linux-kernel - Re: [PATCH 14/16] vfs: Implement mount_super

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Message-ID: <8737qhpifz.fsf@x220.int.ebiederm.org>
Date:	Tue, 19 Apr 2016 13:22:24 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security\@kernel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security\@ubuntu.com \>\> security" <security@...ntu.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Willy Tarreau <w@....eu>,
	Aurelien Jarno <aurelien@...el32.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jann Horn <jann@...jh.net>, Greg KH <greg@...ah.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>, Florian Weimer <fw@...eb.enyo.de>
Subject: Re: [PATCH 14/16] vfs: Implement mount_super_once

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Fri, Apr 15, 2016 at 8:35 AM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> The devpts filesystem has a notion of a system or primary instance of
>> devpts.  To retain the notion of a primary system instance of devpts
>> the code needs a way to allow userspace to mount the internally
>> mounted instance of devpts when it is not currently mounted by
>> userspace.  The new helper mount_super_once allows that.
>
> This is where I stopped reading this patch series.
>
> No.
>
> We want to get *rid* of the idiotic "primary instance" crap.

That is actually pretty much the opposite of what you said last time,
but having looked at the cost to maintian a "primary instance" notion
and what will break if we don't I am happy to remove such a notion
from devpts.

> The whole and only point of doing the "which pts filesystem am I
> associated with" for ptmx is to stop the idiotic "one devpts is
> pecial".
>
> I don't want to see 16 random patches.
>
> I want to see *one* patch that makes /dev/ptmx look up the pts
> filesystem, and be done with it.

Now that I know where most of the landmines are in userspace I performed
some limited testing to see to see what the implications are:

Causing every userspace mount of devpts to use mount_nodev means the
following:

- Support for reserving ptys for the system devpts instance using
  /proc/sys/kernel/pty/reserve needs to be removed.

- On CentOS6 devpts will wind up mounted twice /dev/pts.

- Open of /dev/ptmx can use devpts_mnt to find the devpts filesystem.

In my cursory testing userspace still boots and works desipite those
changes so I am quite happy to go down this path.

Eric