linux-kernel - Re: F_DUPFD_CLOEXEC implementation

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Date:	Mon, 1 Oct 2007 19:49:03 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Al Viro <viro@....linux.org.uk>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Ulrich Drepper <drepper@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: F_DUPFD_CLOEXEC implementation

On Monday 01 October 2007 19:16, Al Viro wrote:
> 	* it's on a bunch of cyclic lists.  Have its neighbor
> go away while you are doing all that crap => boom
> 	* there's that thing call current position...  It gets buggered.
> 	* overwriting it while another task might be in the middle of
> syscall involving it => boom

Hm, I suspected that it's herecy. Any idea how to do it cleanly?

> 	* non-cooperative tasks reading *in* *parallel* from the same
> opened file are going to have a lot more serious problems than agreeing
> on O_NONBLOCK anyway, so I really don't understand what the hell is that for.

They don't even need to read in parallel, just having shared fd is enough.
Think about pipes, sockets and terminals. A real-world scenario:

* a process started from shell (interactive or shell script)
* it sets O_NONBLOCK and does a read from fd 0...
* it gets killed (kill -9, whatever)
* shell suddenly has it's fd 0 in O_NONBLOCK mode
* shell and all subsequent commands started from it unexpectedly have
  O_NONBLOCKed stdin.
--
vda
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