[Import] Introducing libfatdino – low-level Linux library to handle FAT32 volumes at /dev file level

NOTE: This post was imported from my previous blog – v3l0c1r4pt0r.tk. It was originally published on 17th September 2013.

Today it’s finally time to introduce my first Linux project. Its main function is to provide all functions needed to handle block device formatted using FAT32 volume. For now it is early alpha providing only functions for reading. It also contain simple C++ demo that was used initially for testing. Main program repository is placed on github, so I hope all progress will be immediately uploaded and available for everyone. Whole library has been written in C with building scripts in make so it is possible to port it to every environment including BIOS which is as mentioned below one of its targets. Compilation should also be possible on environments other than Linux i.e. Windows. Important: library’s dependencies are glibc (Linux default so it shouldn’t be problem) and PCRE (found on most of the distributions) and there is no configure script as of now to check their presence so if you haven’t them you will get compilation error!

Starting to write this library I had 2 main goals. First one was to write read/write functions for use under Linux environment. This one will be achieved by completing this library. The second is to rewrite part of this lib responsible for reading to use it in pure BIOS environment. This goal will be the most important step to complete my another project that I won’t introduce here, because of really small amount of work done for now.

But you may ask: why FAT32, filesystem that really is the most ancient fs that is already in use? Answer for this question is relatively easy. The first fact that is in favor of FAT is its simplicity comparing to any modern filesystem that has features like permissions or access control lists. The other one is still considerable prevalence especially on every embedded application as cameras, phones (smart too), USB sticks.

Most of the code written here was based on Microsoft’s whitepaper entitled Microsoft Extensible Firmware Initiative FAT32 File System Specification. Few details that wasn’t included in that document is based on other people’s code found on Google so I couldn’t mention the exact names. The most interesting details that Microsoft’s whitepaper doesn’t contain is:

  • creation date and time values in directory entry structure
  • way that drivers (linux driver too) determine if file or directory that doesn’t contain Long File Name should be lowercase or uppercase
  • last access value encoding; that one is as of now still unsupported, I’ll try to add it after whole project is done

All functions that is already present in the lib has I think sufficiently detailed description on its header file. Their implementation is also present on other functions and on the demo app so I hope that it will not be problem to anyone to use it properly even though some caption in the demo are in Polish. The reason for that Polish strings is that demo was initially not designed to be public. To use that library some knowledge about FAT filesystem is required, because every function doesn’t contain all code needed to perform actions listed below, i.e. function that reads directory content has no code to list that directory and cannot follow cluster chain so that actions must be implemented by programmer that wants to use the lib. Of course following cluster chain is implemented in another function so there is no need to write it from scratch.

Functionalities implemented

  • reading a file
  • reading directory (with LFN support)
  • getting file/dir location based on its path in the filesystem
  • getting file’s properties

Functionalities to be implemented

  • writing to file
  • moving files
  • creating dirs
  • modifying directory entries
  • writing bootloader code to BPB/BS

Functions which operation isn’t visible to end user aren’t listed above so there is still lot of work to do.