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

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Message-ID: <20220510185137.GA10706@mail.hallyn.com>
Date:   Tue, 10 May 2022 13:51:37 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        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 Tue, May 10, 2022 at 05:51:07PM +0200, Christian Brauner wrote:
> On Tue, May 10, 2022 at 09:10:25AM -0500, Serge Hallyn wrote:
> > On Tue, May 10, 2022 at 12:25:25PM +0200, Christian Brauner wrote:
> > > 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
> > 
> > Yes, I saw that two references were being taken.  And near as I can tell,
> > the second one was never being dropped.  So if you tell me that before this
> > patch the dentries are never freed, then I'm happy.  Otherwise, I'm
> > bothered the fact that no matching dput is being deleted in the code (to
> > match the extra dget being removed).  So where is the code where the final
> > dput was happening, and is it the d_delete() you're adding which is making
> > it so that that dput won't be called now?
> 
> * So consider mounting securityfs _without this patch applied_:
> 
>   mount -t securityfs /sfs
>   
>   and assume we only have a single user that creates a file "foo" via
>   
>   securityfs_create_file()
>   {
>   	lookup_one_len();	// first dget()
>   	dget();			// second dget()
>   }
>   
>   now assume that user at some point calls
>   
>   void securityfs_remove()
>   {
>   	if (d_is_dir(dentry))
>   		simple_rmdir(dir, dentry);	// first dput()
>   	else
>   		simple_unlink(dir, dentry);	// first dput()
>   	dput(dentry);				// second dput()
>   }
> 
> * Now consider mounting securityfs _with this patch applied_:
> 
>   securityfs_create_file()
>   {
>   	lookup_one_len();	// first dget()
>   }
>   
>   void securityfs_remove()
>   {
>   	dget();					// second dget() 
>   	if (d_is_dir(dentry))
>   		simple_rmdir(dir, dentry);	// first dput()
>   	else
>   		simple_unlink(dir, dentry);	// first dput()
>   	dput(dentry);				// second dput()
>   }

Oh - I was thinking about the new d_delete, but I guess that doesn't matter.

thanks,
-serge