mount.fuse3(8) - man - phpman

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fuse(8) System Manager's Manual fuse(8)

NAME
fuse - configuration and mount options for FUSE file systems

DESCRIPTION
FUSE (Filesystem in Userspace) is a simple interface for userspace programs to export a vir‐
tual filesystem to the Linux kernel. FUSE also aims to provide a secure method for non privi‐
leged users to create and mount their own filesystem implementations.

DEFINITIONS
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 filesys‐
tems.

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.

CONFIGURATION
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 be‐
low).

These limits are enforced by the fusermount3 helper, so they can be avoided by filesystems
that run as root.

OPTIONS
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 defer‐
ring all permission checking to the filesystem. The check by the kernel is done in ad‐
dition to any permission checks by the filesystem, and both have to succeed for an op‐
eration 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 in‐
ternally by the filesystem.

max_read=N
With this option the maximum size of read operations can be set. The default is infi‐
nite, 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 filesys‐
tem 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 fs‐‐
name 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.

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 af‐
ter 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 permis‐
sion bits are the ones missing from the given umask value. The value is given in oc‐
tal 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 sec‐
ond. 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 check‐
ing 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 speci‐
fied, 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 launch‐
ing 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 mali‐
cious file system data.

FUSE MODULES (STACKING)
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.

SECURITY
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 limita‐
tions, 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 usu‐
ally is)

3. No other user (including root) can access the contents of the mounted filesystem.

NOTE
FUSE filesystems are unmounted using the fusermount3(1) command (fusermount3 -u mountpoint).

AUTHORS
FUSE is currently maintained by Nikolaus Rath <Nikolaus AT rath.org>

The original author of FUSE is Miklos Szeredi <mszeredi AT inf.hu>.

This man page was originally written by Bastien Roucaries <roucaries.bastien+de‐
bian AT gmail.com> for the Debian GNU/Linux distribution.

SEE ALSO
fusermount3(1) fusermount(1) mount(8) fuse(4)

fuse(8)