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linux-7.2.3/Documentation/filesystems/fuse/fuse-io-uring.rst
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.. SPDX-License-Identifier: GPL-2.0
=======================================
FUSE-over-io-uring design documentation
=======================================
This documentation covers basic details how the fuse
kernel/userspace communication through io-uring is configured
and works. For generic details about FUSE see fuse.rst.
This document also covers the current interface, which is
still in development and might change.
Limitations
===========
As of now not all requests types are supported through io-uring, userspace
is required to also handle requests through /dev/fuse after io-uring setup
is complete. Specifically notifications (initiated from the daemon side)
and interrupts.
Fuse io-uring configuration
===========================
Fuse kernel requests are queued through the classical /dev/fuse
read/write interface - until io-uring setup is complete.
In order to set up fuse-over-io-uring fuse-server (user-space)
needs to submit SQEs (opcode = IORING_OP_URING_CMD) to the /dev/fuse
connection file descriptor. Initial submit is with the sub command
FUSE_URING_REQ_REGISTER, which will just register entries to be
available in the kernel.
Once at least one entry per queue is submitted, kernel starts
to enqueue to ring queues.
Note, every CPU core has its own fuse-io-uring queue.
Userspace handles the CQE/fuse-request and submits the result as
subcommand FUSE_URING_REQ_COMMIT_AND_FETCH - kernel completes
the requests and also marks the entry available again. If there are
pending requests waiting the request will be immediately submitted
to the daemon again.
Initial SQE
-----------::
| | FUSE filesystem daemon
| |
| | >io_uring_submit()
| | IORING_OP_URING_CMD /
| | FUSE_URING_CMD_REGISTER
| | [wait cqe]
| | >io_uring_wait_cqe() or
| | >io_uring_submit_and_wait()
| |
| >fuse_uring_cmd() |
| >fuse_uring_register() |
Sending requests with CQEs
--------------------------::
| | FUSE filesystem daemon
| | [waiting for CQEs]
| "rm /mnt/fuse/file" |
| |
| >sys_unlink() |
| >fuse_unlink() |
| [allocate request] |
| >fuse_send_one() |
| ... |
| >fuse_uring_queue_fuse_req |
| [queue request on fg queue] |
| >fuse_uring_add_req_to_ring_ent() |
| ... |
| >fuse_uring_copy_to_ring() |
| >io_uring_cmd_done() |
| >request_wait_answer() |
| [sleep on req->waitq] |
| | [receives and handles CQE]
| | [submit result and fetch next]
| | >io_uring_submit()
| | IORING_OP_URING_CMD/
| | FUSE_URING_CMD_COMMIT_AND_FETCH
| >fuse_uring_cmd() |
| >fuse_uring_commit_fetch() |
| >fuse_uring_commit() |
| >fuse_uring_copy_from_ring() |
| [ copy the result to the fuse req] |
| >fuse_uring_req_end() |
| >fuse_request_end() |
| [wake up req->waitq] |
| >fuse_uring_next_fuse_req |
| [wait or handle next req] |
| |
| [req->waitq woken up] |
| <fuse_unlink() |
| <sys_unlink() |
linux-7.2.3/Documentation/filesystems/fuse/fuse-io.rst
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.. SPDX-License-Identifier: GPL-2.0
==============
FUSE I/O Modes
==============
Fuse supports the following I/O modes:
- direct-io
- cached
+ write-through
+ writeback-cache
The direct-io mode can be selected with the FOPEN_DIRECT_IO flag in the
FUSE_OPEN reply.
In direct-io mode the page cache is completely bypassed for reads and writes.
No read-ahead takes place. Shared mmap is disabled by default. To allow shared
mmap, the FUSE_DIRECT_IO_ALLOW_MMAP flag may be enabled in the FUSE_INIT reply.
In cached mode reads may be satisfied from the page cache, and data may be
read-ahead by the kernel to fill the cache. The cache is always kept consistent
after any writes to the file. All mmap modes are supported.
The cached mode has two sub modes controlling how writes are handled. The
write-through mode is the default and is supported on all kernels. The
writeback-cache mode may be selected by the FUSE_WRITEBACK_CACHE flag in the
FUSE_INIT reply.
In write-through mode each write is immediately sent to userspace as one or more
WRITE requests, as well as updating any cached pages (and caching previously
uncached, but fully written pages). No READ requests are ever sent for writes,
so when an uncached page is partially written, the page is discarded.
