On Fri, 27 May 2016, Greg KH wrote:
> On Fri, May 27, 2016 at 12:08:09PM -0400, Robert P. J. Day wrote:
> >
> > continuing down this road of exporting kernel headers, under
> > include/uapi/ (and mostly further under linux/), there's still a
> > *ton* of testing of the __KERNEL__ preprocessor check.
> >
> > now, i realize that when one does "make headers_install", all
> > those files are run through "unifdef" to sanitize almost all of
> > that kernel content, so it's not like it hurts, but is there any
> > reason so much of it is still there? wouldn't it be tidier to get
> > rid of it?
>
> No, because not all of the information in those .h files should be
> exported to userspace. You could either split the files all up into
> two different ones, or just live with the existing infrastructure we
> have. We chose the easy one :)
ok, after further perusal, i can see the misunderstandings i had,
mostly because i'd never read the rationale and design for UAPI
closely enough.
originally, i thought the whole point of the uapi/ directories was
to factor out, as thoroughly as possible, user space-only header file
content, and certainly there are header files where there is a
beautiful, clean break between kernel space stuff and user space
stuff, so splitting such a file into two parts is easy.
so my first mistake was thinking that there should be very little
__KERNEL__ checking in the uapi/ directories. there are, of course,
cases where it's just not feasible to do a perfect partitioning, like
in cases where a single structure definition might contain:
#ifdef __KERNEL__
some member
#else
some other member
#endif
so, sure, that sort of thing would be retained in a uapi/ header file,
and would be finally sanitized as part of the headers install process.
what i'm massively embarrassed to admit is that i didn't know the
uapi/ directories are part of the normal kernel header file search
path:
# Use USERINCLUDE when you must reference the UAPI directories only.
USERINCLUDE := \
-I$(srctree)/arch/$(hdr-arch)/include/uapi \
-Iarch/$(hdr-arch)/include/generated/uapi \
-I$(srctree)/include/uapi \
-Iinclude/generated/uapi \
-include $(srctree)/include/linux/kconfig.h
# Use LINUXINCLUDE when you must reference the include/ directory.
# Needed to be compatible with the O= option
LINUXINCLUDE := \
-I$(srctree)/arch/$(hdr-arch)/include \
-Iarch/$(hdr-arch)/include/generated/uapi \
-Iarch/$(hdr-arch)/include/generated \
$(if $(KBUILD_SRC), -I$(srctree)/include) \
-Iinclude \
$(USERINCLUDE)
which means that rather than trying to refactor a header file into
kernel and user space parts, you can just say, "ah, the heck with it,"
and move it into uapi/, where it will be the *only* version of that
header file in the kernel source tree, suitable for use in both kernel
and user space. case in point:
$ find . -name virtio_blk.h
./include/uapi/linux/virtio_blk.h
$
and in cases like that, sure, there could be all sorts of __KERNEL__
checking going on to distinguish kernel space content from user space
content.
rday
--
========================================================================
Robert P. J. Day Ottawa, Ontario, CANADA
http://crashcourse.ca
Twitter: http://twitter.com/rpjday
LinkedIn: http://ca.linkedin.com/in/rpjday
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