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

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Date:	Tue, 19 Apr 2016 23:21:11 -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 10:43:03PM -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