In writeback-cache mode (enabled by the FUSE_WRITEBACK_CACHE flag) writes go to
the cache only, which means that the write(2) syscall can often complete very
fast. Dirty pages are written back implicitly (background writeback or page
reclaim on memory pressure) or explicitly (invoked by close(2), fsync(2) and
when the last ref to the file is being released on munmap(2)). This mode
assumes that all changes to the filesystem go through the FUSE kernel module
(size and atime/ctime/mtime attributes are kept up-to-date by the kernel), so
it's generally not suitable for network filesystems. If a partial page is
written, then the page needs to be first read from userspace. This means, that
even for files opened for O_WRONLY it is possible that READ requests will be
generated by the kernel.
linux-7.2.3/Documentation/filesystems/fuse/fuse-passthrough.rst
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.. SPDX-License-Identifier: GPL-2.0
================
FUSE Passthrough
================
Introduction
============
FUSE (Filesystem in Userspace) passthrough is a feature designed to improve the
performance of FUSE filesystems for I/O operations. Typically, FUSE operations
involve communication between the kernel and a userspace FUSE daemon, which can
incur overhead. Passthrough allows certain operations on a FUSE file to bypass
the userspace daemon and be executed directly by the kernel on an underlying
"backing file".
This is achieved by the FUSE daemon registering a file descriptor (pointing to
the backing file on a lower filesystem) with the FUSE kernel module. The kernel
then receives an identifier (``backing_id``) for this registered backing file.
When a FUSE file is subsequently opened, the FUSE daemon can, in its response to
the ``OPEN`` request, include this ``backing_id`` and set the
``FOPEN_PASSTHROUGH`` flag. This establishes a direct link for specific
operations.
Currently, passthrough is supported for operations like ``read(2)``/``write(2)``
(via ``read_iter``/``write_iter``), ``splice(2)``, and ``mmap(2)``.
Enabling Passthrough
====================
To use FUSE passthrough:
1. The FUSE filesystem must be compiled with ``CONFIG_FUSE_PASSTHROUGH``
enabled.
2. The FUSE daemon, during the ``FUSE_INIT`` handshake, must negotiate the
``FUSE_PASSTHROUGH`` capability and specify its desired
``max_stack_depth``.
3. The (privileged) FUSE daemon uses the ``FUSE_DEV_IOC_BACKING_OPEN`` ioctl
on its connection file descriptor (e.g., ``/dev/fuse``) to register a
backing file descriptor and obtain a ``backing_id``.
4. When handling an ``OPEN`` or ``CREATE`` request for a FUSE file, the daemon
replies with the ``FOPEN_PASSTHROUGH`` flag set in
``fuse_open_out::open_flags`` and provides the corresponding ``backing_id``
in ``fuse_open_out::backing_id``.
5. The FUSE daemon should eventually call ``FUSE_DEV_IOC_BACKING_CLOSE`` with
the ``backing_id`` to release the kernel's reference to the backing file
when it's no longer needed for passthrough setups.
Privilege Requirements
======================
Setting up passthrough functionality currently requires the FUSE daemon to
possess the ``CAP_SYS_ADMIN`` capability. This requirement stems from several
security and resource management considerations that are actively being
discussed and worked on. The primary reasons for this restriction are detailed
below.
Resource Accounting and Visibility
----------------------------------
The core mechanism for passthrough involves the FUSE daemon opening a file
descriptor to a backing file and registering it with the FUSE kernel module via
the ``FUSE_DEV_IOC_BACKING_OPEN`` ioctl. This ioctl returns a ``backing_id``
associated with a kernel-internal ``struct fuse_backing`` object, which holds a
reference to the backing ``struct file``.
A significant concern arises because the FUSE daemon can close its own file
descriptor to the backing file after registration. The kernel, however, will
still hold a reference to the ``struct file`` via the ``struct fuse_backing``
object as long as it's associated with a ``backing_id`` (or subsequently, with
an open FUSE file in passthrough mode).
This behavior leads to two main issues for unprivileged FUSE daemons:
1. **Invisibility to lsof and other inspection tools**: Once the FUSE
daemon closes its file descriptor, the open backing file held by the kernel
becomes "hidden." Standard tools like ``lsof``, which typically inspect
process file descriptor tables, would not be able to identify that this
file is still open by the system on behalf of the FUSE filesystem. This
makes it difficult for system administrators to track resource usage or
debug issues related to open files (e.g., preventing unmounts).
