linux-kernel - Re: UAPI headers including non-UAPI headers by accident?

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Message-ID: <CALCETrWqH+Tc=yN5CTWQ+wfsrbRon6_qwe3FHC82AG031VQeoA@mail.gmail.com>
Date:	Tue, 23 Jun 2015 14:20:46 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: UAPI headers including non-UAPI headers by accident?

On Tue, Jun 23, 2015 at 1:44 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Thursday 18 June 2015 11:57:56 Andy Lutomirski wrote:
>> On Thu, Jun 18, 2015 at 12:58 AM, Arnd Bergmann <arnd@...db.de> wrote:
>> > On Thursday 18 June 2015 09:52:36 Michael Kerrisk wrote:
>> >> [CC += David]
>> >>
>> >> On 2 June 2015 at 18:36, Andy Lutomirski <luto@...nel.org> wrote:
>> >> > include/uapi/linux/signal.h starts with:
>> >> >
>> >> > #ifndef _UAPI_LINUX_SIGNAL_H
>> >> > #define _UAPI_LINUX_SIGNAL_H
>> >> >
>> >> > #include <asm/signal.h>
>> >> > #include <asm/siginfo.h>
>> >> >
>> >> > This causes it to include <asm/signal.h>, which is not the same thing
>> >> > as <uapi/asm/signal.h>.  Changing that will break userspace use of
>> >> > this header, though, as the uapi/ won't get removed.
>> >> >
>> >> > What's the correct fix?  This is causing trouble with a UML build for me.
>> >>
>> >> Perhaps David has some insight, since he architected the original UAPI split.
>> >
>> > The uapi headers are installed without the uapi prefix. This means
>> > that inside of the kernel, we get
>> >
>> >
>> >  linux/signal.h
>> >  -> uapi/linux/signal.h
>> >     -> asm/signal.h
>> >        -> uapi/asm/signal.h
>> >
>> > while in the installed headers we just get
>> >
>> >     linux/signal.h
>> >        -> asm/signal.h
>> >
>> > This all looks right to me: user space only sees the exported portions
>> > under the traditional names, while the kernel sees both the kernel-side
>> > and user-side definitions from the same path.
>> >
>>
>> It seems counterintuitive and error-prone to me that including
>> <uapi/linux/signal.h> would pull in non-UAPI asm/signal.h declarations
>> in the kernel but not when used from userspace.  It would make it very
>> easy to break the header such that it's only broken in a userspace
>> context.
>>
>
> It's an artifact of how the files were originally generated, as the UAPI
> headers used to be part of the normal headers, with #ifdef __KERNEL__ around
> the parts that are not in include/uapi now.
>
> For some reason, we now have device drivers including the uapi headers
> directly, which was probably done as an accident. Maybe we can just
> change them all back to use the normal header file names and add a
> checkpatch warning in case someone else tries to use the uapi headers
> directly?

I don't really mind when a driver includes a UAPI header.  IMO the
problematic case is when a UAPI header includes a non-UAPI header.

--Andy
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