osteffenrh / vfat-rs

A simple VFAT implementation written in rust.

VFAT / FAT32

A simple VFAT implementation written in rust, and mostly tested against Linux's vfat driver.

It aims to be simple to understand and easy to use in a custom kernel.

no_std

This component was first developed with no_std in mind. std is mostly supported behind a feature flag. Check example/simple.rs for a usage example.

Run example

To run the example, first create a vfat fs using tests/setup.sh then run the example file using:

cargo run --example simple --feature std

Testing

To run the setup.sh script, I've added an exception for my user in the sudoers file:

fponzi ALL=(ALL) NOPASSWD: /usr/bin/mount,/usr/bin/umount

On github actions (CI) it just works, because the user has passwordless sudo. Then all tests can be run with cargo test. Each test in vfat.rs will create and delete a vfat filesystem.

Utils:

You can check whether the file contains a valid MBR via gdisk:

$ gdisk -l fat32.fs

and you can get info about the filesystem with fdisk:

$ fdisk -l fat32.fs

Some stupid script to flush fs changes:

sudo umount /mnt/test
sudo mount -o loop,offset=$((2048*512)) fat32.fs /mnt/test/
ls -l /mnt/test

Check the changes:

sudo dosfsck -w -r -l -v -r /dev/loop13

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TODO

• Have a better update entry which allows to support renaming.
• Test: What happens if there are no free clusters (memory is full)?

Long todo:

• Write to backup FAT as well.
• Free cluster summary update when allocating clusters.

Future improvements.

• Currently, the device mutex is shared behind an ARC reference. Maybe, also having the whole FS behind arc would save quite some space when returning files and directories. Because they get a copy of the Vfat struct.
• Get rid of alloc dependency? It's mostly used for String support rn.

FAQ

• What happens if I have a "File" handle and meanwhile someone deletes this file and I try to read from a deleted file? This case should be taken care of by the application using this library.

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Useful docs:

• https://www.win.tue.nl/~aeb/linux/fs/fat/fat-1.html
• Exfat specification: https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification#1-introduction

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To mount with 777 permission:

sudo mount -o loop,offset=$((2048*512)),uid=1000,gid=1000,dmask=0000,fmask=0001 fat32.fs /mnt/test/