Description
Goal:
To implement a simple filesystem via FUSE to gain a better understanding of filesystems.
Background:
[FUSE](https://github.com/libfuse/libfuse) is a Linux kernel extension that allows for a user space program to provide the implementations for the various file-related syscalls.
We will be using FUSE to create our own file system, managed via a single file that represents our disk device.
Through FUSE and our implementation, it will be possible to interact with our newly created file system using standard UNIX/Linux programs in a transparent way.
From a user interface perspective, our file system will be a two-level directory system, with the following restrictions/simplifications:
* The root directory will only contain other subdirectories, and no regular files
* The subdirectories will only contain regular files, and no subdirectories of their own
* All files will be full access (i.e., chmod 0666), with permissions to be mainly ignored
* Many file attributes such as creation and modification times will not be accurately stored
* Files cannot be truncated
* From an implementation perspective, the file system will keep data on “disk” via a linked allocation strategy with a file allocation table (FAT), outlined below.
Installing FUSE
FUSE consists of two major components: A kernel module that has already been installed, and a set of libraries and example programs that you need to install.
First, copy the source code to your /u/OSLab/USERNAME directory:
“`
cd /u/OSLab/USERNAME
cp /u/OSLab/original/fuse-2.7.0.tar.gz .
tar xvfz fuse-2.7.0.tar.gz
cd fuse-2.7.0
“`
Now, we do the normal configure, compile, install procedure on UNIX, but omit the install step since that needs to be done as a superuser and has already been done.
“`
./configure
make
“`
(The third step would be make install, but if you try it, you will be met with many access denied errors.)
FUSE EXAMPLE
Now, we’ll walk through one of the examples. Enter the following:
“`
cd /u/OSLab/USERNAME/
cd fuse-2.7.0/examples
mkdir testmount
ls -al testmount
./hello testmount
ls -al testmount
“`
If you encounter an error when running `./hello testmount` (e.g., “fuse: failed to exec fusermount: Permission denied”), you will need to source the `/opt/set_specific_profile.sh` using the following command:
“`
source /opt/set_specific_profile.sh
“`
After successfully running `./hello testmount`, you should see 3 entries within it: `.`, `..`, and `hello`. We just created this directory, and thus it was empty, so where did hello come from? Obviously the hello application we just ran could have created it, but what it actually did was lie to the operating system when the OS asked for the contents of that directory. So let’s see what happens when we try to display the contents of the file.
“`
cat testmount/hello
“`
You should get the familiar hello world quotation. If we cat a file that doesn’t really exist, how do we get meaningful output? The answer comes from the fact that the hello application also gets notified of the attempt to read and open the fictional file “hello” and thus can return the data as if it was really there.
Examine the contents of `hello.c` in your favorite text editor, and look at the implementations of `readdir()` and read to see that it is just returning hard coded data back to the system.
The final thing we always need to do is to unmount the file system we just used when we are done or need to make changes to the program. Do so by:
“`
fusermount -u testmount
“`
FUSE High-level Description
The hello application we ran in the above example is a particular FUSE file system provided as a sample to demonstrate a few of the main ideas behind FUSE. The first thing we did was to create an empty directory to serve as a mount point. A mount point is a location in the UNIX hierarchical file system where a new device or file system is located. As an analogy, in Windows, “My Computer” is the mount point for your hard disks and CD-ROMs, and if you insert a USB drive or MP3 player, it will show up there as well. In UNIX, we can have mount points at any location in the file system tree.
Running the hello application and passing it the location of where we want the new file system mounted initiates FUSE and tells the kernel that any file operations that occur under our now mounted directory will be handled via FUSE and the hello application. When we are done using this file system, we simply tell the OS that it no longer is mounted by issuing the above fusermount -u command. At that point the OS goes back to managing that directory by itself.
Specification
Your job is to create the cs1550 file system as a FUSE application that provides the interface described in the first section. A code skeleton has been provided under the examples directory as `cs1550.c`. It is automatically built when you type `make` in the examples directory.
The cs1550 file system should be implemented using a single file, managed by the real file system in the directory that contains the cs1550 application. This file should keep track of the directories and the file data. We will consider the disk to have 512 byte blocks.