2. **Bypassing RLIMIT_NOFILE**: The FUSE daemon process is subject to
resource limits, including the maximum number of open file descriptors
(``RLIMIT_NOFILE``). If an unprivileged daemon could register backing files
and then close its own FDs, it could potentially cause the kernel to hold
an unlimited number of open ``struct file`` references without these being
accounted against the daemon's ``RLIMIT_NOFILE``. This could lead to a
denial-of-service (DoS) by exhausting system-wide file resources.
The ``CAP_SYS_ADMIN`` requirement acts as a safeguard against these issues,
restricting this powerful capability to trusted processes.
**NOTE**: ``io_uring`` solves this similar issue by exposing its "fixed files",
which are visible via ``fdinfo`` and accounted under the registering user's
``RLIMIT_NOFILE``.
Filesystem Stacking and Shutdown Loops
--------------------------------------
Another concern relates to the potential for creating complex and problematic
filesystem stacking scenarios if unprivileged users could set up passthrough.
A FUSE passthrough filesystem might use a backing file that resides:
* On the *same* FUSE filesystem.
* On another filesystem (like OverlayFS) which itself might have an upper or
lower layer that is a FUSE filesystem.
These configurations could create dependency loops, particularly during
filesystem shutdown or unmount sequences, leading to deadlocks or system
instability. This is conceptually similar to the risks associated with the
``LOOP_SET_FD`` ioctl, which also requires ``CAP_SYS_ADMIN``.
To mitigate this, FUSE passthrough already incorporates checks based on
filesystem stacking depth (``sb->s_stack_depth`` and ``fc->max_stack_depth``).
For example, during the ``FUSE_INIT`` handshake, the FUSE daemon can negotiate
the ``max_stack_depth`` it supports. When a backing file is registered via
``FUSE_DEV_IOC_BACKING_OPEN``, the kernel checks if the backing file's
filesystem stack depth is within the allowed limit.
The ``CAP_SYS_ADMIN`` requirement provides an additional layer of security,
ensuring that only privileged users can create these potentially complex
stacking arrangements.
General Security Posture
------------------------
As a general principle for new kernel features that allow userspace to instruct
the kernel to perform direct operations on its behalf based on user-provided
file descriptors, starting with a higher privilege requirement (like
``CAP_SYS_ADMIN``) is a conservative and common security practice. This allows
the feature to be used and tested while further security implications are
evaluated and addressed.
linux-7.2.3/Documentation/filesystems/fuse/fuse.rst
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.. SPDX-License-Identifier: GPL-2.0
=============
FUSE Overview
=============
Definitions
===========
Userspace filesystem:
A filesystem in which data and metadata are provided by an ordinary
userspace process. The filesystem can be accessed normally through
the 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 mounting
user. 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. The
connection exists until either the daemon dies, or the filesystem is
umounted. Note that detaching (or lazy umounting) the filesystem
does *not* break the connection, in this case it will exist until
the 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 kernel
module (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 of
filesystems. A good example is sshfs: a secure network filesystem
using the sftp protocol.
The userspace library and utilities are available from the
`FUSE homepage: <https://github.com/libfuse/>`_
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 first
argument 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 the
mount system call is interpreted as the name of the device.
Mount options
=============
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').
rootmode=M
The file mode of the filesystem's root in octal representation.
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, the
filesystem is free to implement its access policy or leave it to
the underlying file access mechanism (e.g. in case of network
filesystems). This option enables permission checking, restricting
access 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 access
to the user mounting the filesystem. This option is by default only
allowed 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 is
limited anyway to 32 pages (which is 128kbyte on i386).
blksize=N
Set the block size for the filesystem. The default is 512. This
option is only valid for 'fuseblk' type mounts.
Control filesystem
==================
There's a control filesystem for FUSE, which can be mounted by::
mount -t fusectl none /sys/fs/fuse/connections
Mounting it under the '/sys/fs/fuse/connections' directory makes it
backwards compatible with earlier versions.
Under the fuse control filesystem each connection has a directory
named 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 to
userspace or being processed by the filesystem daemon. If there is
no filesystem activity and 'waiting' is non-zero, then the
filesystem is hung or deadlocked.
abort
Writing anything into this file will abort the filesystem
connection. This means that all waiting requests will be aborted an
error returned for all aborted and new requests.
max_background
The maximum number of background requests that can be outstanding
at a time. When the number of background requests reaches this limit,
further requests will be blocked until some are completed, potentially
causing I/O operations to stall.
congestion_threshold
The threshold of background requests at which the kernel considers
the filesystem to be congested. When the number of background requests
exceeds this value, the kernel will skip asynchronous readahead
operations, reducing read-ahead optimizations but preserving essential
I/O, as well as suspending non-synchronous writeback operations
(WB_SYNC_NONE), delaying page cache flushing to the filesystem.
