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

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Date:	Tue, 19 Apr 2016 22:43:03 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security\@kernel.org" <security@...nel.org>,
	"security\@ubuntu.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.

Al Viro <viro@...IV.linux.org.uk> writes:

> On Tue, Apr 19, 2016 at 10:04:20PM -0500, Eric W. Biederman wrote:
>> +#ifdef CONFIG_UNIX98_PTYS
>> +int path_pts(struct path *path)
>> +{
>> +	/* Find "pts" in the same directory as the input path */
>> +	struct dentry *child, *parent;
>> +	struct qstr this;
>> +	int ret;
>> +
>> +	ret = path_parent_directory(path);
>> +	if (ret)
>> +		return ret;
>> +
>> +	parent = path->dentry;
>> +	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. 

>> +	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.

>> +	child = d_lookup(parent, &this);
>> +	inode_unlock(parent->d_inode);
>> +	if (!child)
>> +		return -ENOENT;
>
> Take a look at d_hash_and_lookup(), BTW...

Yes.  That does look like a reasonable simplification.

Eric