MINIX FAT32 service

This is a fork of the MINIX 3.3.0 source with an implementation for fat32, a service that allows other processes ask about info for FAT32 partitions and read their contents. This is not a filesystem driver, and you cannot mount FAT32 partitions with it. A userspace tool for communicating with the service is provided.

Repo contents

Inside this repo is a snapshot of the /usr/src/minix tree with the FAT32 service added inside /usr/src/minix/servers/fat32 and added to all the necessary other files.

Compiling

Ensure that the source for your Minix system is the same as the one this fork is based on. You can check the initial commit for differences. Then, copy the contents of the usr/src/minix directory in the repo to your own /usr/src/minix/ directory. To build everything, cd to /usr/src/releasetools and run make hdboot. This will build your boot images in /boot. Reboot the system and run ps ax | grep fat32 to ensure fat32 is running (its pid should be 13).

Usage

This server is able to lets you traverse a FAT32 partition and read files off of it. It includes support for long filename entries and should hopefully be able to read any valid FAT32 filesystem. A big unsupported feature are wide-character filenames: the filenames are simply truncated to ASCII. The API is lower-level than a filesystem driver, and there isn't a libc-level API. Instead, user programs must make direct syscalls. Fortunately, I have provided a C++11 wrapper around this API inside fatori, that you can use verbatim or copy.

API

fatori/fat32.cpp contains the code for the wrapper, so you can take a look there to see what it actually does. You can take a look at the public interface inside fatori/fat32.hpp. The C++ API has the following classes:

• maybe<T>. Utility class that can either hold a value of type T or "nothing". This is returned by API calls that can return either some result or "no more results". If is_some is false, the maybe holds "nothing". If is_some is true, the maybe holds "something", which is stored inside maybe.value.
• fs. You construct an object of this type directly. The only parameter to the constructor is the path to a block device or file where the filesystem resides. (Be sure to give the block device of the partition, not of the whole drive, as fat32 cannot read the partition headers).
• dir. Represents a FAT32 directory. You get an object of this type by calling fs.open_root_dir(), which gives you the root directory of the partition, or dir.open_subdir(), which opens a subdirectory of this directory. The directory allows you to read the next entry in it by calling dir.next_entry(), which returns information about the next file/directory in the given directory (if there's no more, it returns a maybe<entry> with is_some set to false). This advances the cursor. If the returned entry is a directory, you can immediately call dir.open_subdir(), which will return a dir object representing the directory that corresponds to the entry that was just read.
• file. If the last entry read from a directory was a file, calling dir.open_file() will return a file object for you to work with, corresponding to the file that was just read as an entry. You have only one function of interest: file.read_block() allocates a cluster-sized buffer and reads the next cluster of the file. It returns a maybe<vector<uint8_t>> with is_some set to false if you've reached the end of the file. Otherwise, the contents are returned.

The API is fully RAII and properly throws exceptions if any operation fails.

fatori

fatori is a small user-space program which allows you to inspect the contents of a FAT32 partition easily. Run ./fatori <fat32-block-device-or-file>. You should get a prompt. Following are the commands fatori accepts. All paths are relative to the partition root.

• ls /path/to/dir. Shows the contents of a directory.
• tree /path/to/dir. Recursively shows the contents of a directory in a tree-like format.
• cat /path/to/file. Prints the contents of a given file.
• stat /path/to/file-or-dir. Shows available information about a given file or directory.
• exit.

The code uses the aforementioned C++ API and the source code lives at fatori/fatori.cpp, so you can take a look at how the API is used.

License

The MINIX code contained in this repo is copyrighted by The MINIX project and release under the MINIX license. I claim no ownership of it.

My own code on top of the MINIX source code is also released under the same MINIX license (clone of the BSD license).