From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1M1Taa-0002bY-KH
	for qemu-devel@nongnu.org; Tue, 05 May 2009 18:58:16 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1M1TaV-0002bM-9v
	for qemu-devel@nongnu.org; Tue, 05 May 2009 18:58:15 -0400
Received: from [199.232.76.173] (port=60845 helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1M1TaV-0002bJ-42
	for qemu-devel@nongnu.org; Tue, 05 May 2009 18:58:11 -0400
Received: from mail2.shareable.org ([80.68.89.115]:38237)
	by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32)
	(Exim 4.60) (envelope-from <jamie@shareable.org>) id 1M1TaU-0008D6-JI
	for qemu-devel@nongnu.org; Tue, 05 May 2009 18:58:10 -0400
Date: Tue, 5 May 2009 23:58:09 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH] linux-user: implement pipe2 syscall
Message-ID: <20090505225809.GJ7574@shareable.org>
References: <20090505133048.GA29646@kos.to>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20090505133048.GA29646@kos.to>
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Riku Voipio <riku.voipio@iki.fi>
Cc: qemu-devel@nongnu.org

Riku Voipio wrote:
> implement pipe2 syscall. instead of calling pipe2 directly
> (which was introduced in 2.6.27), emulate the flag functionality
> with fcntl.

This is wrong with multiple threads if the flag contains FD_CLOEXEC.
If that situation is possible, please don't do this.

The point of pipe2() with FD_CLOEXEC is to be atomic: make sure
another thread can never see the file descriptor with FD_CLOEXEC not set.

If you can't guarantee that, it's better to return ENOSYS as every
application using pipe2() like this has a fallback to use pipe() and
FD_CLOEXEC itself, and probably has application logic to protect
against the race condition.

If there's only one thread, or if you can arrange to block any
concurrent clone/fork/execve calls in other threads (in QEMU) during
the race window, then it's fine to emulate it with fcntl.

-- Jamie