Work out why linux FAT driver likes to read all sorts of (unused) data

[ryankurte]

when running using rust fatfs (see tests) reads are as expected, however, when linked via usbd_scsi linux likes to walk a bunch of unused chunks for reasons yet unknown.

this doesn't seem to impact usability, but, it'd be nice to know what's up.

[ryankurte]

fatcat shows the disk configuration is as expected:

➜  ~ fatcat -i fat16test.img 
FAT Filesystem information

Filesystem type: FAT16   
OEM name: UF2 UF2 
Total sectors: 7998
Total data clusters: 8192
Data size: 4194304 (4M)
Disk size: 4094976 (3.90527M)
Bytes per sector: 512
Sectors per cluster: 1
Bytes per cluster: 512
Reserved sectors: 1
Root entries: 64
Root clusters: 4
Sectors per FAT: 32
Fat size: 16384 (16K)
FAT1 start address: 0000000000000200
FAT2 start address: 0000000000004200
Data start address: 0000000000008200
Root directory cluster: 0
Disk label: BLUEPILL   

Free clusters: 7999/8192 (97.644%)
Free space: 4095488 (3.90576M)
Used space: 98816 (96.5K)

though when listing directories fatcat and linux seemingly disagree on available files, and fatcat cannot retrieve data for the first.

➜  ~ ls -al /media/ryan/BLUEPILL 
total 71
drwxr-xr-x  2 ryan ryan  2048 Jan  1  1970 .
drwxr-x---+ 7 root root  4096 Apr 13 09:36 ..
-r--r--r--  1 ryan ryan     7 Jan  1  1980 a.txt
-rw-r--r--  1 ryan ryan 65536 Jan  1  1980 b.bin

➜  ~ sudo fatcat fat16test.img -l /    
Listing path /
Directory cluster: 0
f 0/0/1980 00:00:00  BLUEPILL                       c=0 s=0 (0B)
f 0/0/1980 00:00:00  b.bin                          c=3 s=65536 (64K)

image copied from current device:
fat16test.img.zip

[ryankurte]

directories seem to look a little weird, i would have expected to see a.txt in cluster 0 / the root directory, but, do not

➜  ghostfat git:(main) sudo fatcat /dev/sdd -l /
Listing path /
Directory cluster: 0
f 0/0/1980 00:00:00  GHOSTFAT                       c=0 s=0 (0B)
f 19/4/2022 09:10:14  b.bin                          c=3 s=56 (56B)
➜  ghostfat git:(main) sudo fatcat /dev/sdd -L 0
Listing cluster 0
Directory cluster: 0
f 0/0/1980 00:00:00  GHOSTFAT                       c=0 s=0 (0B)
f 19/4/2022 09:10:14  b.bin                          c=3 s=56 (56B)
➜  ghostfat git:(main) sudo fatcat /dev/sdd -L 1
Listing cluster 1
Directory cluster: 1
➜  ghostfat git:(main) sudo fatcat /dev/sdd -L 2
Listing cluster 2
Directory cluster: 2
f 0/0/1980 00:00:00  12345
                        c=0 s=0 (0B)

➜  ghostfat git:(main) sudo fatcat /dev/sdd -o
Computing FAT cache...
Building the chains...
Found 4 chains

Running the recursive differential analysis...
Exploring 0

Having a look at the chains...
Exploring 2
There is 1 orphaned elements:
* Directory clusters 2 to 2: 1 elements, 0B

Estimation of orphan files total sizes: 0 (0B)