From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) Subject: Re: [RFC] fix devpts mount behavior Date: Tue, 24 Jan 2012 12:24:01 -0800 Message-ID: References: <20120124000517.GA28878@sergelap> <20120124002555.GA29534@sergelap> <20120124010758.GJ23916@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20120124010758.GJ23916-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org> (Al Viro's message of "Tue, 24 Jan 2012 01:07:58 +0000") List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Al Viro Cc: containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Dave Hansen , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andy Whitcroft , sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org, Linus Torvalds List-Id: containers.vger.kernel.org Al Viro writes: > On Mon, Jan 23, 2012 at 04:41:25PM -0800, Linus Torvalds wrote: > >> Right. I think the opportunity for problems should be pretty small. >> >> And it's not like the pty itself wouldn't continue to work - it's just >> that programs like /usr/bin/tty wouldn't be able to *find* it. >> >> Although who knows - maybe there is some other subtle interaction. > > FWIW, the subtle and nasty part in all that is that you can mknod /dev/ptmx > and it *will* work, refering to the "initial" instance. That's what > concerns me about the chroot scenarios - > mknod /jail/dev/ptmx c 5 2 > mkdir /jail/dev/pts > mount -t devpts /jail/dev/pts > chroot /jail > works fine right now, but with that change behaviour will be all wrong - > opening /dev/ptmx inside of jail will grab you a pts, all right, but > it will *not* show up in (jail) /dev/pts/* as it does with the current > kernel. > > Note that if you replace that mknod with symlink pts/ptmx /jail/dev/ptmx > the things will keep working. However, that will _only_ work for kernels > with DEVPTS_MULTIPLE_INSTANCES - without it you won't get ptmx inside > devpts (which is arguably wrong, BTW) For testing I would recommend looking at the distro chroot build cases. It looks like relatively recent udev still creates /dev/ptmx and does not create the symlink. So we might get into the awkward situation of /dev/ptmx not matching /dev/pts/ptmx with something as simple as initramfs mounting /dev/pts and then initscripts mounting /dev/pts. Eric