* To create a 5MB disk image, execute the following:
“`
dd bs=1K count=5K if=/dev/zero of=.disk
“`
This will create a file initialized to contain all zeros, named .disk. You only need to do this once, or every time you want to completely destroy the disk. (This is our “format” command.)
* In order to manage the free or empty space, you will need to create bookkeeping block(s) on our disk that records what blocks have been previously allocated or not.
* Since the disk contains blocks that are directories and blocks that are file data, we need to be able to find and identify what a particular block represents. In our file system, the root only contains other directories, so we will use block 0 of .disk to hold the directory entry of the root and, from there, find our subdirectories.
* The root directory entry will be a struct defined as below (the actual one we provide in the code has additional attributes and padding to force the structure to be 512 bytes):
“`c
struct cs1550_root_directory
{
int nDirectories; //How many subdirectories are in the root
//Needs to be less than MAX_DIRS_IN_ROOT
struct cs1550_directory
{
char dname[MAX_FILENAME + 1]; //directory name (plus space for nul)
long nStartBlock; //where the directory block is on disk
} directories[MAX_DIRS_IN_ROOT]; //There is an array of these
} ;
“`
Since we are limiting our root to be one block in size, there is a limit of how many subdirectories we can create, `MAX_DIRS_IN_ROOT`.
Each subdirectory will have an entry in the directories array with its name and the block index of the subdirectory’s directory entry.
* Directories will be stored in our .disk file as a single block-sized `cs1550_directory_entry` structure per subdirectory.
The structure is defined below (again the actual one we provide in the code has additional attributes and padding to force the structure to be 512 bytes):
“`c
struct cs1550_directory_entry
{
int nFiles; //How many files are in this directory.
//Needs to be less than MAX_FILES_IN_DIR
struct cs1550_file_directory
{
char fname[MAX_FILENAME + 1]; //filename (plus space for nul)
char fext[MAX_EXTENSION + 1]; //extension (plus space for nul)
size_t fsize; //file size
long nStartBlock; //where the first block is on disk
} files[MAX_FILES_IN_DIR]; //There is an array of these
};
“`
Since we require each directory entry to only take up a single disk block, we are limited to a fixed number of files per directory.
Each file entry in the directory has a filename in 8.3 (name.extension) format. We also need to record the total size of the file, and the location of the file’s first block on disk.
* Files will be stored alongside the directories in `.disk`. Data blocks are 512-byte structs of the format:
“`c
struct cs1550_disk_block
{
//The next disk block, if needed. This is the next pointer in the linked
//allocation list
long nNextBlock;
//All the space in the block can be used for actual data
//storage.
char data[MAX_DATA_IN_BLOCK];
};
“`
* This is how the resulting system is logically structured:
The root points to directory Dir1, which has two files, FileA and FileB. FileB spans two dis-contiguous blocks. FileC is referred to from some other directory, not shown.
* To be able to have a simple functioning file system, we need to handle a minimum set of operations on files and directories. The functions are listed here in the order that I suggest you implement them in. The last four do not need implemented beyond what the skeleton code has already.
The syscalls need to return success or failure. Success is indicated by 0 and appropriate errors by the negation of the error code, as listed on the corresponding function’s man page.
* `cs1550_getattr`
* _Description:_ This function should look up the input path to determine if it is a directory or a file. If it is a directory, return the appropriate permissions. If it is a file, return the appropriate permissions as well as the actual size. This size must be accurate since it is used to determine EOF and thus read may not be called.
* _UNIX Equivalent:_ `man -s 2 stat`
* _Return values:_
* 0 on success, with a correctly set structure
* ENOENT if the file is not found
* `cs1550_mkdir`
* _Description:_ This function should add the new directory to the root level, and should update the .disk file appropriately.
* _UNIX Equivalent:_ `man -s 2 mkdir`
* _Return values:_
* 0 on success
* ENAMETOOLONG if the name is beyond 8 chars
* EPERM if the directory is not under the root dir only
* EEXIST if the directory already exists
* `cs1550_readdir`
* _Description:_ This function should look up the input path, ensuring that it is a directory, and then list the contents.
