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From:   =?ISO-8859-1?Q?J=E9r=F4me?= Pouiller <jerome.pouiller@silabs.com>
To:     Hauke Mehrtens <hauke@hauke-m.de>
Cc:     johannes.berg@intel.com, ndesaulniers@google.com,
        pihsun@chromium.org, linux-wireless@vger.kernel.org
Subject: Re: [PATCH] wireless: Use linux/stddef.h instead of stddef.h
Date:   Wed, 27 May 2020 16:03:38 +0200
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On Tuesday 26 May 2020 23:02:56 CEST Hauke Mehrtens wrote:
> On 5/25/20 11:35 AM, J=E9r=F4me 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 in
> >> mainline kernel. I still would like to fix this problem in the kernel.
> >>
> >> Fixes: 6989310f5d43 ("wireless: Use offsetof instead of custom macro."=
)
> >> 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/wirele=
ss.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
> >=20
> > Hello,
> >=20
> > This patch also solves a warning I encountered when I launched sparse
> > (with make C=3D1):
> >=20
> >   /usr/lib/gcc-cross/arm-linux-gnueabihf/9/include/stddef.h:406:9: warn=
ing: preprocessor token offsetof redefined
> >   ./include/linux/stddef.h:17:9: this was the original definition
> >=20
> > However, I wonder if it useful to keep 'include <stddef.h>' when this f=
ile
> > is included from userspace. If it does not compile anymore, there is
> > problem somewhere else, no?
> >=20
> > (Globally, I always find suspicious a kernel source that includes a sys=
tem
> > header)
> Hi,
>=20
> 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 include
> ./include/uapi/linux/stddef.h which does not define offsetof.
>=20
> I think we need the #ifdef __KERNEL__ to make this work in the kernel
> and in the user space.

In this case, maybe the problem should be resolved directly in
uapi/linux/stddef.h?

I have found other headers that use offsetof and may have the same problem
(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
J=E9r=F4me Pouiller