linux-kernel - Re: [PATCH] devpts: Make each mount of devpts an independent filesystem.

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Date:	Wed, 20 Apr 2016 05:11:13 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Willy Tarreau <w@....eu>,
	Aurelien Jarno <aurelien@...el32.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jann Horn <jann@...jh.net>, Greg KH <greg@...ah.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>, Florian Weimer <fw@...eb.enyo.de>
Subject: Re: [PATCH] devpts: Make each mount of devpts an independent
 filesystem.

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...

> >> +	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.