To list the contents, you need to use the `filler()` function. For example: `filler(buf, “.”, NULL, 0);` adds the current directory to the listing generated by `ls -a`.
In general, you will only need to change the second parameter to be the name of the file or directory you want to add to the listing.
* _UNIX Equivalent:_ `man -s 2 readdir`
However it’s not exactly equivalent
* _Return values:_
* 0 on success
* ENOENT if the directory is not valid or found
* `cs1550_rmdir`
* This function should not be modified.
* `cs1550_mknod`
* _Description:_ This function should add a new file to a subdirectory, and should update the .disk file appropriately with the modified directory entry structure.
* _UNIX Equivalent:_ `man -s 2 mknod`
* _Return values:_
* 0 on success
* ENAMETOOLONG if the name is beyond 8.3 chars
* EPERM if the file is trying to be created in the root dir
* EEXIST if the file already exists
* `cs1550_write`
* _Description:_ This function should write the data in buf into the file denoted by path, starting at offset.
* _UNIX Equivalent:_ `man -s 2 write`
* _Return values:_
* size on success
* EFBIG if the offset is beyond the file size (but handle appends)
* `cs1550_read`
* _Description:_ This function should read the data in the file denoted by path into buf, starting at offset.
* _UNIX Equivalent:_ `man -s 2 read`
* _Return values:_
* size read on success
* EISDIR if the path is a directory
* `cs1550_unlink`
* This function should not be modified. Files thus cannot be deleted. However, we can still see the linked allocation of dis-contiguous blocks via appends.
* `cs1550_truncate`
* This function should not be modified.
* `cs1550_open`
* This function should not be modified, as you get the full path every time any of the other functions are called.
* `cs1550_flush`
* This function should not be modified.
* The cs1550.c file is included as part of the Makefile in the examples directory, so building your changes is as simple as typing make.
* One suggestion for testing is to launch a FUSE application with the `-d` option (`./cs1550 -d testmount`). This will keep the program in the foreground, and it will print out every message that the application receives, and interpret the return values that you’re getting back. Just open a second terminal window and try your testing procedures. Note if you do a CTRL+C in this window, you may not need to unmount the file system, but on crashes (transport errors) you definitely need to.
* Your first steps will involve simply testing with `ls` and `mkdir`. When that works, try using echo and redirection to write to a file. `cat` will read from a file, and you will eventually even be able to launch `pico` on a file.
* Remember that you may want to delete your `.disk` file if it becomes corrupted. You can use the commands `od -x` to see the contents in hex of a file, or the command `strings` to grab human readable text out of a binary file.
Submission Guidelines:
* **DO NOT** add your `.disk` file to your repository
* **DO NOT SUBMIT** any IDE package files.
* Your repository must include:
* Your `cs1550.c` source file
* A valid Makefile for your filesystem program
* You must be able to compile your filesystem program program by running `make`.
* You must fill out info_sheet.txt.
* Be sure to remember to push the latest copy of your code back to your GitHub repository before the the assignment is due. At the deadline, the repositories will automatically be copied for grading. Whatever is present in your GitHub repository at that time will be considered your submission for this assignment.
Additional Notes/Hints:
* The root directory is equivalent to your mount point. The FUSE application does not see the directory tree outside of this position. All paths are translated automatically for you.
* `sscanf(path, “/%[^/]/%[^.].%s”, directory, filename, extension);` or you can use `strtok()`
* Your application is part of userspace, and as such you are free to use whatever C Standard Libraries you wish, including the file handling ones.
* Remember to always close your disk file after you open it in a function. Since the program doesn’t terminate until you unmount the file system, if you’ve opened a file for writing and not closed it, no other function can open that file simultaneously.
* Remember to open your files for binary access.
* Without the -d option, FUSE will be launched without knowledge of the directory you started it in, and thus won’t be able to find your .disk file, if it is referenced via a relative path. This is okay, we will grade with the -d option enabled.
Grading Rubric:
* Free block bookkeeping done appropriately: 15
* `cs1550_getattr`: 15
* `cs1550_mkdir`: 15
* `cs1550_readdir`: 10
* `cs1550_mknod`: 10
* `cs1550_write`: 15
* `cs1550_read`: 15
* Assignment info sheet/submission: 5
