libfuse ======= Warning: unresolved security issue ---------------------------------- Be aware that FUSE has an unresolved security bug ([bug #15](https://github.com/libfuse/libfuse/issues/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. About ----- 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. Installation ------------ ./configure make -j8 make install You may also need to add `/usr/local/lib` to `/etc/ld.so.conf` and/or run *ldconfig*. If you're building from the git repository (instead of using a release tarball), you also need to run `./makeconf.sh` to create the `configure` script. You'll also need a fuse kernel module (Linux kernels 2.6.14 or later contain FUSE support). For more details see the file `INSTALL` Security implications --------------------- 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_other` and `allow_root` mount options in `fuse.conf`) Building your own filesystem ------------------------------ FUSE comes with several example file systems in the `examples` directory. For example, the *fusexmp* example mirrors 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. Getting Help ------------ If you need help, please ask on the <fuse-devel@lists.sourceforge.net> 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/main/issues.