From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.hallyn.com (mail.hallyn.com [178.63.66.53]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 867737B for ; Mon, 9 May 2022 20:03:14 +0000 (UTC) Received: by mail.hallyn.com (Postfix, from userid 1001) id AA9D9936; Mon, 9 May 2022 14:54:14 -0500 (CDT) Date: Mon, 9 May 2022 14:54:14 -0500 From: "Serge E. Hallyn" To: Stefan Berger Cc: linux-integrity@vger.kernel.org, zohar@linux.ibm.com, serge@hallyn.com, christian.brauner@ubuntu.com, containers@lists.linux.dev, dmitry.kasatkin@gmail.com, ebiederm@xmission.com, krzysztof.struczynski@huawei.com, roberto.sassu@huawei.com, mpeters@redhat.com, lhinds@redhat.com, lsturman@redhat.com, puiterwi@redhat.com, jejb@linux.ibm.com, jamjoom@us.ibm.com, linux-kernel@vger.kernel.org, paul@paul-moore.com, rgb@redhat.com, linux-security-module@vger.kernel.org, jmorris@namei.org, jpenumak@redhat.com, Christian Brauner , John Johansen , Matthew Garrett , Micah Morton , Kentaro Takeda , Jarkko Sakkinen Subject: Re: [PATCH v12 01/26] securityfs: rework dentry creation Message-ID: <20220509195414.GA30894@mail.hallyn.com> References: <20220420140633.753772-1-stefanb@linux.ibm.com> <20220420140633.753772-2-stefanb@linux.ibm.com> Precedence: bulk X-Mailing-List: containers@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220420140633.753772-2-stefanb@linux.ibm.com> User-Agent: Mutt/1.9.4 (2018-02-28) On Wed, Apr 20, 2022 at 10:06:08AM -0400, Stefan Berger wrote: > From: Christian Brauner > > 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 :) > until it is removed. Since it is virtually guaranteed that there is at > least one user of securityfs that has created dentries the initial > securityfs mount cannot go away until all dentries have been removed. > > Since most of the users of the initial securityfs mount don't go away > until the system is shutdown the initial securityfs won't go away when > unmounted. Instead a mount will usually surface the same superblock as > before. The additional dget() doesn't matter in this scenario since it > is required that all dentries have been cleaned up by the respective > users before the superblock can be destroyed, i.e. superblock shutdown > is tied to the lifetime of the associated dentries. > > However, in order to support ima namespaces we need to extend securityfs > to support being mounted outside of the initial user namespace. For > namespaced users the pinning logic doesn't make sense. Whereas in the > initial namespace the securityfs instance and the associated data > structures of its users can't go away for reason explained earlier users > of non-initial securityfs instances do go away when the last users of > the namespace are gone. > > So for those users we neither want to duplicate the pinning logic nor > make the global securityfs instance display different information based > on the namespace. Both options would be really messy and hacky. > > Instead we will simply give each namespace its own securityfs instance > similar to how each ipc namespace has its own mqueue instance and all > entries in there are cleaned up on umount or when the last user of the > associated namespace is gone. > > This means that the superblock's lifetime isn't tied to the dentries. > Instead the last umount, without any fds kept open, will trigger a clean > shutdown. But now the additional dget() gets in the way. Instead of > being able to rely on the generic superblock shutdown logic we would > need to drop the additional dentry reference during superblock shutdown > for all associated users. That would force the use of a generic > coordination mechanism for current and future users of securityfs which > is unnecessary. Simply remove the additional dget() in > securityfs_dentry_create(). > > In securityfs_remove() we will call dget() to take an additional > reference on the dentry about to be removed. After simple_unlink() or > simple_rmdir() have dropped the dentry refcount we can call d_delete() > which will either turn the dentry into negative dentry if our earlier > dget() is the only reference to the dentry, i.e. it has no other users, > or remove it from the hashqueues in case there are additional users. > > All of these changes should not have any effect on the userspace > semantics of the initial securityfs mount. > > Signed-off-by: Christian Brauner > Cc: John Johansen > Cc: Matthew Garrett > Cc: Micah Morton > Cc: Kentaro Takeda > Cc: James Morris > Cc: Jarkko Sakkinen > Signed-off-by: Stefan Berger > Reviewed-by: Mimi Zohar > --- > security/inode.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/security/inode.c b/security/inode.c > index 6c326939750d..13e6780c4444 100644 > --- a/security/inode.c > +++ b/security/inode.c > @@ -159,7 +159,6 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode, > inode->i_fop = fops; > } > d_instantiate(dentry, inode); > - dget(dentry); > inode_unlock(dir); > return dentry; > > @@ -302,10 +301,12 @@ void securityfs_remove(struct dentry *dentry) > dir = d_inode(dentry->d_parent); > inode_lock(dir); > if (simple_positive(dentry)) { > + dget(dentry); > if (d_is_dir(dentry)) > simple_rmdir(dir, dentry); > else > simple_unlink(dir, dentry); > + d_delete(dentry); > dput(dentry); > } > inode_unlock(dir); > -- > 2.34.1