linux-kernel - Re: [PATCH v12 01/26] securityfs: rework dentry creation

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Message-ID: <20220510102525.hlt2rm3k3hg5r6gg@wittgenstein>
Date:   Tue, 10 May 2022 12:25:25 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Stefan Berger <stefanb@...ux.ibm.com>,
        linux-integrity@...r.kernel.org, zohar@...ux.ibm.com,
        christian.brauner@...ntu.com, containers@...ts.linux.dev,
        dmitry.kasatkin@...il.com, ebiederm@...ssion.com,
        krzysztof.struczynski@...wei.com, roberto.sassu@...wei.com,
        mpeters@...hat.com, lhinds@...hat.com, lsturman@...hat.com,
        puiterwi@...hat.com, jejb@...ux.ibm.com, jamjoom@...ibm.com,
        linux-kernel@...r.kernel.org, paul@...l-moore.com, rgb@...hat.com,
        linux-security-module@...r.kernel.org, jmorris@...ei.org,
        jpenumak@...hat.com, John Johansen <john.johansen@...onical.com>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Micah Morton <mortonm@...omium.org>,
        Kentaro Takeda <takedakn@...data.co.jp>,
        Jarkko Sakkinen <jarkko@...nel.org>
Subject: Re: [PATCH v12 01/26] securityfs: rework dentry creation

On Mon, May 09, 2022 at 02:54:14PM -0500, Serge Hallyn wrote:
> On Wed, Apr 20, 2022 at 10:06:08AM -0400, Stefan Berger wrote:
> > From: Christian Brauner <brauner@...nel.org>
> > 
> > When securityfs creates a new file or directory via
> > securityfs_create_dentry() it will take an additional reference on the
> > newly created dentry after it has attached the new inode to the new
> > dentry and added it to the hashqueues.
> > If we contrast this with debugfs which has the same underlying logic as
> > securityfs. It uses a similar pairing as securityfs. Where securityfs
> > has the securityfs_create_dentry() and securityfs_remove() pairing,
> > debugfs has the __debugfs_create_file() and debugfs_remove() pairing.
> > 
> > In contrast to securityfs, debugfs doesn't take an additional reference
> > on the newly created dentry in __debugfs_create_file() which would need
> > to be put in debugfs_remove().
> > 
> > The additional dget() isn't a problem per se. In the current
> > implementation of securityfs each created dentry pins the filesystem via
> 
> Is 'via' an extra word here or is there a missing word?
> 
> I'll delay the rest of my response as the missing word may answer my
> remaining question :)

It can be both. It should either be removed or it should be followed by
"securityfs_create_dentry()". securityfs_create_dentry() takes two
references one in lookup_one_len() and another one explicitly via
dget(). The latter one isn't needed. Some of that has been covered in an
earlier thread:
https://lore.kernel.org/lkml/20220105101815.ldsm4s5yx7pmuiil@wittgenstein