Definitions¶

It looks like there is a kernel module for fuse, fuse.ko, and this needs to be loaded to the kernel. # kldload fuse.ko. Lastly, edit /boot/loader.conf to load the module each boot: # vi /boot/loader.conf - fusefsload='YES' Now mounting USB drive with ext4 filesystem is working! The reason this occurs is because the data is first cached by the user application that originally issued the I/O call (FUSE), and second by the sdcard daemon (EXT4 FS). Problem #3 – Incomplete. Benchmarks Of ZFS-FUSE On Linux Against EXT4, Btrfs Software: 30 Aug 2010: Using Disk Compression With Btrfs To Enhance. Software: 28 Aug 2010: Btrfs, EXT4 & ZFS On A Solid-State Drive Storage: 09 Aug 2010: Benchmarking ZFS On FreeBSD vs. EXT4 & Btrfs On. Software: 27 Jul 2010: Running ZFS With CAM-based ATA On FreeBSD 8.1 Software: 26. You need a specific fuse driver for each file system type, as the point of fuse is to have the file system code running in user-land. So any pre-existing kernel driver code, running in kernel-land (ext2, ext3, ext4, xfs.) can't be used 'as-is' by fuse.

Userspace filesystem:

A filesystem in which data and metadata are provided by an ordinaryuserspace process. The filesystem can be accessed normally throughthe kernel interface.

Filesystem daemon:

The process(es) providing the data and metadata of the filesystem.

Non-privileged mount (or user mount):

A userspace filesystem mounted by a non-privileged (non-root) user.The filesystem daemon is running with the privileges of the mountinguser. NOTE: this is not the same as mounts allowed with the “user”option in /etc/fstab, which is not discussed here.

Filesystem connection:

A connection between the filesystem daemon and the kernel. Theconnection exists until either the daemon dies, or the filesystem isumounted. Note that detaching (or lazy umounting) the filesystemdoes not break the connection, in this case it will exist untilthe last reference to the filesystem is released.

Mount owner:

The user who does the mounting.

User:

The user who is performing filesystem operations.

What is FUSE?¶

FUSE is a userspace filesystem framework. It consists of a kernelmodule (fuse.ko), a userspace library (libfuse.*) and a mount utility(fusermount).

One of the most important features of FUSE is allowing secure,non-privileged mounts. This opens up new possibilities for the use offilesystems. A good example is sshfs: a secure network filesystemusing the sftp protocol.

The userspace library and utilities are available from theFUSE homepage:

Filesystem type¶

The filesystem type given to mount(2) can be one of the following:

fuse

This is the usual way to mount a FUSE filesystem. The firstargument of the mount system call may contain an arbitrary string,which is not interpreted by the kernel.

fuseblk

The filesystem is block device based. The first argument of themount system call is interpreted as the name of the device.

Mount options¶

fd=N

The file descriptor to use for communication between the userspacefilesystem and the kernel. The file descriptor must have beenobtained by opening the FUSE device (‘/dev/fuse’).

rootmode=M

The file mode of the filesystem’s root in octal representation.

Fuse Ext4 Mac

user_id=N

The numeric user id of the mount owner.

group_id=N

The numeric group id of the mount owner.

default_permissions

By default FUSE doesn’t check file access permissions, thefilesystem is free to implement its access policy or leave it tothe underlying file access mechanism (e.g. in case of networkfilesystems). This option enables permission checking, restrictingaccess based on file mode. It is usually useful together with the‘allow_other’ mount option.

allow_other

This option overrides the security measure restricting file accessto the user mounting the filesystem. This option is by default onlyallowed to root, but this restriction can be removed with a(userspace) configuration option.

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 islimited anyway to 32 pages (which is 128kbyte on i386).

blksize=N

Set the block size for the filesystem. The default is 512. Thisoption is only valid for ‘fuseblk’ type mounts.

Control filesystem¶

There’s a control filesystem for FUSE, which can be mounted by:

Mounting it under the ‘/sys/fs/fuse/connections’ directory makes itbackwards compatible with earlier versions.

Under the fuse control filesystem each connection has a directorynamed by a unique number.

For each connection the following files exist within this directory:

waiting

The number of requests which are waiting to be transferred touserspace or being processed by the filesystem daemon. If there isno filesystem activity and ‘waiting’ is non-zero, then thefilesystem is hung or deadlocked.

abort

Writing anything into this file will abort the filesystemconnection. This means that all waiting requests will be aborted anerror returned for all aborted and new requests.

Only the owner of the mount may read or write these files.

Interrupting filesystem operations¶

If a process issuing a FUSE filesystem request is interrupted, thefollowing will happen:

If the request is not yet sent to userspace AND the signal isfatal (SIGKILL or unhandled fatal signal), then the request isdequeued and returns immediately.
If the request is not yet sent to userspace AND the signal is notfatal, then an interrupted flag is set for the request. Whenthe request has been successfully transferred to userspace andthis flag is set, an INTERRUPT request is queued.
If the request is already sent to userspace, then an INTERRUPTrequest is queued.

INTERRUPT requests take precedence over other requests, so theuserspace filesystem will receive queued INTERRUPTs before any others.

Fuse Vs Ext4

The userspace filesystem may ignore the INTERRUPT requests entirely,or may honor them by sending a reply to the original request, withthe error set to EINTR.

It is also possible that there’s a race between processing theoriginal request and its INTERRUPT request. There are two possibilities:

The INTERRUPT request is processed before the original request isprocessed
The INTERRUPT request is processed after the original request hasbeen answered

If the filesystem cannot find the original request, it should wait forsome timeout and/or a number of new requests to arrive, after which itshould reply to the INTERRUPT request with an EAGAIN error. In case1) the INTERRUPT request will be requeued. In case 2) the INTERRUPTreply will be ignored.

Aborting a filesystem connection¶

It is possible to get into certain situations where the filesystem isnot responding. Reasons for this may be:

Broken userspace filesystem implementation
Network connection down
Accidental deadlock
Malicious deadlock

(For more on c) and d) see later sections)

In either of these cases it may be useful to abort the connection tothe filesystem. There are several ways to do this:

Kill the filesystem daemon. Works in case of a) and b)
Kill the filesystem daemon and all users of the filesystem. Worksin all cases except some malicious deadlocks
Use forced umount (umount -f). Works in all cases but only iffilesystem is still attached (it hasn’t been lazy unmounted)
Abort filesystem through the FUSE control filesystem. Mostpowerful method, always works.

How do non-privileged mounts work?¶

Fuse Ext4

Since the mount() system call is a privileged operation, a helperprogram (fusermount) is needed, which is installed setuid root.

The implication of providing non-privileged mounts is that the mountowner must not be able to use this capability to compromise thesystem. Obvious requirements arising from this are:

mount owner should not be able to get elevated privileges with thehelp of the mounted filesystem
mount owner should not get illegitimate access to information fromother users’ and the super user’s processes
mount owner should not be able to induce undesired behavior inother users’ or the super user’s processes

How are requirements fulfilled?¶

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2021.06