Hi Andrii,

I love your patch! Yet something to improve:

[auto build test ERROR on bpf-next/master]
[cannot apply to bpf/master cgroup/for-next v5.6-rc6 next-20200320]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]

url:    https://github.com/0day-ci/linux/commits/Andrii-Nakryiko/Add-support-for-cgroup-bpf_link/20200321-045848
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
config: m68k-sun3_defconfig (attached as .config)
compiler: m68k-linux-gcc (GCC) 9.2.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=9.2.0 make.cross ARCH=m68k 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All error/warnings (new ones prefixed by >>):

   In file included from include/linux/kernel.h:11,
                    from include/linux/list.h:9,
                    from include/linux/timer.h:5,
                    from include/linux/workqueue.h:9,
                    from include/linux/bpf.h:9,
                    from kernel/bpf/syscall.c:4:
   kernel/bpf/syscall.c: In function 'link_update':
>> include/linux/kernel.h:994:51: error: dereferencing pointer to incomplete type 'struct bpf_cgroup_link'
     994 |  BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \
         |                                                   ^~
   include/linux/compiler.h:330:9: note: in definition of macro '__compiletime_assert'
     330 |   if (!(condition))     \
         |         ^~~~~~~~~
   include/linux/compiler.h:350:2: note: in expansion of macro '_compiletime_assert'
     350 |  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
         |  ^~~~~~~~~~~~~~~~~~~
   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/kernel.h:994:2: note: in expansion of macro 'BUILD_BUG_ON_MSG'
     994 |  BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \
         |  ^~~~~~~~~~~~~~~~
   include/linux/kernel.h:994:20: note: in expansion of macro '__same_type'
     994 |  BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \
         |                    ^~~~~~~~~~~
>> kernel/bpf/syscall.c:3612:13: note: in expansion of macro 'container_of'
    3612 |   cg_link = container_of(link, struct bpf_cgroup_link, link);
         |             ^~~~~~~~~~~~
   In file included from <command-line>:
>> include/linux/compiler_types.h:129:35: error: invalid use of undefined type 'struct bpf_cgroup_link'
     129 | #define __compiler_offsetof(a, b) __builtin_offsetof(a, b)
         |                                   ^~~~~~~~~~~~~~~~~~
   include/linux/stddef.h:17:32: note: in expansion of macro '__compiler_offsetof'
      17 | #define offsetof(TYPE, MEMBER) __compiler_offsetof(TYPE, MEMBER)
         |                                ^~~~~~~~~~~~~~~~~~~
   include/linux/kernel.h:997:21: note: in expansion of macro 'offsetof'
     997 |  ((type *)(__mptr - offsetof(type, member))); })
         |                     ^~~~~~~~
>> kernel/bpf/syscall.c:3612:13: note: in expansion of macro 'container_of'
    3612 |   cg_link = container_of(link, struct bpf_cgroup_link, link);
         |             ^~~~~~~~~~~~
>> kernel/bpf/syscall.c:3618:9: error: implicit declaration of function 'cgroup_bpf_replace'; did you mean 'cgroup_bpf_offline'? [-Werror=implicit-function-declaration]
    3618 |   ret = cgroup_bpf_replace(cg_link->cgroup, cg_link,
         |         ^~~~~~~~~~~~~~~~~~
         |         cgroup_bpf_offline
   cc1: some warnings being treated as errors

vim +994 include/linux/kernel.h

cf14f27f82af78 Alexei Starovoitov 2018-03-28  984  
^1da177e4c3f41 Linus Torvalds     2005-04-16  985  /**
^1da177e4c3f41 Linus Torvalds     2005-04-16  986   * container_of - cast a member of a structure out to the containing structure
^1da177e4c3f41 Linus Torvalds     2005-04-16  987   * @ptr:	the pointer to the member.
^1da177e4c3f41 Linus Torvalds     2005-04-16  988   * @type:	the type of the container struct this is embedded in.
^1da177e4c3f41 Linus Torvalds     2005-04-16  989   * @member:	the name of the member within the struct.
^1da177e4c3f41 Linus Torvalds     2005-04-16  990   *
^1da177e4c3f41 Linus Torvalds     2005-04-16  991   */
^1da177e4c3f41 Linus Torvalds     2005-04-16  992  #define container_of(ptr, type, member) ({				\
c7acec713d14c6 Ian Abbott         2017-07-12  993  	void *__mptr = (void *)(ptr);					\
c7acec713d14c6 Ian Abbott         2017-07-12 @994  	BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&	\
c7acec713d14c6 Ian Abbott         2017-07-12  995  			 !__same_type(*(ptr), void),			\
c7acec713d14c6 Ian Abbott         2017-07-12  996  			 "pointer type mismatch in container_of()");	\
c7acec713d14c6 Ian Abbott         2017-07-12  997  	((type *)(__mptr - offsetof(type, member))); })
^1da177e4c3f41 Linus Torvalds     2005-04-16  998  

:::::: The code at line 994 was first introduced by commit
:::::: c7acec713d14c6ce8a20154f9dfda258d6bcad3b kernel.h: handle pointers to arrays better in container_of()

:::::: TO: Ian Abbott <abbotti@mev.co.uk>
:::::: CC: Linus Torvalds <torvalds@linux-foundation.org>

---
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