From: ebiederm@xmission.com (Eric W. Biederman)
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Hurley <peter@hurleysoftware.com>, Greg KH <greg@kroah.com>,
        Jiri Slaby <jslaby@suse.com>, Aurelien Jarno <aurelien@aurel32.net>,
        Andy Lutomirski <luto@amacapital.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@debian.org, Willy Tarreau <w@1wt.eu>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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	<1459819769-30387-1-git-send-email-ebiederm@xmission.com>
	<20160405025425.GK17997@ZenIV.linux.org.uk>
	<20160405030333.GL17997@ZenIV.linux.org.uk>
Date: Fri, 08 Apr 2016 13:54:01 -0500
In-Reply-To: <20160405030333.GL17997@ZenIV.linux.org.uk> (Al Viro's message of
	"Tue, 5 Apr 2016 04:03:33 +0100")
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Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup
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Al Viro <viro@ZenIV.linux.org.uk> writes:

> On Tue, Apr 05, 2016 at 03:54:25AM +0100, Al Viro wrote:
>
>> That, I take it, is a lookup for .. and buggering off if it fails *or* if
>> we had been in caller's root or something that overmount it?  Not that the
>> latter had been possible - root is a directory and can be overmounted only
>> by another such, and we are called from ->open() of a device node.
>> 
>> > +	/* Remember the result of this permission check for later */
>> > +	ret = inode_permission(path.dentry->d_inode, MAY_EXEC);
>> > +	if (path_pts(&path))
>> > +		goto fail;
>> 
>> Egads, man - you've just introduced a special function for looking up
>> something named "pts" in a given directory!
>> 
>> The reason not to use kern_path() would be what, the fact that it doesn't
>> allow starting at given location?  So let's make a variant that would - and
>> rather than bothering with RCU, just go for something like (completely
>> untested)
>
> Ah...  Right, that would demand exec permissions on the starting point.
> Still, this is incredibly ugly ;-/  I'll try to come up with something
> more tolerable, but this "path_pts" thing is too ugly to live.
> Seriously.

Given that I can think of no other reason than this special case to ever
want to use this code.  I figured having something incredibily special
case and obviously so was the way to go.  Then at least no one would
mistake it for a general purpose facility.

Eric