From 034dd340b08be1f2f0477ad16131d609f9dbd53c Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sun, 30 Aug 2026 09:22:00 -0700
Subject: Merge tag 'trace-v7.3-2' of
 git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure
---
 Documentation/filesystems/fuse/fuse-io-uring.rst   | 131 ++++++
 Documentation/filesystems/fuse/fuse-io.rst         |  45 +++
 .../filesystems/fuse/fuse-passthrough.rst          | 133 +++++++
 Documentation/filesystems/fuse/fuse.rst            | 440 +++++++++++++++++++++
 Documentation/filesystems/fuse/index.rst           |  15 +
 .../filesystems/fuse/uapi/fuse-uapi-io-uring.rst   | 126 ++++++
 6 files changed, 890 insertions(+)
 create mode 100644 Documentation/filesystems/fuse/fuse-io-uring.rst
 create mode 100644 Documentation/filesystems/fuse/fuse-io.rst
 create mode 100644 Documentation/filesystems/fuse/fuse-passthrough.rst
 create mode 100644 Documentation/filesystems/fuse/fuse.rst
 create mode 100644 Documentation/filesystems/fuse/index.rst
 create mode 100644 Documentation/filesystems/fuse/uapi/fuse-uapi-io-uring.rst

(limited to 'Documentation/filesystems/fuse')

diff --git a/Documentation/filesystems/fuse/fuse-io-uring.rst b/Documentation/filesystems/fuse/fuse-io-uring.rst
new file mode 100644
index 000000000..29f980575
--- /dev/null
+++ b/Documentation/filesystems/fuse/fuse-io-uring.rst
@@ -0,0 +1,131 @@
+.. 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.
+
+For the userspace protocol, see
+Documentation/filesystems/fuse/uapi/fuse-uapi-io-uring.rst.
+
+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()                            |
+
+Buffer pools
+============
+
+Without a buffer pool, every entry needs to pass a dedicated payload buffer
+large enough for the maximum payload size. A buffer pool decouples entries
+from payload buffers. The server hands the kernel one contiguous buffer pool
+of memory and when the kernel sends the server a request, it indicates the
+offset into the pool for that request's payload. Internally, the kernel is
+able to manage/optimize the buffer pool memory however it likes.
+
+A server may also register the pool region with io_uring as a fixed buffer.
+The backing pages are then pinned once, avoiding per-request pinning and
+address translation. This also allows servers to use the same registered
+buffers for subsequent backing store I/O through io-uring, keeping data
+in the same pinned pages without additional pinning / mapping overhead.
+
+Zero-copy
+=========
+
+Zero-copy lets the server read from / write to the client's pages (pinned
+user pages for direct I/O, or page-cache folios for buffered I/O) without an
+intermediary payload copy. This requires CAP_SYS_ADMIN privileges.
+
+When a fuse request arrives for a file that opted into zero-copy, the kernel
+registers the relevant pages (pinned user pages for direct i/o or underlying
+page cache folios for buffered i/o) into a sparse slot in the server's
+io_uring registered buffer table. The server can then operate on these pages
+directly using io-uring fixed buffer operations (eg read_fixed / write_fixed)
+and the kernel unregisters these pages when the request completes.
+Non-page-backed args (eg op out headers) will go through the payload buffer as
+normal.
diff --git a/Documentation/filesystems/fuse/fuse-io.rst b/Documentation/filesystems/fuse/fuse-io.rst
new file mode 100644
index 000000000..d736ac4cb
--- /dev/null
+++ b/Documentation/filesystems/fuse/fuse-io.rst
@@ -0,0 +1,45 @@
+.. 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.
diff --git a/Documentation/filesystems/fuse/fuse-passthrough.rst b/Documentation/filesystems/fuse/fuse-passthrough.rst
new file mode 100644
index 000000000..2b0e7c2da
--- /dev/null
+++ b/Documentation/filesystems/fuse/fuse-passthrough.rst
@@ -0,0 +1,133 @@
+.. 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.
diff --git a/Documentation/filesystems/fuse/fuse.rst b/Documentation/filesystems/fuse/fuse.rst
new file mode 100644
index 000000000..0fbd5a03f
--- /dev/null
+++ b/Documentation/filesystems/fuse/fuse.rst
@@ -0,0 +1,440 @@
+.. 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.
diff --git a/Documentation/filesystems/fuse/index.rst b/Documentation/filesystems/fuse/index.rst
new file mode 100644
index 000000000..3dada6c40
--- /dev/null
+++ b/Documentation/filesystems/fuse/index.rst
@@ -0,0 +1,15 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================================================
+FUSE (Filesystem in Userspace) Technical Documentation
+======================================================
+
+.. toctree::
+   :maxdepth: 2
+   :numbered:
+
+   fuse
+   fuse-io
+   fuse-io-uring
+   fuse-passthrough
+   uapi/fuse-uapi-io-uring
diff --git a/Documentation/filesystems/fuse/uapi/fuse-uapi-io-uring.rst b/Documentation/filesystems/fuse/uapi/fuse-uapi-io-uring.rst
new file mode 100644
index 000000000..8367be7ea
--- /dev/null
+++ b/Documentation/filesystems/fuse/uapi/fuse-uapi-io-uring.rst
@@ -0,0 +1,126 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================
+FUSE-over-io-uring uapi documentation
+=====================================
+
+Commands
+========
+
+``enum fuse_uring_cmd``:
+
+``FUSE_IO_URING_CMD_ADD_QUEUE``
+  Create a queue identified by ``fuse_uring_cmd_req.qid``. Queue-wide
+  options are passed in ``fuse_uring_cmd_req.flags``:
+
+  ``FUSE_URING_ZERO_COPY``
+    Enable zero-copy on this queue. Requires ``CAP_SYS_ADMIN`` and a buffer
+    pool, which is added separately via ``ADD_BUFPOOL`` before registering
+    entries (see `Zero-copy`_).