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, the
following will happen:
- If the request is not yet sent to userspace AND the signal is
fatal (SIGKILL or unhandled fatal signal), then the request is
dequeued and returns immediately.
- If the request is not yet sent to userspace AND the signal is not
fatal, then an interrupted flag is set for the request. When
the request has been successfully transferred to userspace and
this flag is set, an INTERRUPT request is queued.
- If the request is already sent to userspace, then an INTERRUPT
request is queued.
INTERRUPT requests take precedence over other requests, so the
userspace filesystem will receive queued INTERRUPTs before any others.
The userspace filesystem may ignore the INTERRUPT requests entirely,
or may honor them by sending a reply to the *original* request, with
the error set to EINTR.
It is also possible that there's a race between processing the
original request and its INTERRUPT request. There are two possibilities:
1. The INTERRUPT request is processed before the original request is
processed
2. The INTERRUPT request is processed after the original request has
been answered
If the filesystem cannot find the original request, it should wait for
some timeout and/or a number of new requests to arrive, after which it
should reply to the INTERRUPT request with an EAGAIN error. In case
1) the INTERRUPT request will be requeued. In case 2) the INTERRUPT
reply will be ignored.
Aborting a filesystem connection
================================
It is possible to get into certain situations where the filesystem is
not responding. Reasons for this may be:
a) Broken userspace filesystem implementation
b) Network connection down
c) Accidental deadlock
d) Malicious deadlock
(For more on c) and d) see later sections)
In either of these cases it may be useful to abort the connection to
the 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. Works
in all cases except some malicious deadlocks
- Use forced umount (umount -f). Works in all cases but only if
filesystem is still attached (it hasn't been lazy unmounted)
- Abort filesystem through the FUSE control filesystem. Most
powerful method, always works.
How do non-privileged mounts work?
==================================
Since the mount() system call is a privileged operation, a helper
program (fusermount) is needed, which is installed setuid root.
The implication of providing non-privileged mounts is that the mount
owner must not be able to use this capability to compromise the
system. Obvious requirements arising from this are:
A) mount owner should not be able to get elevated privileges with the
help of the mounted filesystem
B) mount owner should not get illegitimate access to information from
other users' and the super user's processes
C) mount owner should not be able to induce undesired behavior in
other users' or the super user's processes
How are requirements fulfilled?
===============================
A) The mount owner could gain elevated privileges by either:
1. creating a filesystem containing a device file, then opening this device
2. creating a filesystem containing a suid or sgid application, then executing this application
The solution is not to allow opening device files and ignore
setuid and setgid bits when executing programs. To ensure this
fusermount always adds "nosuid" and "nodev" to the mount options
for non-privileged mounts.
B) If another user is accessing files or directories in the
filesystem, the filesystem daemon serving requests can record the
exact sequence and timing of operations performed. This
information is otherwise inaccessible to the mount owner, so this
counts as an information leak.
The solution to this problem will be presented in point 2) of C).
C) There are several ways in which the mount owner can induce
undesired behavior in other users' processes, such as:
1) mounting a filesystem over a file or directory which the mount
owner could otherwise not be able to modify (or could only
make limited modifications).
This is solved in fusermount, by checking the access
permissions on the mountpoint and only allowing the mount if
the mount owner can do unlimited modification (has write
access to the mountpoint, and mountpoint is not a "sticky"
directory)
2) Even if 1) is solved the mount owner can change the behavior
of other users' processes.
i) It can slow down or indefinitely delay the execution of a
filesystem operation creating a DoS against the user or the
whole system. For example a suid application locking a
system file, and then accessing a file on the mount owner's
filesystem could be stopped, and thus causing the system
file to be locked forever.
ii) It can present files or directories of unlimited length, or
directory structures of unlimited depth, possibly causing a
system process to eat up diskspace, memory or other
resources, again causing *DoS*.
The solution to this as well as B) is not to allow processes
to access the filesystem, which could otherwise not be
monitored or manipulated by the mount owner. Since if the
mount owner can ptrace a process, it can do all of the above
without using a FUSE mount, the same criteria as used in
ptrace can be used to check if a process is allowed to access
the filesystem or not.
Note that the *ptrace* check is not strictly necessary to
prevent C/2/i, it is enough to check if mount owner has enough
privilege to send signal to the process accessing the
filesystem, since *SIGSTOP* can be used to get a similar effect.
I think these limitations are unacceptable?
