linux-kernel - Re: [PATCH 01/16] devpts: Attempting to get it right

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Message-ID: <5716CC33.8070302@hurleysoftware.com>
Date:	Tue, 19 Apr 2016 17:24:19 -0700
From:	Peter Hurley <peter@...leysoftware.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"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>,
	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 01/16] devpts: Attempting to get it right

On 04/19/2016 04:35 PM, Linus Torvalds wrote:
> On Tue, Apr 19, 2016 at 3:06 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>>
>> I have work inspired by this rolled into my code.  I will post shortly
>> after a little more testing.
> 
> Actually, I have a slightly fixed version in my tree. I've been
> running this on my own machines for a few days, just to verify, along
> with some testing.
> 
> The fixes are some cleanups of the header file (the !UNIX98 section
> that nobody uses was bogus), and fixing "devpts_get_ref()" to get the
> "struct file *" argument too. The current code doesn't need it, but
> the code to actually look up the right pts/ directory from the ptmx
> file open needs it because that's where the path is - passing in just
> the inode isn't sufficient).
> 
> Anyway, I think I'll just merge my branch instead of sending out
> another emailed patch, because I don't think that patch is
> controversial or unsafe. It doesn't actually change any semantics, and
> only does cleanups. If it breaks something due to the rules about
> private_data being different for slave and master side pty's, then the
> old code was broken too - it used to always put a inode pointer in
> there, but they were very different inode pointers.

Yes, the old code was broken. The new code always uses the ptmx inode.
Which is a stopgap measure to prevent the devpts instance from
teardown when the only open filp is /dev/tty.

What I want to do instead (but not for -stable) is bump the inode
reference at controlling tty association instead (either first
qualifying open or ioctl(TIOCSCTTY)). That way it would always be
the slave inode. This method is non-trivial though because it makes
disassociation more complicated.

Regards,
Peter Hurley