linux-kernel - Re: F_DUPFD_CLOEXEC implementation

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Date:	Mon, 1 Oct 2007 12:04:58 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Denys Vlasenko <vda.linux@...glemail.com>
cc:	Al Viro <viro@....linux.org.uk>,
	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 Mon, 1 Oct 2007, Denys Vlasenko wrote:

> 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.

I told you how in the previous email. You cannot use the:

1) set O_NONBLOCK
2) read/write
3) unset O_NONBLOCK

in a racy-free fashion, w/out wrapping it with a lock (thing that we 
don't want to do).



PS: send/recv are socket functions, and you really don't want to overload 
    them for other fds.



- Davide


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