davide125 / btrfs-fuse

A read-only, license friendly, FUSE based btrfs implementation

btrfs-fuse

About

This is a read-only btrfs implementation using FUSE (Filesystem in Userspace).

Although btrfs is already in mainline Linux kernel, there are still use-cases for such read-only btrfs implementation:

• Educational purpose

  Let new developers get a quick way to learn how a complex and modern filesystem works.

• For certain bootloaders

  Certain bootloaders need code base compatible with their license.

• As a last resort method for subpage/multipage support

  Currently (v5.16-rc) Linux kernel can only support sectorsize == pagesize , and 4K sectorsize with 64K page size.

  Thus this project can act as a last resort method to read data from filesystem with unsupported sectorsize.

Build

This project uses meson build system.

$ cd btrfs-fuse
$ meson setup build
$ cd build
$ ninja

This project has the following dependency:

• uuid

  For uuid parsing

• libb2

  For BLAKE2 checksum support

• libcrypto

  For SHA256 checksum support

• libxxhash

  For XXHASH checksum support

Limitation

Currently btrfs-fuse has the following btrfs features missing:

• RAID5/6 support
• Compressioned data read
• xattr/fattr support

Above features are still under active development.

When such missing features is hit, btrfs-fuse would return -EOPNOTSUPP.

While there are still some other FUSE related feature missing:

• Multi-thread support

  Low priority feature, as bootloader won't really need that feature, and extra concurrency code can confuse new readers.

• Proper subvolume inode address space

  This is due to FUSE limitation, that one FUSE must has the same stat::st_dev. In kernel btrfs returns different stat::st_dev for different subvolumes, but in FUSE we don't have the ability do the same thing.

Usage

$ btrfs-fuse [<fuse options>] <device> [<extra devs> ...] <mnt>

Please note that, if multiple devices are passed into btrfs-fuse and contains different file systems, btrfs-fuse will use the last device to initialize the mount.

That's to say, for the following example:

$ mkfs.btrfs -f /dev/test/scratch1
$ mkfs.btrfs -f /dev/test/scratch2
$ btrfs-fuse /dev/test/scratch1 /dev/test/scratch2 /tmp/mount

Then only btrfs on /dev/test/scratch2 will be mounted onto /tmp/mount.

License

All files at the root directory is under MIT license.

Files under libs directory is under their own licenses. Mostly GPL-2.0+ or GPL-2.0-only.

Those external libs include:

• crc32c.[ch]

  For CRC32C checksum support.

  Cross-ported from btrfs-progs, which is cross-ported from older kernel, which is still under GPL-2.0+ license.

• list.h

  For kernel style list implementation.

  Cross-ported from btrfs-progs, which is cross-ported from kernel, and under GPL-2.0-only license.

• rbtree.[ch] and rbtree_augmented.h

  For kernel style rb-tree implementation.

  Cross-ported from btrfs-progs, which is cross-ported from kernel, and under GPL-2.0+ license.