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

Christian Brauner
2022-05-10T15:51:16Z

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@kernel.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()
}