kernfs: Add support for always empty directories.

commit ea015218f2f7ace2dad9cedd21ed95bdba2886d7 upstream.

Add a new function kernfs_create_empty_dir that can be used to create
directory that can not be modified.

Update the code to use make_empty_dir_inode when reporting a
permanently empty directory to the vfs.

Update the code to not allow adding to permanently empty directories.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 39 insertions, 1 deletions

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index fffca9517321..2d48d28e1640 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -592,6 +592,9 @@ int kernfs_add_one(struct kernfs_node *kn)
                 goto out_unlock;
 
         ret = -ENOENT;
+        if (parent->flags & KERNFS_EMPTY_DIR)
+                goto out_unlock;
+
         if ((parent->flags & KERNFS_ACTIVATED) && !kernfs_active(parent))
                 goto out_unlock;
 
@@ -783,6 +786,38 @@ struct kernfs_node *kernfs_create_dir_ns(struct kernfs_node *parent,
         return ERR_PTR(rc);
 }
 
+/**
+ * kernfs_create_empty_dir - create an always empty directory
+ * @parent: parent in which to create a new directory
+ * @name: name of the new directory
+ *
+ * Returns the created node on success, ERR_PTR() value on failure.
+ */
+struct kernfs_node *kernfs_create_empty_dir(struct kernfs_node *parent,
+                                            const char *name)
+{
+        struct kernfs_node *kn;
+        int rc;
+
+        /* allocate */
+        kn = kernfs_new_node(parent, name, S_IRUGO|S_IXUGO|S_IFDIR, KERNFS_DIR);
+        if (!kn)
+                return ERR_PTR(-ENOMEM);
+
+        kn->flags |= KERNFS_EMPTY_DIR;
+        kn->dir.root = parent->dir.root;
+        kn->ns = NULL;
+        kn->priv = NULL;
+
+        /* link in */
+        rc = kernfs_add_one(kn);
+        if (!rc)
+                return kn;
+
+        kernfs_put(kn);
+        return ERR_PTR(rc);
+}
+
 static struct dentry *kernfs_iop_lookup(struct inode *dir,
                                         struct dentry *dentry,
                                         unsigned int flags)
@@ -1254,7 +1289,8 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
         mutex_lock(&kernfs_mutex);
 
         error = -ENOENT;
-        if (!kernfs_active(kn) || !kernfs_active(new_parent))
+        if (!kernfs_active(kn) || !kernfs_active(new_parent) ||
+            (new_parent->flags & KERNFS_EMPTY_DIR))
                 goto out;
 
         error = 0;
diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index 2da8493a380b..756dd56aaf60 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -296,6 +296,8 @@ static void kernfs_init_inode(struct kernfs_node *kn, struct inode *inode)
         case KERNFS_DIR:
                 inode->i_op = &kernfs_dir_iops;
                 inode->i_fop = &kernfs_dir_fops;
+                if (kn->flags & KERNFS_EMPTY_DIR)
+                        make_empty_dir_inode(inode);
                 break;
         case KERNFS_FILE:
                 inode->i_size = kn->attr.size;