Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance
To: Andy Lutomirski <luto@amacapital.net>,
        "Eric W. Biederman" <ebiederm@xmission.com>
References: <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>
 <20151211210400.GL20997@ZenIV.linux.org.uk>
 <87h9jovgf7.fsf@x220.int.ebiederm.org>
 <CALCETrWVbWz-cOaOZrcU1MLPyviknssEWY5OoKLVqkoDi-9kjA@mail.gmail.com>
Cc: 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: <566B4931.5080806@zytor.com>
Date: Fri, 11 Dec 2015 14:07:45 -0800
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On 12/11/15 13:48, Andy Lutomirski wrote:
> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>> Al Viro <viro@ZenIV.linux.org.uk> writes:
>>
>>> On Fri, Dec 11, 2015 at 01:40:40PM -0600, Eric W. Biederman wrote:
>>>
>>>> +    inode = path.dentry->d_inode;
>>>> +    filp->f_path = path;
>>>> +    filp->f_inode = inode;
>>>> +    filp->f_mapping = inode->i_mapping;
>>>> +    path_put(&old);
>>>
>>> Don't.  You are creating a fairly subtle constraint on what the code in
>>> fs/open.c and fs/namei.c can do, for no good reason.  You can bloody
>>> well maintain the information you need without that.
>>
>> There is a good reason.  We can not write a race free version of ptsname
>> without it.
> 
> As long as this is for new userspace code, would it make sense to just
> add a new ioctl to ask "does this ptmx fd match this /dev/pts fd?"
> 

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.

	-hpa

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