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_tex_mkfs_get_size_for_inodes(size_tninodes) {
// space for inodessize_trequired=ninodes*EX_BLOCK_SIZE;
// space for inodes bitmaprequired+=round_to_block(ninodes / 8);
// space for data of inodesrequired+=ninodes*EX_DIRECT_BLOCKS*EX_BLOCK_SIZE;
// space for data bitmaprequired+=round_to_block(ninodes*EX_DIRECT_BLOCKS / 8);
// space for super blockrequired+=round_to_block(sizeof(structex_super_block));
returnrequired;
}

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