The little filesystem in user-space

A FUSE wrapper that puts the littlefs in user-space.

FUSE - https://github.com/libfuse/libfuse
littlefs - https://github.com/geky/littlefs

This project allows you to mount littlefs directly in a host PC. This allows you to easily debug an embedded system using littlefs on removable storage, or even debug littlefs itself, since the block device can be viewed in a hex-editor simultaneously.

littlefs-fuse uses FUSE to interact with the host OS kernel, which means it can be compiled into a simple user program without kernel modifications. This comes with a performance penalty, but works well for the littlefs, since littlefs is intended for embedded systems.

Currently littlefs-fuse has been tested on the following OSs:

• Linux
• FreeBSD

Usage on Linux

littlefs-fuse requires FUSE version 2.6 or higher, you can find your FUSE version with:

fusermount -V

In order to build against FUSE, you will need the package libfuse-dev:

sudo apt-get install libfuse-dev

Once you have cloned littlefs-fuse, you can compile the program with make:

make

This should have built the lfs program in the top-level directory.

From here we will need a block device. If you don't have removable storage handy, you can use a file-backed block device with Linux's loop devices:

sudo chmod a+rw /dev/loop0                  # make loop device user accessible
dd if=/dev/zero of=image bs=512 count=2048  # create a 1MB image
losetup /dev/loop0 image                    # attach the loop device

littlefs-fuse has two modes of operation, formatting and mounting.

To format a block device, pass the --format flag. Note! This will erase any data on the block device!

./lfs --format /dev/loop0

To mount, run littlefs-fuse with a block device and a mountpoint:

mkdir mount
./lfs /dev/loop0 mount

Once mounted, the littlefs filesystem will be accessible through the mountpoint. You can now use the littlefs like you would any other filesystem:

cd mount
echo "hello" > hi.txt
ls
cat hi.txt

After using littlefs, you can unmount and detach the loop device:

cd ..
umount mount
sudo losetup -d /dev/loop0

Usage on FreeBSD

littlefs-fuse requires FUSE version 2.6 or higher, you can find your FUSE version with:

pkg info fusefs-libs | grep Version

Once you have cloned littlefs-fuse, you can compile the program with make:

gmake

This should have built the lfs program in the top-level directory.

From here we will need a block device. If you don't have removable storage handy, you can use a file-backed block device with FreeBSD's loop devices:

dd if=/dev/zero of=imageBSD bs=1m count=1   # create a 1 MB image
sudo mdconfig -at vnode -f image            # attach the loop device
sudo chmod 666 /dev/mdX                     # make loop device user accessible,
                                            # where mdX is device created with mdconfig command

littlefs-fuse has two modes of operation, formatting and mounting.

To format a block device, pass the --format flag. Note! This will erase any data on the block device!

./lfs --format /dev/md0

To mount, run littlefs-fuse with a block device and a mountpoint:

mkdir mount
./lfs /dev/md0 mount

Once mounted, the littlefs filesystem will be accessible through the mountpoint. You can now use the littlefs like you would any other filesystem:

cd mount
echo "hello" > hi.txt
ls
cat hi.txt

After using littlefs, you can unmount and detach the loop device:

cd ..
umount mount
sudo mdconfig -du 0

Limitations

As an embedded filesystem, littlefs is designed to be simple. By default, this comes with a number of limitations compared to more PC oriented filesystems:

• No timestamps, this will cause some programs, such as make to fail
• No user permissions, this is why all of the files show up bright green in ls, all files are accessible by anyone
• No symbolic links or special device files, currently only regular and directory file-types are implemented

Tips

If the littlefs was formatted with different geometry than the physical block device, you can override what littlefs-fuse detects. lfs -h lists all available options:

./lfs --block_size=512 --format /dev/loop0
./lfs --block_size=512 /dev/loop0 mount

You can run littlefs-fuse in debug mode to get a log of the kernel interactions with littlefs-fuse. Any printfs in the littlefs driver will end up here:

./lfs -d /dev/loop0 mount

You can even run littlefs-fuse in gdb to debug the filesystem under user operations. Note! When gdb is halted this will freeze any programs interacting with the filesystem!

make DEBUG=1 clean all                # build with debug info
gdb --args ./lfs -d /dev/loop0 mount  # run with gdb

Using xxd or other hex-editory is very useful for inspecting the block device while debugging. You can even run xxd from inside gdb using gdb's ! syntax:

dd if=/dev/loop0 bs=512 count=1 skip=0 | xxd -g1