linux-kernel - Re: [RFC] fix devpts mount behavior

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Date:	Tue, 24 Jan 2012 17:16:01 -0600
From:	Serge Hallyn <serge.hallyn@...onical.com>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	containers@...ts.linux-foundation.org,
	Dave Hansen <haveblue@...ibm.com>,
	linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andy Whitcroft <apw@...onical.com>, sukadev@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC] fix devpts mount behavior

Quoting Kay Sievers (kay.sievers@...y.org):
> On Tue, Jan 24, 2012 at 23:02, Serge E. Hallyn <serge@...lyn.com> wrote:
> > Quoting Eric W. Biederman (ebiederm@...ssion.com):
> 
> >> It looks like relatively recent udev still creates /dev/ptmx and does
> >
> > Boy, it does, and it's stubborn about it.  Removing the /lib/udev/rules.d
> > entry doesn't stop it.  (this is after I've had an init job replace the
> > devtmpfs-created ptmx entry with a symlink)
> 
> Udev has nothing to do with that. The kernel creates that device node.
> Udev does not carry any rules you could remove, to name device nodes,
> it only set permissions and creates symlinks to device nodes.

That's odd, because I was sure I deleted the node after the kernel created
it.

But it sounds like I must have done it wrong.

> It will never replace a kernel-created device node with a symlink,
> there is no way to express that. If you don't want a device node
> there, you need to change the kernel, to not export
> /sys/class/tty/ptmx/ the way it is today.
> 
> > So current distros (well, Ubuntu and Fedora at least) would need to at least
> > (a) fix udev,
> 
> To do what?

Nothing, as I'm sure you're right above  :)

> > (b) change the default devpts mount (done from initramfs) to
> > add ptmxmode=666,
> 
> > (c) (if not done in udev) create the /dev/ptmx symlink.
> 
> Udev can only create symlinks to devices the driver-core creates, not
> to devices inside a custom filesystem.

I see.

> > For safety I'd recommend creating /dev/pts/ptmx with
> > DEVPTS_MULTIPLE_INSTANCES=n (or dropping that support), and by default
> > setting ptmxmode to 666 as that's what udev does.
> 
> The mode for ptmx is set by the kernel itself, and does not even need
> udev to do that:
>   $ cat /sys/class/tty/ptmx/uevent
>   MAJOR=5
>   MINOR=2
>   DEVNAME=ptmx
>   DEVMODE=0666

That has nothing to do with /dev/pts/ptmx, whose perms are set based on
the '-o ptmxmode=" argument, and default to 000 if not specified.  If
/dev/ptmx is going to be a symlink to /dev/pts/ptmx, then we have to set
the /dev/pts/ptmx perms to not be 000, or users won't be able to create
ptys.

-serge
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