From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 6C16123C2 for ; Tue, 10 May 2022 10:26:36 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 54F86C385C6; Tue, 10 May 2022 10:26:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1652178396; bh=F7UCs6MY2v5gEg3OOGEyNsq6zFa48822pWX1xYsPpAk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=KE55aPGkw7dfdqaaR2oZ817K6C3dkPClEwJ3JeUvvOJmWZX4gCeduI9qV8eRTpXol KXG8kx/AHmcxSmbMjABfiauAOzG7NrBLtGGa4vidbKdX7yQJV49RY1KC479Jfg8rQW uxW1XxT4Ib8cyv18w/VrY48xQGyN3PtIYFhoFUUWHnN9WfJztQ8pnnJE6YEh2Y6c44 jmxpXtKftWBsRTttyP+5qbCAge1vSDjFgHok7Z61R0emmGbtcKc3352ffBsfQzQHps 72WHQlOYZ0PTdkHFjvE47D3nGDHy4wFV0oNoqe4kydgYEBTVkIuermvbJaTYYNGBKh /9Dk6cl+oMRVQ== Date: Tue, 10 May 2022 12:26:27 +0200 From: Christian Brauner To: "Serge E. Hallyn" Cc: Stefan Berger , linux-integrity@vger.kernel.org, zohar@linux.ibm.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, John Johansen , Matthew Garrett , Micah Morton , Kentaro Takeda , Jarkko Sakkinen Subject: Re: [PATCH v12 01/26] securityfs: rework dentry creation Message-ID: <20220510102627.t7tzxkuigt7w7fyw@wittgenstein> References: <20220420140633.753772-1-stefanb@linux.ibm.com> <20220420140633.753772-2-stefanb@linux.ibm.com> <20220509195414.GA30894@mail.hallyn.com> <20220509203618.GA31408@mail.hallyn.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=utf-8 Content-Disposition: inline In-Reply-To: <20220509203618.GA31408@mail.hallyn.com> On Mon, May 09, 2022 at 03:36:18PM -0500, Serge Hallyn wrote: > On Mon, May 09, 2022 at 02:54:14PM -0500, Serge E. Hallyn wrote: > > 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); > > Hm, so I realize your patch isn't introducing this, but is the > fact that we ignore the possible -ENOTEMPTY return value of > simple_rmdir() not a problem? > > > > else > > > simple_unlink(dir, dentry); > > > + d_delete(dentry); > > I'm mostly trying to convince myself that the fact that there is not > a matching dput being removed (to match the dget being removed above) > is ok. I do think it is, but that belief seems to dictate that right > now dentries must never be being released. > > Otherwise, it seems like there must be cases where the next dput could > be called on a dentry that has been freed. I think that's answered by Amir in the next mail already. So I'm skipping to that part of the thread.