The FILE_NAME_INFORMATION struct fits only one character.

So you need to allocate more space as you need it. To quote:

```
If ZwQueryInformationFile fails because of a buffer overflow, drivers that implement FileNameInformation should return as many WCHAR characters of the file name as will fit in the buffer and specify the full length that is required in the FileNameLength parameter of the FILE_NAME_INFORMATION structure. You should reissue the query by using the file name length so that you can retrieve the full file name. Drivers that do not follow this pattern might require a gradual increase in length until they retrieve the full file name. For more information about working with files, see Using Files in a Driver.
```