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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | DESCRIPTION | DEFINITIONS | CONFIGURATION | OPTIONS | FUSE MODULES (STACKING) | SECURITY | NOTE | AUTHORS | SEE ALSO | COLOPHON |
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fuse(8) System Manager's Manual fuse(8)
fuse - configuration and mount options for FUSE file systems
FUSE (Filesystem in Userspace) is a simple interface for
userspace programs 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.
FUSE The in-kernel filesystem that forwards requests to a user-
space process.
filesystem
The user-space process that responds to requests received
from the kernel.
libfuse
The shared library that most (user-space) filesystems use
to communicate with FUSE (the kernel filesystem). libfuse
also provides the fusermount3 (or fusermount if you have
older version of libfuse) helper to allow non-privileged
users to mount filesystems.
filesystem owner
The user that starts the filesystem and instructs the
kernel to associate it with a particular mountpoint. The
latter is typically done by the filesystem itself on
start-up. When using libfuse, this is done by calling the
fusermount3 utility.
client Any process that interacts with the mountpoint.
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).
These limits are enforced by the fusermount3 helper, so they can
be avoided by filesystems that run as root.
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
This option instructs the kernel to perform its own
permission check instead of deferring all permission
checking to the filesystem. The check by the kernel is
done in addition to any permission checks by the
filesystem, and both have to succeed for an operation to
be allowed. The kernel performs a standard UNIX permission
check (based on mode bits and ownership of the directory
entry, and uid/gid of the client).
This mount option is activated implicitly if the
filesystem enables ACL support during the initial feature
negotiation when opening the device fd. In this case, the
kernel performs both ACL and standard unix permission
checking.
Filesystems that do not implement any permission checking
should generally add this option internally.
allow_other
This option overrides the security measure restricting
file access to the filesystem owner, so that all users
(including root) can access the files.
rootmode=M
Specifies the file mode of the filesystem's root (in octal
representation).
blkdev Mount a filesystem backed by a block device. This is a
privileged option. The device must be specified with the
fsname=NAME option.
blksize=N
Set the block size for the filesystem. This option is only
valid for 'fuseblk' type mounts. The default is 512.
In most cases, this option should not be specified by the
filesystem owner but set internally by the filesystem.
max_read=N
With this option the maximum size of read operations can
be set. The default is infinite, but typically the kernel
enforces its own limit in addition to this one. A value of
zero corresponds to no limit.
This option should not be specified by the filesystem
owner. The correct (or optimum) value depends on the
filesystem implementation and should thus be set by the
filesystem internally.
This mount option is deprecated in favor of direct
negotiation over the device fd (as done for e.g. the
maximum size of write operations). For the time being,
libfuse-using filesystems that want to limit the read size
must therefore use this mount option and set the same
value again in the init() handler.
fd=N The file descriptor to use for communication between the
userspace filesystem and the kernel. The file descriptor
must have been obtained by opening the FUSE device
(/dev/fuse).
This option should not be specified by the filesystem
owner. It is set by libfuse (or, if libfuse is not used,
must be set by the filesystem itself).
user_id=N
group_id=N Specifies the numeric uid/gid of the mount
owner.
This option should not be specified by the filesystem
owner. It is set by libfuse (or, if libfuse is not used,
must be set by the filesystem itself).
fsname=NAME
Sets the filesystem source (first field in /etc/mtab). The
default is the name of the filesystem process.
subtype=TYPE
Sets the filesystem type (third field in /etc/mtab). The
default is the name of the filesystem process. If the
kernel supports it, /etc/mtab and /proc/mounts will show
the filesystem type as fuse.TYPE
If the kernel doesn't support subtypes, the source field
will be TYPE#NAME, or if fsname option is not specified,
just TYPE.
libfuse-specific mount options:
These following options are not actually passed to the kernel but
interpreted by libfuse. They can be specified for all filesystems
that use libfuse:
allow_root
This option is similar to allow_other but file access is
limited to the filesystem owner and root. This option and
allow_other are mutually exclusive.
auto_unmount
This option enables automatic release of the mountpoint if
filesystem terminates for any reason. Normally the
filesystem is responsible for releasing the mountpoint,
which means that the mountpoint becomes inaccessible if
the filesystem terminates without first unmounting.
This option is dangerous and should only be used after
careful consideration of the risks.
Automatically unmounting the filesystem means that if the
filesystem crashes the mountpoint may suddenly appear
empty, which may have unintended consequences. For
example, a running backup and mirroring program may
conclude that all the data in the filesystem has been
deleted and proceed to propagate this deletion to the
backup / remote system. If the mountpoint instead becomes
inaccessible (the default), most programs will behave
correctly (report an error).
This feature may also accidentally unmount the wrong
filesystem due to race conditions. For example, if another
filesystem was mounted underneath the same mountpoint, or
if a new filesystem is mounted after the FUSE process has
crashed, it may accidentally get unmounted.
At the moment, this option implies that the filesystem
will also be mounted with nodev and nosuid (even when
mounted by root). This restriction may be lifted in the
future.
High-level mount options:
These following options are not actually passed to the kernel but
interpreted by libfuse. They can only be specified for
filesystems that use the high-level libfuse API:
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 is an alternative to kernel_cache. Instead of
unconditionally keeping cached data, the cached data is
invalidated on open(2) if the modification time or the
size of the file has changed since it was last opened.
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).
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_timeout=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
returned 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
noforget
remember=T
Normally, libfuse assigns inodes to paths only for as long
as the kernel is aware of them. With this option inodes
are instead assigned for at least T seconds (or, in the
case of noforget, the life-time of the filesystem). This
will require more memory, but may be necessary when using
applications that make use of inode numbers.
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.
mount.fuse3 options:
These options are interpreted by mount.fuse3 and are thus only
available when mounting a file system via mount.fuse3 (such as
when mounting via the generic mount(1) command or /etc/fstab).
Supported options are:
setuid=USER
Switch to USER and its primary group before launching the
FUSE file system process. mount.fuse3 must be run as root
or with CAP_SETUID and CAP_SETGID for this to work.
drop_privileges
Perform setup of the FUSE file descriptor and mounting the
file system before launching the FUSE file system process.
mount.fuse3 requires privilege to do so, i.e. must be run
as root or at least with CAP_SYS_ADMIN and CAP_SETPCAP. It
will launch the file system process fully unprivileged,
i.e. without capabilities(7) and prctl(2) flags set up
such that privileges can't be reacquired (e.g. via setuid
or fscaps binaries). This reduces risk in the event of the
FUSE file system process getting compromised by malicious
file system data.
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
possible values). Default is UTF-8.
to_code=CHARSET
Character set to convert to. Default is determined by the
current 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 fusermount3 program is installed set-user-gid to fuse. This
is done to allow users from fuse group 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:
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 fusermount3(1) command
(fusermount3 -u mountpoint).
FUSE is currently maintained by Nikolaus Rath <[email protected]>
The original author of FUSE is Miklos Szeredi
<[email protected]>.
This man page was originally written by Bastien Roucaries
<[email protected]> for the Debian GNU/Linux
distribution.
fusermount3(1) fusermount(1) mount(8) fuse(4)
This page is part of the libfuse (Filesystem in Userspace)
project. Information about the project can be found at
⟨https://github.com/libfuse/libfuse⟩. If you have a bug report
for this manual page, see
⟨https://github.com/libfuse/libfuse/issues⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/libfuse/libfuse.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
fuse(8)
Pages that refer to this page: fusermount3(1), mount(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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