linux-kernel - Re: [PATCH v2] ipc: provide generic compat versions of IPC syscalls

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Date:	Fri, 9 Dec 2011 13:48:52 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Ralf Baechle <ralf@...ux-mips.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux390@...ibm.com, "David S. Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Christoph Hellwig <hch@....de>,
	Lucas De Marchi <lucas.demarchi@...fusion.mobi>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"J. Bruce Fields" <bfields@...hat.com>, NeilBrown <neilb@...e.de>,
	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	sparclinux@...r.kernel.org
Subject: Re: [PATCH v2] ipc: provide generic compat versions of IPC syscalls

On Fri, 9 Dec 2011 10:29:07 -0500
Chris Metcalf <cmetcalf@...era.com> wrote:

> When using the "compat" APIs, architectures will generally want to
> be able to make direct syscalls to msgsnd(), shmctl(), etc., and
> in the kernel we would want them to be handled directly by
> compat_sys_xxx() functions, as is true for other compat syscalls.
> 
> However, for historical reasons, several of the existing compat IPC
> syscalls do not do this.  semctl() expects a pointer to the fourth
> argument, instead of the fourth argument itself.  msgsnd(), msgrcv()
> and shmat() expect arguments in different order.
> 
> This change adds an __ARCH_WANT_OLD_COMPAT_IPC define that can be
> set in <asm/compat.h> to preserve this behavior for ports that use it
> (x86, sparc, powerpc, s390, and mips).  No actual semantics are changed
> for those architectures, and there is only a minimal amount of code
> refactoring in ipc/compat.c.
> 
> Newer architectures like tile (and perhaps future architectures such
> as arm64 and unicore64) should not supply this define, and thus can
> avoid having any IPC-specific code at all in their architecture-specific
> compat layer.  In the same vein, if this define is omitted, IPC_64 mode
> is assumed, since that's what the <asm-generic> headers expect.
> 
> The workaround code in "tile" for msgsnd() and msgrcv() is removed
> with this change; it also fixes the bug that shmat() and semctl() were
> not being properly handled.

What would we need to do to get all architectures using the new
interfaces, and remove __ARCH_WANT_OLD_COMPAT_IPC?

Regarding the implementation: rather than patching the header
files, it would be more conventional (and arguably better) to add

	select ARCH_WANT_OLD_COMPAT_IPC

to arch/*/Kconfig, then use CONFIG_ARCH_WANT_OLD_COMPAT_IPC.
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