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From:   Hauke Mehrtens <hauke@hauke-m.de>
Subject: Re: [PATCH] wireless: Use linux/stddef.h instead of stddef.h
To:     =?UTF-8?B?SsOpcsO0bWUgUG91aWxsZXI=?= <jerome.pouiller@silabs.com>
Cc:     johannes.berg@intel.com, ndesaulniers@google.com,
        pihsun@chromium.org, linux-wireless@vger.kernel.org
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Message-ID: <ab7a0c7b-6a38-66cc-d1c4-6bc4fd627227@hauke-m.de>
Date:   Thu, 11 Jun 2020 16:00:31 +0200
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On 5/27/20 4:03 PM, J=C3=A9r=C3=B4me Pouiller wrote:
> On Tuesday 26 May 2020 23:02:56 CEST Hauke Mehrtens wrote:
>> On 5/25/20 11:35 AM, J=C3=A9r=C3=B4me Pouiller wrote:
>>> On Thursday 21 May 2020 22:14:22 CEST Hauke Mehrtens wrote:
>>>> When compiling inside the kernel include linux/stddef.h instead of
>>>> stddef.h. When I compile this header file in backports for power PC =
I
>>>> run into a conflict with ptrdiff_t. I was unable to reproduce this i=
n
>>>> mainline kernel. I still would like to fix this problem in the kerne=
l.
>>>>
>>>> Fixes: 6989310f5d43 ("wireless: Use offsetof instead of custom macro=
=2E")
>>>> Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
>>>> ---
>>>>  include/uapi/linux/wireless.h | 6 +++++-
>>>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/include/uapi/linux/wireless.h b/include/uapi/linux/wire=
less.h
>>>> index a2c006a364e0..24f3371ad826 100644
>>>> --- a/include/uapi/linux/wireless.h
>>>> +++ b/include/uapi/linux/wireless.h
>>>> @@ -74,7 +74,11 @@
>>>>  #include <linux/socket.h>              /* for "struct sockaddr" et =
al  */
>>>>  #include <linux/if.h>                  /* for IFNAMSIZ and co... */=

>>>>
>>>> -#include <stddef.h>                     /* for offsetof */
>>>> +#ifdef __KERNEL__
>>>> +#      include <linux/stddef.h>        /* for offsetof */
>>>> +#else
>>>> +#      include <stddef.h>              /* for offsetof */
>>>> +#endif
>>>
>>> Hello,
>>>
>>> This patch also solves a warning I encountered when I launched sparse=

>>> (with make C=3D1):
>>>
>>>   /usr/lib/gcc-cross/arm-linux-gnueabihf/9/include/stddef.h:406:9: wa=
rning: preprocessor token offsetof redefined
>>>   ./include/linux/stddef.h:17:9: this was the original definition
>>>
>>> However, I wonder if it useful to keep 'include <stddef.h>' when this=
 file
>>> is included from userspace. If it does not compile anymore, there is
>>> problem somewhere else, no?
>>>
>>> (Globally, I always find suspicious a kernel source that includes a s=
ystem
>>> header)
>> Hi,
>>
>> The wireless.h makes use of offsetof and this is defined in system
>> stddef.h and in include/linux/stddef.h. When we only include
>> linux/stddef.h it will work fine when compiled inside the kernel, but =
it
>> could cause problems when used in user space, because this would inclu=
de
>> ./include/uapi/linux/stddef.h which does not define offsetof.
>>
>> I think we need the #ifdef __KERNEL__ to make this work in the kernel
>> and in the user space.
>=20
> In this case, maybe the problem should be resolved directly in
> uapi/linux/stddef.h?
>=20
> I have found other headers that use offsetof and may have the same prob=
lem
> (currently, they do not include stddef.h. So, some of the macros do not=

> compile):
>    - include/uapi/linux/fuse.h
>    - include/uapi/linux/genwqe/genwqe_card.h
>    - include/uapi/linux/gfs2_ondisk.h
>=20
Hi,

Would you prefer to add this to include/uapi/linux/stddef.h:
#ifndef offsetof
#define offsetof(TYPE, MEMBER)	((size_t)&((TYPE *)0)->MEMBER)
#endif

and then include linux/stddef.h in all header files which use offsetof?

include/uapi/linux/fuse.h checks for __KERNEL__ and then includes
linux/types.h or stdint.h, my initial approach would be to do this in a
similar way for stddef.h.


What is the kernel policy of including system header files like stddef.h?=


Hauke


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