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From:   Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Date:   Tue, 23 Mar 2021 14:46:35 +0900
Message-ID: <CAMZ6RqK0-7DaoscTgKD+APDxtPw1q0Dz0Kef_doa0PZOnBav=w@mail.gmail.com>
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:     Rong Chen <rong.a.chen@intel.com>
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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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)))?

(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
> >>>>
> >>> _______________________________________________
> >>> kbuild-all mailing list -- kbuild-all@lists.01.org
> >>> To unsubscribe send an email to kbuild-all-leave@lists.01.org
> >>
>