Warning: unresolved security issue
Be aware that FUSE has an unresolved security bug (bug #15): the permission check for accessing a cached directory is only done once when the directory entry is first loaded into the cache. Subsequent accesses will re-use the results of the first check, even if the directory permissions have since changed, and even if the subsequent access is made by a different user.
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 completely disable caching of directory
entries. Alternatively, the severity of the bug can be somewhat
reduced by not using the
allow_other mount option.
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. After extracting the tarball, build and install with
./configure make -j8 make install
You may also need to add
run ldconfig. If you're building from the git repository (instead of
using a release tarball), you also need to run
You'll also need a fuse kernel module (Linux kernels 2.6.14 or later contain FUSE support).
For more details see the file
If you run
make install, the fusermount program is installed
set-user-id to root. This is done to allow normal users to mount
their own filesystem implementations.
There must however be some limitations, in order to prevent Bad User from doing nasty things. Currently those limitations are:
The user can only mount on a mountpoint, for which it has write permission
The mountpoint is not 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_rootmount options in
Building your own filesystem
FUSE comes with several example file systems in the
directory. For example, the fusexmp example mirrors the contents of
the root directory under the mountpoint. Start from there and adapt
The documentation of the API functions and necessary callbacks is
mostly contained in the files
include/fuse.h (for the high-level
include/fuse_lowlevel.h (for the low-level API). An
autogenerated html version of the API is available in the
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.