mkfat.py suspected to present phantom clusters

[Jakub Jermář]

This is a follow-up ticket on ticket #122, which was closed as a result of making srv/fat resistant against the phantom clusters.

The investigation revealed that the FAT file system image created by the mkfat.py utility has the total number of sectors (i.e. a boot sector record member) smaller than the number of sectors inferred from the sectors per fat (also a boot sector record member).

[Jakub Jermář]

Some documentation sources [1] seem to document this behavior, although the wording is not very clear:

"The last sector of the FAT is not necessarily all part of the FAT.
The FAT stops at the cluster number in the last FAT sector that
corresponds to the entry for cluster number CountofClusters + 1 (see
the CountofClusters computation earlier), and this entry is not
necessarily at the end of the last FAT sector. FAT code should not
make any assumptions about what the contents of the last FAT sector
are after the CountofClusters + 1 entry. FAT format code should zero
the bytes after this entry though."

[1] Microsoft Extensible Firmware Initiative: FAT32 File System

Specification — FAT: General Overview of On-Disk Format, version
1.03, Microsoft Corporation, December 6, 2000