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@xxxxxxxxxx> > > > > > > > > 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() }
