format and options for the fuse file systems
FUSE (Filesystem in Userspace) is a simple interface for userspace pro‐ grams to export a virtual filesystem to the Linux kernel. FUSE also aims to provide a secure method for non privileged users to create and mount their own filesystem implementations.
Some options regarding mount policy can be set in the file /etc/fuse.conf. Currently these options are: mount_max = NNN Set the maximum number of FUSE mounts allowed to non-root users. The default is 1000. user_allow_other Allow non-root users to specify the allow_other or allow_root mount options (see below).
Most of the generic mount options described in mount are supported (ro, rw, suid, nosuid, dev, nodev, exec, noexec, atime, noatime, sync, async, dirsync). Filesystems are mounted with nodev,nosuid by default, which can only be overridden by a privileged user. General mount options: These are FUSE specific mount options that can be specified for all filesystems: default_permissions By default FUSE doesn't check file access permissions, the filesystem is free to implement it's access policy or leave it to the underlying file access mechanism (e.g. in case of network filesystems). This option enables permission checking, restrict‐ ing access based on file mode. This is option is usually useful together with the allow_other mount option. allow_other This option overrides the security measure restricting file ac‐ cess to the user mounting the filesystem. So all users (includ‐ ing root) can access the files. This option is by default only allowed to root, but this restriction can be removed with a con‐ figuration option described in the previous section. allow_root This option is similar to allow_other but file access is limited to the user mounting the filesystem and root. This option and allow_other are mutually exclusive. kernel_cache This option disables flushing the cache of the file contents on every open(2). This should only be enabled on filesystems, where the file data is never changed externally (not through the mounted FUSE filesystem). Thus it is not suitable for network filesystems and other intermediate filesystems. NOTE: if this option is not specified (and neither direct_io) data is still cached after the open(2), so a read(2) system call will not always initiate a read operation. auto_cache This option enables automatic flushing of the data cache on open(2). The cache will only be flushed if the modification time or the size of the file has changed. large_read Issue large read requests. This can improve performance for some filesystems, but can also degrade performance. This option is only useful on 2.4.X kernels, as on 2.6 kernels requests size is automatically determined for optimum performance. direct_io This option disables the use of page cache (file content cache) in the kernel for this filesystem. This has several affects: 1. Each read(2) or write(2) system call will initiate one or more read or write operations, data will not be cached in the kernel. 2. The return value of the read() and write() system calls will correspond to the return values of the read and write opera‐ tions. This is useful for example if the file size is not known in advance (before reading it). max_read=N With this option the maximum size of read operations can be set. The default is infinite. Note that the size of read requests is limited anyway to 32 pages (which is 128kbyte on i386). max_readahead=N Set the maximum number of bytes to read-ahead. The default is determined by the kernel. On linux-2.6.22 or earlier it's 131072 (128kbytes) max_write=N Set the maximum number of bytes in a single write operation. The default is 128kbytes. Note, that due to various limitations, the size of write requests can be much smaller (4kbytes). This limitation will be removed in the future. async_read Perform reads asynchronously. This is the default sync_read Perform all reads (even read-ahead) synchronously. hard_remove The default behavior is that if an open file is deleted, the file is renamed to a hidden file (.fuse_hiddenXXX), and only re‐ moved when the file is finally released. This relieves the filesystem implementation of having to deal with this problem. This option disables the hiding behavior, and files are removed immediately in an unlink operation (or in a rename operation which overwrites an existing file). It is recommended that you not use the hard_remove option. When hard_remove is set, the following libc functions fail on un‐ linked files (returning errno of ENOENT): read(2), write(2), fsync(2), close(2), f*xattr(2), ftruncate(2), fstat(2), fch‐ mod(2), fchown(2) debug Turns on debug information printing by the library. fsname=NAME Sets the filesystem source (first field in /etc/mtab). The de‐ fault is the mount program name. subtype=TYPE Sets the filesystem type (third field in /etc/mtab). The default is the mount program name. If the kernel suppports it, /etc/mtab and /proc/mounts will show the filesystem type as fuse.TYPE If the kernel doesn't support subtypes, the source filed will be TYPE#NAME, or if fsname option is not specified, just TYPE. use_ino Honor the st_ino field in kernel functions getattr() and fill_dir(). This value is used to fill in the st_ino field in the stat(2), lstat(2), fstat(2) functions and the d_ino field in the readdir(2) function. The filesystem does not have to guaran‐ tee uniqueness, however some applications rely on this value be‐ ing unique for the whole filesystem. readdir_ino If use_ino option is not given, still try to fill in the d_ino field in readdir(2). If the name was previously looked up, and is still in the cache, the inode number found there will be used. Otherwise it will be set to -1. If use_ino option is given, this option is ignored. nonempty Allows mounts over a non-empty file or directory. By default these mounts are rejected to prevent accidental covering up of data, which could for example prevent automatic backup. umask=M Override the permission bits in st_mode set by the filesystem. The resulting permission bits are the ones missing from the given umask value. The value is given in octal representation. uid=N Override the st_uid field set by the filesystem (N is numeric). gid=N Override the st_gid field set by the filesystem (N is numeric). blkdev Mount a filesystem backed by a block device. This is a privi‐ leged option. The device must be specified with the fsname=NAME option. entry_timeout=T The timeout in seconds for which name lookups will be cached. The default is 1.0 second. For all the timeout options, it is possible to give fractions of a second as well (e.g. entry_time‐ out=2.8) negative_timeout=T The timeout in seconds for which a negative lookup will be cached. This means, that if file did not exist (lookup retuned ENOENT), the lookup will only be redone after the timeout, and the file/directory will be assumed to not exist until then. The default is 0.0 second, meaning that caching negative lookups are disabled. attr_timeout=T The timeout in seconds for which file/directory attributes are cached. The default is 1.0 second. ac_attr_timeout=T The timeout in seconds for which file attributes are cached for the purpose of checking if auto_cache should flush the file data on open. The default is the value of attr_timeout intr Allow requests to be interrupted. Turning on this option may result in unexpected behavior, if the filesystem does not sup‐ port request interruption. intr_signal=NUM Specify which signal number to send to the filesystem when a re‐ quest is interrupted. The default is hardcoded to USR1. modules=M1[:M2...] Add modules to the filesystem stack. Modules are pushed in the order they are specified, with the original filesystem being on the bottom of the stack.
Modules are filesystem stacking support to high level API. Filesystem modules can be built into libfuse or loaded from shared object iconv Perform file name character set conversion. Options are: from_code=CHARSET Character set to convert from (see iconv -l for a list of possi‐ ble values). Default is UTF-8. to_code=CHARSET Character set to convert to. Default is determined by the cur‐ rent locale. subdir Prepend a given directory to each path. Options are: subdir=DIR Directory to prepend to all paths. This option is mandatory. rellinks Transform absolute symlinks into relative norellinks Do not transform absolute symlinks into relative. This is the default.
The fusermount program is installed set-user-gid to fuse. This is done to allow users from fuse group to mount their own filesystem implemen‐ tations. There must however be some limitations, in order to prevent Bad User from doing nasty things. Currently those limitations are: 1. The user can only mount on a mountpoint, for which it has write permission 2. The mountpoint is not a sticky directory which isn't owned by the user (like /tmp usually is) 3. No other user (including root) can access the contents of the mounted filesystem.
FUSE filesystems are unmounted using the fusermount(1) command (fuser‐ mount -u mountpoint).
The main author of FUSE is Miklos Szeredi