fs: Add helper functions for permanently empty directories.

commit fbabfd0f4ee2e8847bf56edf481249ad1bb8c44d upstream.

To ensure it is safe to mount proc and sysfs I need to check if
filesystems that are mounted on top of them are mounted on truly empty
directories.  Given that some directories can gain entries over time,
knowing that a directory is empty right now is insufficient.

Therefore add supporting infrastructure for permantently empty
directories that proc and sysfs can use when they create mount points
for filesystems and fs_fully_visible can use to test for permanently
empty directories to ensure that nothing will be gained by mounting a
fresh copy of proc or sysfs.

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

1 files changed, 96 insertions, 0 deletions

diff --git a/fs/libfs.c b/fs/libfs.c
index cb1fb4b9b637..02813592e121 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1093,3 +1093,99 @@ simple_nosetlease(struct file *filp, long arg, struct file_lock **flp,
         return -EINVAL;
 }
 EXPORT_SYMBOL(simple_nosetlease);
+
+
+/*
+ * Operations for a permanently empty directory.
+ */
+static struct dentry *empty_dir_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
+{
+        return ERR_PTR(-ENOENT);
+}
+
+static int empty_dir_getattr(struct vfsmount *mnt, struct dentry *dentry,
+                                 struct kstat *stat)
+{
+        struct inode *inode = d_inode(dentry);
+        generic_fillattr(inode, stat);
+        return 0;
+}
+
+static int empty_dir_setattr(struct dentry *dentry, struct iattr *attr)
+{
+        return -EPERM;
+}
+
+static int empty_dir_setxattr(struct dentry *dentry, const char *name,
+                              const void *value, size_t size, int flags)
+{
+        return -EOPNOTSUPP;
+}
+
+static ssize_t empty_dir_getxattr(struct dentry *dentry, const char *name,
+                                  void *value, size_t size)
+{
+        return -EOPNOTSUPP;
+}
+
+static int empty_dir_removexattr(struct dentry *dentry, const char *name)
+{
+        return -EOPNOTSUPP;
+}
+
+static ssize_t empty_dir_listxattr(struct dentry *dentry, char *list, size_t size)
+{
+        return -EOPNOTSUPP;
+}
+
+static const struct inode_operations empty_dir_inode_operations = {
+        .lookup         = empty_dir_lookup,
+        .permission     = generic_permission,
+        .setattr        = empty_dir_setattr,
+        .getattr        = empty_dir_getattr,
+        .setxattr       = empty_dir_setxattr,
+        .getxattr       = empty_dir_getxattr,
+        .removexattr    = empty_dir_removexattr,
+        .listxattr      = empty_dir_listxattr,
+};
+
+static loff_t empty_dir_llseek(struct file *file, loff_t offset, int whence)
+{
+        /* An empty directory has two entries . and .. at offsets 0 and 1 */
+        return generic_file_llseek_size(file, offset, whence, 2, 2);
+}
+
+static int empty_dir_readdir(struct file *file, struct dir_context *ctx)
+{
+        dir_emit_dots(file, ctx);
+        return 0;
+}
+
+static const struct file_operations empty_dir_operations = {
+        .llseek         = empty_dir_llseek,
+        .read           = generic_read_dir,
+        .iterate        = empty_dir_readdir,
+        .fsync          = noop_fsync,
+};
+
+
+void make_empty_dir_inode(struct inode *inode)
+{
+        set_nlink(inode, 2);
+        inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO;
+        inode->i_uid = GLOBAL_ROOT_UID;
+        inode->i_gid = GLOBAL_ROOT_GID;
+        inode->i_rdev = 0;
+        inode->i_size = 2;
+        inode->i_blkbits = PAGE_SHIFT;
+        inode->i_blocks = 0;
+
+        inode->i_op = &empty_dir_inode_operations;
+        inode->i_fop = &empty_dir_operations;
+}
+
+bool is_empty_dir_inode(struct inode *inode)
+{
+        return (inode->i_fop == &empty_dir_operations) &&
+                (inode->i_op == &empty_dir_inode_operations);
+}