From: Peter Hurley <peter@hurleysoftware.com>
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance
To: "Eric W. Biederman" <ebiederm@xmission.com>
References: <43AD2BA7-B594-4299-95F3-D86FD38AF21B@zytor.com>
 <87egexpf4o.fsf@x220.int.ebiederm.org>
 <CA+55aFw9Bg+Zh_T4zP487n3ieaxoMHgZ_nNJVdpSR4kQK9gQ9w@mail.gmail.com>
 <1CB621EF-1647-463B-A144-D611DB150E15@zytor.com>
 <20151208223135.GA8352@kroah.com> <87oae0h2bo.fsf@x220.int.ebiederm.org>
 <56677DE3.5040705@zytor.com> <20151209012311.GA11794@kroah.com>
 <84B136DF-55E4-476A-9CB2-062B15677EE5@zytor.com>
 <20151209013859.GA12442@kroah.com> <20151209083225.GA30452@1wt.eu>
 <87wpskyds7.fsf_-_@x220.int.ebiederm.org>
Cc: Greg KH <greg@kroah.com>, Jiri Slaby <jslaby@suse.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Aurelien Jarno <aurelien@aurel32.net>,
        Andy Lutomirski <luto@amacapital.net>,
        Florian Weimer <fw@deneb.enyo.de>, Al Viro <viro@zeniv.linux.org.uk>,
        Serge Hallyn <serge.hallyn@ubuntu.com>, Jann Horn <jann@thejh.net>,
        "security@kernel.org" <security@kernel.org>,
        "security@ubuntu.com >> security" <security@ubuntu.com>,
        security@debian.org, Willy Tarreau <w@1wt.eu>,
        linux-kernel@vger.kernel.org
Message-ID: <566F1CD7.20502@hurleysoftware.com>
Date: Mon, 14 Dec 2015 11:47:35 -0800
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On 12/11/2015 11:40 AM, Eric W. Biederman wrote:
> Forcing newinstance for every mount of the devpts filesystem actually
> requires the association between /dev/ptmx and the currently mounted
> instance of devpts at /dev/pts.  Simply remembering the first mount of
> the devpts filesystem and associating that with /dev/ptmx is not
> enough.  I am aware of at least one instance where an initramfs mounts
> devpts before the main system instance of devpts is mounted.

Can you point me to that usage please?

I ask because there's a patch to move devpts init from module initcall
up to fs initcall (neither devpts nor the pty driver is actually built
as a module anyway), and I'd like to look at what the consequences
might be for that userspace configuration.


> 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)?

Regards,
Peter Hurley

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