linux-kernel - Re: Syslets, Threadlets, generic AIO support, v6

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Date:	Wed, 30 May 2007 23:53:54 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Davide Libenzi <davidel@...ilserver.org>,
	Ingo Molnar <mingo@...e.hu>,
	Ulrich Drepper <drepper@...hat.com>,
	Jeff Garzik <jeff@...zik.org>,
	Zach Brown <zach.brown@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Syslets, Threadlets, generic AIO support, v6

Linus Torvalds a écrit :
> 
> On Wed, 30 May 2007, Eric Dumazet wrote:
>>> No, Davide, the problem is that some applications depend on getting
>>> _specific_ file descriptors.
>> Fix the application, and not adding kernel bloat ?
> 
> No. The application is _correct_. It's how file descriptors are defined to 
> work. 
> 
>> Then you can also exclude multi-threading, since a thread (even not inside
>> glibc) can also use socket()/pipe()/open()/whatever and take the zero file
>> descriptor as well.
> 
> Totally different. That's an application internal issue. It does *not* 
> mean that we can break existing standards.
> 
>> The only hardcoded thing in Unix is 0, 1 and 2 fds.
> 
> Wrong. I already gave an example of real code that just didn't bother to 
> keep track of which fd's it had open, and closed them all. Partly, in 
> fact, because you can't even _know_ which fd's you have open when somebody 
> else just execve's you.

If someone really cares, /proc/self/fd can help. But one shouldn't care at all.

About the things that the process can do before execing() a process, file 
descriptors outside of 0,1,2 are the most obvious thing, but you also have 
alarm(), or stupid rlimits.

> 
> You can call it buggy, but the fact is, if you do, you're SIMPLY WRONG. 
> 
> You cannot just change years and years of coding practice, and standard 
> documentations. The behaviour of file descriptors is a fact. Ignoring that 
> fact because you don't like it is naïve and simply not realistic.

I want to change nothing. Current situation is fine and well documented, thank 
you.

If a program does "for (i = 0; i < NR_OPEN; i++) close(i);", this 
*will*/*should* work as intended : close all files descriptors from 0 to 
NR_OPEN. Big deal.

But you wont find in a program :

FILE *fp = fopen("somefile", "r");
for (i = 0; i < NR_OPEN; i++)
     close(i);
while (fgets(buff, sizeof(buff), fp)) {
}


You and/or others want to add fd namespaces and other hacks.

I saw on this thread suspicious examples, I am waiting for a real one, 
justifying all this stuff.

After file descriptors separation, I guess we'll need memory space separation 
as well, signal separations (SIGALRM comes to mind), uid/gid separation, cpu 
time separation, and so on... setrlimit() layered for every shared lib.


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