linux-kernel - Re: Extending syscalls

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Message-ID: <478FABDF.1020002@zytor.com>
Date:	Thu, 17 Jan 2008 14:26:23 -0500
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jonathan Corbet <corbet@....net>
CC:	Al Viro <viro@...IV.linux.org.uk>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Linux Containers <containers@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Cedric Le Goater <clg@...ibm.com>, drepper@...hat.com,
	Serge Hallyn <serue@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Extending syscalls

Jonathan Corbet wrote:
> 
> Heh, indeed.  But we do seem to have a recurring problem of people
> wanting to extend sys_foo() beyond the confines of its original API.
> I've observed a few ways of doing that:
> 
>  - create sys_foo2() (or sys_foo64(), or sys_fooat(), or sys_pfoo(),
>    or...) and add the new stuff there.
> 
> The first approach has traditionally been the most popular.  If we have
> a consensus that this is the way to extend system calls in the future,
> it would be nice to set that down somewhere.  We could avoid a lot of
> API blind alleys that way.
> 

I would argue it is the right approach.  It lets the kernel system call 
entry dispatch directly to the system call for the "new" case, and to a 
compatibility thunk for the "old" case.  It has the following desirable 
properties:

- No overhead for the "new" case.
- Minimal overhead for the "old" case.
- Easily dealt with by tools like strace that examine system calls.

	-hpa
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