linux-kernel - Re: RFC: A revised timerfd API

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Message-ID: <20070922233744.GA2218@hardeman.nu>
Date:	Sun, 23 Sep 2007 01:37:44 +0200
From:	David Härdeman <david@...deman.nu>
To:	Michael Kerrisk <mtk.linux.lists@...il.com>
Cc:	Bernd Eckenfels <ecki@...a.inka.de>, linux-kernel@...r.kernel.org,
	Ulrich Drepper <drepper@...hat.com>, geoff@...are.org.uk,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>,
	Jonathan Corbet <corbet@....net>,
	Randy Dunlap <rdunlap@...otime.net>, vda.linux@...glemail.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	mtk-manpages@....net
Subject: Re: RFC: A revised timerfd API

On Sat, Sep 22, 2007 at 06:07:14PM +0200, Michael Kerrisk wrote:
>On 9/22/07, Bernd Eckenfels <ecki@...a.inka.de> wrote:
>> In article <46F514C9.5010208@....net> you wrote:
>> >  1. This design stretches the POSIX timers API in strange
>> >     ways.
>>
>> Maybe it is possible to reimplement the POSIX API in usermode using the
>> kernel's FD implementation?
>
>It's a clever idea...  Without thinking on it too long, I'm not sure
>whether or not there might be some details which would make this
>difficult.

It seems to be a dangerous idea. It has the potential of breaking 
userspace applications that rely on POSIX timers not creating fd's.

Image code like this:

	/* Close stdin, stdout, stderr */
	close(0);
	close(1);
	close(2);

	/* Oh, a timer would be nice */
	timer_create(x, y, z);

	/* Create new stdin, stdout, stderr */
	fd = open("/dev/null", flags);
	dup(fd);
	dup(fd);

Unless timer_create does some magic to avoid using the lowest available 
fd, this would suddenly break as the timerfd would be fd 0.

-- 
David Härdeman
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