linux-kernel - Re: F_DUPFD_CLOEXEC implementation

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Date:	Tue, 2 Oct 2007 10:28:13 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Davide Libenzi <davidel@...ilserver.org>
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 Monday 01 October 2007 20:04, Davide Libenzi wrote:
> > 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).

I'm confused. I am saying exactly this same thing: that I cannot
do it atomically using standard unix operations, but I still need
to do a nonblocking read. Why are you explaining to me that it
cannot be done? I *know*. I'm asking what API should be
added/extended to make it possible.

I have following proposals:

* make recv(..., MSG_DONTWAIT) work on any fd

Sounds neat, but not trivial to implement in current kernel.

* new fcntl command F_DUPFL: fcntl(fd, F_DUPFL, n):
  Analogous to F_DUPFD, but gives you *unshared* copy of the fd.
  Further seeks, fcntl(fd, F_SETFL, O_NONBLOCK), etc won't affect
  any other process.

How hard would it be implement F_DUPFL in current kernel?
--
vda
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