Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance
To: Andy Lutomirski <luto@amacapital.net>
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Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Greg KH <greg@kroah.com>,
        Jiri Slaby <jslaby@suse.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Aurelien Jarno <aurelien@aurel32.net>,
        Florian Weimer <fw@deneb.enyo.de>,
        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" <linux-kernel@vger.kernel.org>
From: "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <566B4BAA.3030600@zytor.com>
Date: Fri, 11 Dec 2015 14:18:18 -0800
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On 12/11/15 14:12, Andy Lutomirski wrote:
>>
>> For the newinstance case st_dev should match between the master and the
>> slave.  Unfortunately this is not the case for a legacy ptmx, as a
>> stat() on the master descriptor still returns the st_dev, st_rdev, and
>> st_ino for the ptmx device node.
> 
> Sure, but I'm not talking about stat.  I'm saying that we could add a
> new ioctl that works on any ptmx fd (/dev/ptmx or /dev/pts/ptmx) that
> answers the question "does this ptmx logically belong to the given
> devpts filesystem".
> 
> Since it's not stat, we can make it do whatever we want, including
> following a link to the devpts instance that isn't f_path or f_inode.
> 

Sure.  My thinking, though, was whether or not we can do something that
works on legacy kernels, and/or is less intrusive than new ioctls.

What is the actual operation we need?

	-hpa

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