linux-kernel - Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

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Message-ID: <5AFEA8FF-BDEA-40AF-8C45-19F3E9CC36D3@zytor.com>
Date:	Sat, 19 Dec 2015 20:35:29 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	ebiederm@...ssion.com, Peter Hurley <peter@...leysoftware.com>
CC:	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Aurelien Jarno <aurelien@...el32.net>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

On December 19, 2015 8:11:50 PM PST, ebiederm@...ssion.com wrote:
>ebiederm@...ssion.com (Eric W. Biederman) writes:
>
>>>> In that system ptys simply did not work after boot when I tested
>>>> associating /dev/ptmx with the first mount of the devpts
>filesystem.
>>>
>>> Assuming userspace isn't broken by that patch, is a fixed
>association
>>> with first mount otherwise an acceptable solution for magic
>/dev/ptmx
>>> (where /dev/ptmx is not a symlink to /dev/pts/ptmx)?
>>
>> I do not believe a fixed association with the first mount is an
>> acceptable solution for implementing  /dev/ptmx in association with
>> a change to cause mount of devpts to be an independent filesystem.
>> Such an association fails to be backwards compatible with existing
>> userspace, and it is extremely fragile.
>
>Ugh.  After reviewing the userspace code that mounts devpts we have
>to do use a magic /dev/ptmx to solve the issue we are trying to solve.
>
>The fragility of detecting the primary system devpts seems solvable.
>
>CentOS5 and openwrt-15.05 mount devpts, unmount devpts,
>then mount devpts again.  So a rule of mouting the internal devpts if
>it
>isn't mounted would work for those.
>
>CentOS6 uses switch_root and moves it's early mount of devpts onto the
>primary root, and then because devpts is also in /etc/fstab tries and
>fails to mount devpts once more at the same location.  Implying
>newinstance will make that mounting devpts twice.  That sounds solvable
>but I don't see a clean way of detecting that case yet.
>
>Ugh.
>
>I am going to pound my head up against what is needed to find the
>primary system mount of devpts for a bit more and see if I can solve
>that.  Otherwise this exercise is pointless.
>
>Eric

Does it matter if it mounts devpts twice?  It seems like a waste of a minuscule amount of memory, and nothing else.
-- 
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