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>,
        Andy Lutomirski <luto@amacapital.net>, security@debian.org,
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        Aurelien Jarno <aurelien@aurel32.net>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Jann Horn <jann@thejh.net>, Greg KH <greg@kroah.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jiri Slaby <jslaby@suse.com>, Florian Weimer <fw@deneb.enyo.de>
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	<alpine.LFD.2.20.1604161105040.30857@i7>
	<87twixgsnq.fsf@x220.int.ebiederm.org>
	<CA+55aFyOv3jUOsC-JbfeqM-Rs1z1a+QEt2EejpTf-9EfXdttdg@mail.gmail.com>
	<87oa95gevf.fsf_-_@x220.int.ebiederm.org>
	<20160420032541.GE25498@ZenIV.linux.org.uk>
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	<20160420041113.GF25498@ZenIV.linux.org.uk>
Date: Tue, 19 Apr 2016 23:21:11 -0500
In-Reply-To: <20160420041113.GF25498@ZenIV.linux.org.uk> (Al Viro's message of
	"Wed, 20 Apr 2016 05:11:13 +0100")
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Subject: Re: [PATCH] devpts: Make each mount of devpts an independent filesystem.
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Al Viro <viro@ZenIV.linux.org.uk> writes:

> On Tue, Apr 19, 2016 at 10:43:03PM -0500, Eric W. Biederman wrote:
>> >> +	if (!d_can_lookup(parent))
>> >> +		return -ENOENT;
>> >
>> > And how, pray tell, would a parent of anything fail to be a directory?
>> 
>> It is to make that function be visually distinct from path_parentat
>> which does something rather different. 
>
> Huh?  I'm asking how can that condition ever turn out to be true.  Unless
> you really advocate something like
> 	if (2 * 17 != 34)
> 		return -234567;	// to make it visually distinct from foobar(),
> 				// which doesn't have such a test
> your reply doesn't seem to make any sense...

Oh apologies I thought you were asking about the naming of the function,
path_parent_directory.  Yes.  The d_can_lookup does appear to be redundant.

It definitely looks like bedtime for me.

>> >> +	this.name = "pts";
>> >> +	this.len = 3;
>> >> +	this.hash = full_name_hash(this.name, this.len);
>> >> +	if (parent->d_flags & DCACHE_OP_HASH) {
>> >> +		int err = parent->d_op->d_hash(parent, &this);
>> >> +		if (err < 0)
>> >> +			return err;
>> >> +	}
>> >> +	inode_lock(parent->d_inode);
>> >
>> > What the hell for?  What does that lock on parent change for the
>> > dcache lookup you are doing here?
>> 
>> Good point. That is overkill. As we know the dentry is a mount point and
>> must be in the dcache, the customary lock for performing a lookup from
>> the disk is not necessary.
>
> Er...  To avoid reader confusion:
> 	a) d_lookup() does *not* do a filesystem lookup
> 	b) it does not need inode_lock()
> 	c) it (and not a "lookup from the disk") is what's actually being
> called in the code in question.

And since I was stripping down the ordinary filesystem lookup path to
just the pieces needed I apparently wound up with a few extras.

Do you think it would be possible to guarantee an rcu lookup for the
operations in path_pts?  I think needing to perform a follow_mount makes
that impossible to guarantee.

All the caller wants is to find the superblock of the mounted filesystem
and increment sb->s_active.

Eric