===========================================
If a sysadmin trusts the users enough, or can ensure through other
measures, that system processes will never enter non-privileged
mounts, it can relax the last limitation in several ways:
- With the 'user_allow_other' config option. If this config option is
set, the mounting user can add the 'allow_other' mount option which
disables the check for other users' processes.
User namespaces have an unintuitive interaction with 'allow_other':
an unprivileged user - normally restricted from mounting with
'allow_other' - could do so in a user namespace where they're
privileged. If any process could access such an 'allow_other' mount
this would give the mounting user the ability to manipulate
processes in user namespaces where they're unprivileged. For this
reason 'allow_other' restricts access to users in the same userns
or a descendant.
- With the 'allow_sys_admin_access' module option. If this option is
set, super user's processes have unrestricted access to mounts
irrespective of allow_other setting or user namespace of the
mounting user.
Note that both of these relaxations expose the system to potential
information leak or *DoS* as described in points B and C/2/i-ii in the
preceding section.
Kernel - userspace interface
============================
The following diagram shows how a filesystem operation (in this
example unlink) is performed in FUSE. ::
| "rm /mnt/fuse/file" | FUSE filesystem daemon
| |
| | >sys_read()
| | >fuse_dev_read()
| | >request_wait()
| | [sleep on fc->waitq]
| |
| >sys_unlink() |
| >fuse_unlink() |
| [get request from |
| fc->unused_list] |
| >request_send() |
| [queue req on fc->pending] |
| [wake up fc->waitq] | [woken up]
| >request_wait_answer() |
| [sleep on req->waitq] |
| | <request_wait()
| | [remove req from fc->pending]
| | [copy req to read buffer]
| | [add req to fc->processing]
| | <fuse_dev_read()
| | <sys_read()
| |
| | [perform unlink]
| |
| | >sys_write()
| | >fuse_dev_write()
| | [look up req in fc->processing]
| | [remove from fc->processing]
| | [copy write buffer to req]
| [woken up] | [wake up req->waitq]
| | <fuse_dev_write()
| | <sys_write()
| <request_wait_answer() |
| <request_send() |
| [add request to |
| fc->unused_list] |
| <fuse_unlink() |
| <sys_unlink() |
.. note:: Everything in the description above is greatly simplified
There are a couple of ways in which to deadlock a FUSE filesystem.
Since we are talking about unprivileged userspace programs,
something must be done about these.
**Scenario 1 - Simple deadlock**::
| "rm /mnt/fuse/file" | FUSE filesystem daemon
| |
| >sys_unlink("/mnt/fuse/file") |
| [acquire inode semaphore |
| for "file"] |
| >fuse_unlink() |
| [sleep on req->waitq] |
| | <sys_read()
| | >sys_unlink("/mnt/fuse/file")
| | [acquire inode semaphore
| | for "file"]
| | *DEADLOCK*
The solution for this is to allow the filesystem to be aborted.
**Scenario 2 - Tricky deadlock**
This one needs a carefully crafted filesystem. It's a variation on
the above, only the call back to the filesystem is not explicit,
but is caused by a pagefault. ::
| Kamikaze filesystem thread 1 | Kamikaze filesystem thread 2
| |
| [fd = open("/mnt/fuse/file")] | [request served normally]
| [mmap fd to 'addr'] |
| [close fd] | [FLUSH triggers 'magic' flag]
| [read a byte from addr] |
| >do_page_fault() |
| [find or create page] |
| [lock page] |
| >fuse_readpage() |
| [queue READ request] |
| [sleep on req->waitq] |
| | [read request to buffer]
| | [create reply header before addr]
| | >sys_write(addr - headerlength)
| | >fuse_dev_write()
| | [look up req in fc->processing]
| | [remove from fc->processing]
| | [copy write buffer to req]
| | >do_page_fault()
| | [find or create page]
| | [lock page]
| | * DEADLOCK *
The solution is basically the same as above.
An additional problem is that while the write buffer is being copied
to the request, the request must not be interrupted/aborted. This is
because the destination address of the copy may not be valid after the
request has returned.
This is solved with doing the copy atomically, and allowing abort
while the page(s) belonging to the write buffer are faulted with
get_user_pages(). The 'req->locked' flag indicates when the copy is
taking place, and abort is delayed until this flag is unset.
linux-7.2.3/Documentation/filesystems/fuse/index.rst
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.. SPDX-License-Identifier: GPL-2.0
======================================================
FUSE (Filesystem in Userspace) Technical Documentation
======================================================
.. toctree::
:maxdepth: 2
:numbered:
fuse
fuse-io
fuse-io-uring
fuse-passthrough