
>>> Hi Rong,
>>>
>>> On 22.03.21 09:52, Rong Chen wrote:
>>>
>>>> On 3/21/21 10:19 PM, Oliver Hartkopp wrote:
>>>>> Two reminders in two days? ;-)
>>>>>
>>>>> Did you check my answer here?
>>>>> https://lore.kernel.org/lkml/afffeb73-ba4c-ca2c-75d0-9e7899e5cbe1@hartkopp.net/
>>>>>
>>>>>
>>>>> And did you try the partly revert?
>>>> Hi Oliver,
>>>>
>>>> Sorry for the delay, we tried the revert patch and the problem still
>>>> exists,
>>>> we also found that commit c7b74967 changed the error message which
>>>> triggered
>>>> the report.
>>>>
>>>> The problem is that offsetof(struct can_frame, data) !=
>>>> offsetof(struct canfd_frame, data)
>>>> the following struct layout shows that the offset has been changed by
>>>> union:
>>>>
>>>> struct can_frame {
>>>>           canid_t                    can_id;               /* 0     4 */
>>>>           union {
>>>>                   __u8               len;                  /* 4     1 */
>>>>                   __u8               can_dlc;              /* 4     1 */
>>>>           };                                               /* 4     4 */
>>> Ugh! Why did the compiler extend the space for the union to 4 bytes?!?
> Just a random idea but maybe the added padding is due to some
> kind of odd intrication with the __attribute__((__aligned__(8)))
> just below? Does this reproduce if we remove the
> __attribute__((__aligned__(8)))?

Here is the layout without __attribute__((__aligned__(8))),
the union is still extended to 4 bytes:

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 */
         __u8                       data[8];              /* 11     8 */

         /* size: 20, cachelines: 1, members: 6 */
         /* padding: 1 */
         /* last cacheline: 20 bytes */
};

