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

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Date:	Tue, 19 Apr 2016 18:24:12 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security@...ntu.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

On Tue, Apr 19, 2016 at 4:29 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I _violently_ oppose the stupid DEVPTS_MULTIPLE_INSTANCES config option.

So just to show what I want to actually happen, here's the hacky patch
on top of my (now merged) cleanup patch that actually does what I want
devpts to do.

I say it's hacky, because the "follow_mount()" thing there really is
pretty hacky. Al - suggestions for how to do this *right*?

But this actually forcibly removes the whole "newinstance" thing, and
makes every pts mount a new instance, and just relies on "ptmx" doing
the right thing.

In other words, with this patch, you can *literally* do just this (as
root, obviously):

  mkdir test-dir
  cd test-dir

  mknod ptmx c 5 2
  mkdir pts
  mount -t devpts pts pts

and after that it all just works. You can do this:

  ls -l pts

which shows just the other ptmx noode (that is unused and pointless -
I'd actually like to just remove it, but whatever), and then you can
do

  sleep 100 < ptmx &
  sleep 100 < ptmx &
  ls -l pts

and you will magically see those new 0/1 entries in that pts
subdirectory.. It's entirely independent of /dev/pts/, and there's no
magic connection or any magic dis-connection. It all JustWorks(tm).

Note how this works even *outside* of /dev. But it works inside of
/dev equally well.

Now, a *real* patch would

 - solve that "follow_mount()" issue some other way

 - not remove the newinstance code immediately (I did it to show that
even the bootup works with a unmodified distro)

 - actually remove the whole "DEVPTS_MULTIPLE_INSTANCES" config option

 - I'm not happy with devpts_pty_kill(). I would want to clean that up
a bit somehow. I think this is at least partly what Peter Hurley was
talking about. That thing is not pretty.

so this attached patch is by no means meant to be applied as-is. But
it's meant to show what (a) the new organization allows and (b) what I
was going for.

                          Linus

View attachment "patch.diff" of type "text/plain" (5380 bytes)