NTFS_FILE_RECORD_INPUT_BUFFER unless the fileId is invalid.
The MFT is fairly well documented in many places on the web. That said, it is important to note that an MFT FILE-RECORD contains key-value pairs of attributes. Among the things in those attributes are the hardlink names, any reparse-point, and any NTFS streams (like a dir) that are part of that file.
Put another way, a given fileId can refer to multiple "file names" if it has multiple hardlinks pointing to it. So the canonical name is the first hardlink found in the key-value attributes of the $MFT record, which is then walked backward/up through the parent directory tree to build the canonical path to the file.
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