Best Regards,
Rong Chen

>
> (I am not saying that we should permanently remove it, this is
> only a suggestion for troubleshooting).
>
>>>>           __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)));
>>>>
>>>> struct canfd_frame {
>>>>           canid_t                    can_id;               /* 0     4 */
>>>>           __u8                       len;                  /* 4     1 */
>>>>           __u8                       flags;                /* 5     1 */
>>>>           __u8                       __res0;               /* 6     1 */
>>>>           __u8                       __res1;               /* 7     1 */
>>>>           __u8                       data[64]
>>>> __attribute__((__aligned__(8))); /*     8    64 */
>>>>
>>>>           /* size: 72, cachelines: 2, members: 6 */
>>>>           /* forced alignments: 1 */
>>>>           /* last cacheline: 8 bytes */
>>>> } __attribute__((__aligned__(8)))
>>>>
>>>>
>>>> and I tried to add "__attribute__((packed))" to the union, the issue
>>>> is gone:
>>>>
>>>> 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) */
>>> This is pretty strange!
>>>
>>> pahole on my x86_64 machine shows the correct data structure layout:
>>>
>>> struct can_frame {
>>>          canid_t                    can_id;               /* 0     4 */
>>>          union {
>>>                  __u8               len;                  /* 4     1 */
>>>                  __u8               can_dlc;              /* 4     1 */
>>>          };                                               /* 4     1 */
>>>          __u8                       __pad;                /* 5     1 */
>>>          __u8                       __res0;               /* 6     1 */
>>>          __u8                       len8_dlc;             /* 7     1 */
>>>          __u8                       data[8]
>>> __attribute__((__aligned__(8))); /*     8     8 */
>>>
>>>          /* size: 16, cachelines: 1, members: 6 */
>>>          /* forced alignments: 1 */
>>>          /* last cacheline: 16 bytes */
>>> } __attribute__((__aligned__(8)));
>>>
>>> Target: x86_64-linux-gnu
>>> gcc version 10.2.1 20210110 (Debian 10.2.1-6)
>>> Linux 5.12.0-rc3-00070-g8b12a62a4e3e x86_64 GNU/Linux
>>>
>>> So it looks like your compiler does not behave correctly - and I
>>> wonder if it would be the correct approach to add the __packed()
>>> attribute or better fix/change the (ARM) compiler.
> I had a look at the ISO/IEC 9899-1999 (aka C99 standard). In
> section 6.7.2.1 "Structure and union specifiers", there are no
> clauses to forbid this behavior...
> Here are the relevant clauses of that section:
>    * 12 Each non-bit-field member of a structure or union object
>      is aligned in an implementation-defined appropriate to its
>      type.
>    * 13 [...] There may be unnamed padding within a structure
>      object, but not at its beginning.
>    * 14 The size of a union is sufficient to contain the largest
>      of its members. [...]
>    * 15 There may be unnamed padding at the end of a structure or
>      union.
>
> So while I am really curious to understand why the compiler
> behaves like that, technically speaking, it does not violate the
> standard. As such, I think that Mark's patch (which negates
> clause 15) makes sense.
>
>> Hi Oliver,
>>
>> 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.
>>
>> Best Regards,
>> Rong Chen
>>
>>> At least I'm very happy that the BUILD_BUG_ON() triggered correctly -
>>> so it was worth to have it ;-)
>>>
>>> Best regards,
>>> Oliver
>>>
>>>
>>>>> Maybe there's a mismatch in include files - or BUILD_BUG_ON()
>>>>> generally does not work with unions on ARM as assumed here:
>>>>>
>>>>> https://lore.kernel.org/lkml/6e57d5d2-9b88-aee6-fb7a-82e24144d179@hartkopp.net/
>>>>>
>>>>>
>>>>> In both cases I can not really fix the issue.
>>>>> When the partly revert (suggested above) works, this would be a hack
>>>>> too.
>>>>>
>>>>> Best,
>>>>> Oliver
>>>>>
>>>>> On 20.03.21 21:43, kernel test robot wrote:
>>>>>> Hi Oliver,
>>>>>>
>>>>>> FYI, the error/warning still remains.
>>>>>>
>>>>>> tree:
>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>>>>>> master
>>>>>> head:   812da4d39463a060738008a46cfc9f775e4bfcf6
>>>>>> commit: c7b74967799b1af52b3045d69d4c26836b2d41de can: replace
>>>>>> can_dlc as variable/element for payload length
>>>>>> date:   4 months ago
>>>>>> config: arm-randconfig-r016-20210321 (attached as .config)
>>>>>> compiler: arm-linux-gnueabi-gcc (GCC) 9.3.0
>>>>>> reproduce (this is a W=1 build):
>>>>>>           wget
>>>>>> https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross
>>>>>> -O ~/bin/make.cross
>>>>>>           chmod +x ~/bin/make.cross
>>>>>>           #
>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c7b74967799b1af52b3045d69d4c26836b2d41de
>>>>>>
>>>>>>           git remote add linus
>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>>>>>>           git fetch --no-tags linus master
>>>>>>           git checkout c7b74967799b1af52b3045d69d4c26836b2d41de
>>>>>>           # save the attached .config to linux build tree
>>>>>>           COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0
>>>>>> make.cross ARCH=arm
>>>>>>
>>>>>> If you fix the issue, kindly add following tag as appropriate
>>>>>> Reported-by: kernel test robot <lkp@intel.com>
>>>>>>
>>>>>> All errors (new ones prefixed by >>):
>>>>>>
>>>>>>      In file included from <command-line>:
>>>>>>      net/can/af_can.c: In function 'can_init':
>>>>>>>> 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(struct canfd_frame, data)
>>>>>>        315 |  _compiletime_assert(condition, msg,
>>>>>> __compiletime_assert_, __COUNTER__)
>>>>>>            |                                      ^
>>>>>>      include/linux/compiler_types.h:296:4: note: in definition of
>>>>>> macro '__compiletime_assert'
>>>>>>        296 |    prefix ## suffix();    \
>>>>>>            |    ^~~~~~
>>>>>>      include/linux/compiler_types.h:315:2: note: in expansion of
>>>>>> macro '_compiletime_assert'
>>>>>>        315 |  _compiletime_assert(condition, msg,
>>>>>> __compiletime_assert_, __COUNTER__)
>>>>>>            |  ^~~~~~~~~~~~~~~~~~~
>>>>>>      include/linux/build_bug.h:39:37: note: in expansion of macro
>>>>>> 'compiletime_assert'
>>>>>>         39 | #define BUILD_BUG_ON_MSG(cond, msg)
>>>>>> compiletime_assert(!(cond), msg)
>>>>>>            | ^~~~~~~~~~~~~~~~~~
>>>>>>      include/linux/build_bug.h:50:2: note: in expansion of macro
>>>>>> 'BUILD_BUG_ON_MSG'
>>>>>>         50 |  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: "
>>>>>> #condition)
>>>>>>            |  ^~~~~~~~~~~~~~~~
>>>>>>      net/can/af_can.c:891:2: note: in expansion of macro 'BUILD_BUG_ON'
>>>>>>        891 |  BUILD_BUG_ON(offsetof(struct can_frame, len) !=
>>>>>>            |  ^~~~~~~~~~~~
>>>>>>
>>>>>>
>>>>>> vim +/__compiletime_assert_536 +315 include/linux/compiler_types.h
>>>>>>
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  301
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  302  #define
>>>>>> _compiletime_assert(condition, msg, prefix, suffix) \
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  303
>>>>>> __compiletime_assert(condition, msg, prefix, suffix)
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  304
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  305  /**
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  306   * compiletime_assert -
>>>>>> break build and emit msg if condition is false
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  307   * @condition: a
>>>>>> compile-time constant condition to check
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  308   * @msg:       a
>>>>>> message to emit if condition is false
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  309   *
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  310   * In tradition of
>>>>>> POSIX assert, this macro will break the build if the
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  311   * supplied condition
>>>>>> is *false*, emitting the supplied error message if the
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  312   * compiler has support
>>>>>> to do so.
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  313   */
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  314  #define
>>>>>> compiletime_assert(condition, msg) \
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21 @315
>>>>>> _compiletime_assert(condition, msg, __compiletime_assert_,
>>>>>> __COUNTER__)
>>>>>> eb5c2d4b45e3d2 Will Deacon 2020-07-21  316
>>>>>>
>>>>>> :::::: The code at line 315 was first introduced by commit
>>>>>> :::::: eb5c2d4b45e3d2d5d052ea6b8f1463976b1020d5 compiler.h: Move
>>>>>> compiletime_assert() macros into compiler_types.h
>>>>>>
>>>>>> :::::: TO: Will Deacon <will@kernel.org>
>>>>>> :::::: CC: Will Deacon <will@kernel.org>
>>>>>>
>>>>>> ---
>>>>>> 0-DAY CI Kernel Test Service, Intel Corporation
>>>>>> https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org
>>>>>>
>>>>> _______________________________________________
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