fs: Add helper functions for permanently empty directories.
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.
Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/fs/libfs.c b/fs/libfs.c
index cb1fb4b..0281359 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1093,3 +1093,99 @@
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);
+}