Exfuse

Exfuse is a simple Unix filesystem that I created for educational purposes. Currently, its use is limited to the fuse, but is written so that it can be used as a library without it. I describe it as simple because

• An inode uses only the direct blocks
• All blocks are allocated when creating a folder/file
• Files are stored in a folder as a list
• Uses a global lock
• No duplication of key structures (e.g. superblock, root, bitmaps)
• There is nothing like journal log

Limitations

Because the filesystem uses only the direct addressing there is a limitation of file size and number of entries in a directory. File size is limited to EX_DIRECT_BLOCKS * EX_BLOCK_SIZE, which is currently 256 * 4096 = 1MiB and the maximum number of elements in a directory is limited to EX_DIRECT_BLOCKS * EX_BLOCK_SIZE / sizeof (struct ex_dir_entry) which is currently 256 * 4096 / 64 = 16384 entries.

List of implemented functions

    access
    chmod
    chown
    create
    getattr (e.g. stat)
    link
    mkdir
    open
    read
    readdir
    readlink
    rename
    rmdir
    statfs
    symlink
    truncate
    unlink
    utimens
    write

Compilation

mkdir --parent build
cd build
cmake .. -DTESTS=ON
make && make test

Components

exmkfs

It is used to store filesystem structures on a "device". You can specify the maximum number of inodes during the initialization of filesystem, the default value is 256. The minimum device size for the given number of inodes is determined by the following function:

size_t ex_mkfs_get_size_for_inodes(size_t ninodes) {
    // space for inodes
    size_t required = ninodes * EX_BLOCK_SIZE;
    // space for inodes bitmap
    required += round_to_block(ninodes / 8);
    // space for data of inodes
    required += ninodes * EX_DIRECT_BLOCKS * EX_BLOCK_SIZE;
    // space for data bitmap
    required += round_to_block(ninodes * EX_DIRECT_BLOCKS / 8);
    // space for super block
    required += round_to_block(sizeof(struct ex_super_block));

    return required;
}

exdbg

It is used for introspection of data structures. It can now display superblock, inode and structure size information, and can display bitmap and inode data. Bitmap and inode data are written in binary format on stdout, so it is useful to display them with something like xxd.

libexfuse

A library that contains all the filesystem logic. It does not depend on libfuse, its interfaces can be found in src/ex.h. Its primary purpose is to be used in tests.

exfuse

Wrapper around libfuse. It contains main from where it calls fuse_main, to which it passes struct fuse operations. The implementation is in src/wrapper.c .

Usage

Create a filesystem

$ ./exmkfs --device foo --create --inodes 1024 --log-level info

Display information about the superblock

$ ./exdbg --device foo
super:
    root = 40960
    magic = ffaacc
    device_size = 1077977088 (1.004GiB)
data_bitmap:
    head = 16
    last = 31
    address = 8192
    size = 32768
    allocated = 256
    maxitems = 262144
inode_bitmap:
    head = 64
    last = 0
    address = 4096
    size = 128
    allocated = 1
    maxitems = 1024

Display information about the inode

$ ./exdbg --device foo --inode 40960
info: device.c: ex_device_open: device is open: fd=3
inode:
    number: 0
    size: 2144
    magic: abcc
    uid: 1000
    gid: 1000
    address: 40960
    nlinks: 2
    mode: 40705 (dir other=r-x group=--- user=rwx)
    mtime: 1573202060.477585196
    atime: 1573202060.477585196
    ctime: 1573202060.477585196
    entries:
        name: '.', address: 40960
        name: '..', address: 40960

Display the inode data

$ ./exdbg --device foo --inode-data 40960 | xxd
00000000: 00a0 0000 0000 0000 de00 2e00 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 00a0 0000 0000 0000 de00 2e2e 0000 0000  ................
00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000080: 6161 6161 6161 6161 6161 6161 6161 6161  aaaaaaaaaaaaaaaa
00000090: 6161 6161 6161 6161 6161 6161 6161 6161  aaaaaaaaaaaaaaaa

Display the bitmap data

$ ./exdbg --device foo --bitmap-data 64 | xxd
00000000: 0100 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................

Mount the filesystem

mkdir mp
./exfuse -f --device foo mp