linux-kernel - Re: [PATCH v3 2/4] kernfs: use VFS negative dentry caching

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Date:   Fri, 09 Apr 2021 17:34:01 +0800
From:   Ian Kent <raven@...maw.net>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        Brice Goglin <brice.goglin@...il.com>,
        Fox Chen <foxhlchen@...il.com>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        David Howells <dhowells@...hat.com>,
        Eric Sandeen <sandeen@...deen.net>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3 2/4] kernfs: use VFS negative dentry caching

On Fri, 2021-04-09 at 16:26 +0800, Ian Kent wrote:
> On Fri, 2021-04-09 at 01:35 +0000, Al Viro wrote:
> > On Fri, Apr 09, 2021 at 09:15:06AM +0800, Ian Kent wrote:
> > > +		parent = kernfs_dentry_node(dentry->d_parent);
> > > +		if (parent) {
> > > +			const void *ns = NULL;
> > > +
> > > +			if (kernfs_ns_enabled(parent))
> > > +				ns = kernfs_info(dentry->d_parent-
> > > > d_sb)->ns;
> > 
> > 	For any dentry d, we have d->d_parent->d_sb == d->d_sb.  All
> > the time.
> > If you ever run into the case where that would not be true, you've
> > found
> > a critical bug.
> 
> Right, yes.
> 
> > > +			kn = kernfs_find_ns(parent, dentry-
> > > > d_name.name, ns);
> > > +			if (kn)
> > > +				goto out_bad;
> > > +		}
> > 
> > Umm...  What's to prevent a race with successful rename(2)?  IOW,
> > what's
> > there to stabilize ->d_parent and ->d_name while we are in that
> > function?
> 
> Indeed, glad you looked at this.
> 
> Now I'm wondering how kerfs_iop_rename() protects itself from
> concurrent kernfs_rename_ns() ... 

As I thought ... I haven't done an exhaustive search but I can't find
any file system that doesn't call back into kernfs from
kernfs_syscall_ops (if provided at kernfs root creation).

I don't see anything that uses kernfs that defines a .rename() op
but if there was one it would be expected to call back into kernfs
at which point it would block on kernfs_mutex (kernfs_rwsem) until
it's released.

So I don't think there can be changes in this case due to the lock
taken just above the code your questioning.

I need to think a bit about whether the dentry being negative (ie.
not having kernfs node) could allow bad things to happen ...

Or am I misunderstanding the race your pointing out here?

Ian