ovl: add ovl_mmap()

Implement stacked mmap.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/file.c

@@ -255,6 +255,37 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 	return ret;
 }
 
+static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct file *realfile = file->private_data;
+	const struct cred *old_cred;
+	int ret;
+
+	if (!realfile->f_op->mmap)
+		return -ENODEV;
+
+	if (WARN_ON(file != vma->vm_file))
+		return -EIO;
+
+	vma->vm_file = get_file(realfile);
+
+	old_cred = ovl_override_creds(file_inode(file)->i_sb);
+	ret = call_mmap(vma->vm_file, vma);
+	revert_creds(old_cred);
+
+	if (ret) {
+		/* Drop reference count from new vm_file value */
+		fput(realfile);
+	} else {
+		/* Drop reference count from previous vm_file value */
+		fput(file);
+	}
+
+	ovl_file_accessed(file);
+
+	return ret;
+}
+
 const struct file_operations ovl_file_operations = {
 	.open		= ovl_open,
 	.release	= ovl_release,
@@ -262,4 +293,5 @@ const struct file_operations ovl_file_operations = {
 	.read_iter	= ovl_read_iter,
 	.write_iter	= ovl_write_iter,
 	.fsync		= ovl_fsync,
+	.mmap		= ovl_mmap,
 };