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

["Serge E. Hallyn" <serge@hallyn.com>]

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 :)

> 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 <brauner@kernel.org>
> Cc: John Johansen <john.johansen@canonical.com>
> Cc: Matthew Garrett <mjg59@srcf.ucam.org>
> Cc: Micah Morton <mortonm@chromium.org>
> Cc: Kentaro Takeda <takedakn@nttdata.co.jp>
> Cc: James Morris <jmorris@namei.org>
> Cc: Jarkko Sakkinen <jarkko@kernel.org>
> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
> ---
>  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