linux-kernel - Re: [PATCH 04/13] Always expose MAP

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Date:	Thu, 17 Sep 2015 11:13:31 +0100
From:	David Howells <dhowells@...hat.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	dhowells@...hat.com, "Kirill A. Shutemov" <kirill@...temov.name>,
	Palmer Dabbelt <palmer@...belt.com>, arnd@...db.de,
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Subject: Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace

Josh Triplett <josh@...htriplett.org> wrote:

> > Sure. And 0 is perfectly fine value for the flag. Like with MAP_FILE.
> 
> Rephrasing: the flag should always exist with the correct value.
> Whether the kernel handles it or not, the kernel *headers* shouldn't
> change to match the kernel, not least of which because they don't
> necessarily match the running kernel.  Just like we define the
> prototypes for syscalls that the running kernel may return ENOSYS for.

Josh is correct.

CONFIG_xxx *should* *not* be seen in UAPI headers, except inside #ifdef
__KERNEL__ guards under special circumstances - and #ifdef __KERNEL__ guards
*should* *not* be seen in UAPI headers except under special circumstances.

In terms of such special circumstances, take a peek in
include/uapi/linux/acct.h at struct acct with this:

	/* m68k had no padding here. */
	#if !defined(CONFIG_M68K) || !defined(__KERNEL__)
		__u16		ac_ahz;			/* AHZ */
	#endif

in the middle of it...

Or include/{,uapi/}linux/agpgart.h where it defines two different but
same-named variants of several structs.

Now, some of these - particularly things like the latter - can be fixed by
someone who has the time.

David
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