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...

From: Vincent MAILHOL
Date: Tue Mar 23 2021 - 01:47:47 EST

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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@xxxxxxxxx> 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@xxxxxxxxxxxx/
> >>>
> >>>
> >>> 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@xxxxxxxxxxxx/
> >>>
> >>>
> >>> 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@xxxxxxxxx>
> >>>>
> >>>> 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@xxxxxxxxxx>
> >>>> :::::: CC: Will Deacon <will@xxxxxxxxxx>
> >>>>
> >>>> ---
> >>>> 0-DAY CI Kernel Test Service, Intel Corporation
> >>>> https://lists.01.org/hyperkitty/list/kbuild-all@xxxxxxxxxxxx
> >>>>
> >>> _______________________________________________
> >>> kbuild-all mailing list -- kbuild-all@xxxxxxxxxxxx
> >>> To unsubscribe send an email to kbuild-all-leave@xxxxxxxxxxxx
> >>
>