FUSE (Filesystem in Userspace) is an interface for userspace programs to export a filesystem to the Linux kernel. The FUSE project consists of two components: the fuse kernel module (maintained in the regular kernel repositories) and the libfuse userspace library (maintained in this repository). libfuse provides the reference implementation for communicating with the FUSE kernel module.
A FUSE file system is typically implemented as a standalone application that links with libfuse. libfuse provides functions to mount the file system, unmount it, read requests from the kernel, and send responses back. libfuse offers two APIs: a “high-level”, synchronous API, and a “low-level” asynchronous API. In both cases, incoming requests from the kernel are passed to the main program using callbacks. When using the high-level API, the callbacks may work with file names and paths instead of inodes, and processing of a request finishes when the callback function returns. When using the low-level API, the callbacks must work with inodes and responses must be sent explicitly using a separate set of API functions.
You can download libfuse from https://github.com/libfuse/libfuse/releases. To build and install, we recommend to use Meson (version 0.38 or newer) and Ninja. After extracting the libfuse tarball, create a (temporary) build directory and run Meson:
$ mkdir build; cd build $ meson ..
Normally, the default build options will work fine. If you nevertheless want to adjust them, you can do so with the mesonconf command:
$ mesonconf # list options $ mesonconf -D disable-mtab=true # set an option
To build, test and install libfuse, you then use Ninja:
$ ninja $ sudo python3 -m pytest test/ $ sudo ninja install
Running the tests requires the py.test Python module. Instead of running the tests as root, the majority of tests can also be run as a regular user if util/fusermount3 is made setuid root first:
$ sudo chown root:root util/fusermount3 $ sudo chmod 4755 util/fusermount3 $ python3 -m pytest test/
The fusermount3 program is installed setuid root. This is done to allow normal users to mount their own filesystem implementations.
To limit the harm that malicious users can do this way, fusermount3 enforces the following limitations:
The user can only mount on a mountpoint for which he has write permission
The mountpoint must not be a sticky directory which isn’t owned by the user (like /tmp usually is)
No other user (including root) can access the contents of the mounted filesystem (though this can be relaxed by allowing the use of the allow_other and allow_root mount options in /etc/fuse.conf)
If you intend to use the allow_other mount options, be aware that FUSE has an unresolved security bug: if the default_permissions mount option is not used, the results of the first permission check performed by the file system for a directory entry will be re-used for subsequent accesses as long as the inode of the accessed entry is present in the kernel cache - even if the permissions have since changed, and even if the subsequent access is made by a different user. This is of little concern if the filesystem is accessible only to the mounting user (which has full access to the filesystem anyway), but becomes a security issue when other users are allowed to access the filesystem (since they can exploit this to perform operations on the filesystem that they do not actually have permissions for).
This bug needs to be fixed in the Linux kernel and has been known since 2006 but unfortunately no fix has been applied yet. If you depend on correct permission handling for FUSE file systems, the only workaround is to use
default_permissions (which does not currently support ACLs), or to completely disable caching of directory entry attributes.
FUSE comes with several example file systems in the
examples directory. For example, the passthrough examples mirror the contents of the root directory under the mountpoint. Start from there and adapt the code!
The documentation of the API functions and necessary callbacks is mostly contained in the files
include/fuse.h (for the high-level API) and
include/fuse_lowlevel.h (for the low-level API). An autogenerated html version of the API is available in the
doc/html directory and at http://libfuse.github.io/doxygen.
If you need help, please ask on the email@example.com mailing list (subscribe at https://lists.sourceforge.net/lists/listinfo/fuse-devel).
Please report any bugs on the GitHub issue tracker at https://github.com/libfuse/libfuse/issues.
Professional support is offered via Rath Consulting.