Fix ExFAT parsing with clusters larger than one sector

[Lekensteyn]

The existing exfat.bin testcase is configured with a sector size equal to the cluster size. When that is not the case, parsing the root directory and subdirectory entries fails with an EOFError. Since directory entries are sector-aligned, reduce the block size to match. Fix some mistakes and clarify the documentation while at it.

Fixes a crash reproduced with with exfatprogs version 1.2.6:

truncate -s 4M exfat4m.bin; mkfs.exfat exfat4m.bin
python3 -c 'from dissect.fat import ExFAT; fs = ExFAT(open("exfat4m.bin", "rb")); print(fs.files["/"][1].keys())'

This check for a file and subdirectory also passes now:

# mount exfat4m.bin /mnt
# mkdir /mnt/subdir
# touch /mnt/file.txt /mnt/subdir/sub.txt
# umount /mnt
$ python3 -c 'from dissect.fat import ExFAT; fs = ExFAT(open("exfat4m.bin", "rb")); print(fs.files["/"][1]["subdir"][1].keys())'
odict_keys(['sub.txt'])
$ python3 -c 'from dissect.fat import ExFAT; fs = ExFAT(open("exfat4m.bin", "rb")); print(fs.files["/"][1].keys())'
odict_keys(['subdir', 'file.txt'])

This test file has been included in the test suite.

[Horofic]

Hey @Lekensteyn thank you for the fix! I believe it should be possible to upload files to git-lfs on your fork. They'll get added to this project once the PR is merged. Alternatively I could try to create the file for you to adjust the tests to.

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[Lekensteyn]

@DissectBot agree

[Lekensteyn]

Looks like it worked, the git-lfs object is also visible in this repo!

My local tests pass, fingers crossed that CI will also be happy :)

$ pytest -v -k test_exfat.py | cat
============================= test session starts ==============================
platform linux -- Python 3.12.7, pytest-8.3.4, pluggy-1.5.0 -- /tmp/venv/bin/python
cachedir: .pytest_cache
rootdir: /tmp/dissect.fat
configfile: pyproject.toml
collecting ... collected 8 items / 6 deselected / 2 selected

test_exfat.py::test_exfat PASSED                                         [ 50%]
test_exfat.py::test_exfat_4m PASSED                                      [100%]

======================= 2 passed, 6 deselected in 0.20s ========================

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.00%. Comparing base (fe9d7dc) to head (d681187).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dissect/fat/exfat.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   81.89%   82.00%   +0.10%     
==========================================
  Files           6        6              
  Lines         497      500       +3     
==========================================
+ Hits          407      410       +3     
  Misses         90       90              

Flag	Coverage Δ
unittests	82.00% <90.00%> (+0.10%)	⬆️

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[Horofic]

With this the CI should pass.

[Lekensteyn]

The coverage checker is complaining about this branch. Not much to be done here, this code is unused.

[Horofic]

That’s fine for now. This specific file system implementation is due for a rewrite since it’s not compatible anymore with the rest of the Dissect API.