+
+``FUSE_IO_URING_CMD_ADD_BUFPOOL``
+  Register the payload buffer pool for an existing queue. The server provides
+  a single contiguous region in ``fuse_uring_cmd_req.bufpool.uaddr`` /
+  ``.len``. This command must be issued after ``ADD_QUEUE`` and before
+  registering any payload-carrying entries on that queue.
+  ``fuse_uring_cmd_req.flags`` must be 0. Submitting this command with
+  ``IORING_URING_CMD_FIXED`` marks the pool as registered, which avoids per
+  i/o pinning/unpinning and mapping overhead (see `Buffer pools`_).
+
+``FUSE_IO_URING_CMD_REGISTER``
+  Register a ring entry (a long-lived SQE that carries the request header
+  iovec). For a zero-copy queue, ``fuse_uring_cmd_req.ent_zero_copy_buf_index``
+  indicates the reserved registered buffer table slot this entry uses for
+  zero-copy (see `Zero-copy`_).
+
+``FUSE_IO_URING_CMD_COMMIT_AND_FETCH``
+  Commit the reply for a completed request and fetch the next one. The
+  request is identified by ``fuse_uring_cmd_req.commit_id`` (the value the
+  kernel reported in ``fuse_uring_ent_in_out.commit_id``).
+
+Structures
+==========
+
+``struct fuse_uring_cmd_req`` (80-byte SQE command area):
+
+============================  ==================================================
+Field                         Meaning
+============================  ==================================================
+``flags``                     Command-specific flags (see each command).
+``commit_id``                 Request id, for ``COMMIT_AND_FETCH``.
+``qid``                       Queue index.
+``bufpool.uaddr``             Pool base address, for ``ADD_BUFPOOL``.
+``bufpool.len``               Pool length in bytes, for ``ADD_BUFPOOL``.
+``bufpool.reserved``          Must be 0, for ``ADD_BUFPOOL``.
+``ent_zero_copy_buf_index``   Per-entry zero-copy slot, for ``REGISTER``.
+============================  ==================================================
+
+``struct fuse_uring_ent_in_out`` (reported by the kernel per request):
+
+============================  ==================================================
+Field                         Meaning
+============================  ==================================================
+``flags``                     ``FUSE_URING_ENT_ZERO_COPY`` if zero-copied.
+``commit_id``                 Id to echo back in ``COMMIT_AND_FETCH``.
+``payload_sz``                Total payload size in bytes (see `Zero-copy`_).
+``offset``                    Payload buffer offset within the pool.
+============================  ==================================================
+
+Buffer pools
+============
+Setup:
+
+* Issue ``ADD_QUEUE`` for the qid.
+* Issue ``ADD_BUFPOOL`` with ``bufpool.uaddr`` and ``bufpool.len`` pointing
+  at the region.
+* Register entries with ``REGISTER``.
+
+For every request that has a payload, the kernel reports where the payload
+lives in ``struct fuse_uring_ent_in_out`` (part of
+``struct fuse_uring_req_header``):
+
+``offset``
+  Byte offset, within the pool region, for this request's payload buffer.
+  The server adds this to the pool base address to locate the payload.
+
+``payload_sz``
+  Number of payload bytes for this request.
+
+To use registered buffers, the server registers the pool region with io_uring
+and submits ``ADD_BUFPOOL`` with ``IORING_URING_CMD_FIXED`` set in
+``sqe->uring_cmd_flags`` and the index of the registered bufpool in
+``sqe->buf_index``. Every SQE the server submits afterwards must follow the
+same fixed-buffer protocol, carrying ``IORING_URING_CMD_FIXED`` and that same
+``sqe->buf_index``. The same registered buffer can be reused for the server's
+backing-store I/O as well (e.g. ``IORING_OP_READ_FIXED`` /
+``IORING_OP_WRITE_FIXED``).
+
+Zero-copy
+=========
+Requirements:
+
+* The server must be privileged (``CAP_SYS_ADMIN``).
+* A zero-copy queue: ``ADD_QUEUE`` with the ``FUSE_URING_ZERO_COPY`` flag set.
+* A buffer pool: ``ADD_BUFPOOL``.
+* For each entry, ``REGISTER`` with ``ent_zero_copy_buf_index`` set to the
+  index this entry uses in the server's io_uring registered-buffer table.
+  This is where the kernel registers the request's pages for the server to
+  access (it is separate from the payload pool). On a non-zero-copy queue this
+  field must be 0.
+
+Zero-copy is selected per open file. The server sets the open-file flag in
+the ``FUSE_OPEN`` / ``FUSE_CREATE`` reply:
+
+``FOPEN_IO_URING_ZERO_COPY``
+  Reads/writes on this open file should use zero-copy.
+
+For a request that is zero-copied, the kernel sets ``FUSE_URING_ENT_ZERO_COPY``
+in ``fuse_uring_ent_in_out.flags`` and places the request's pages at the
+entry's ``ent_zero_copy_buf_index``. The server then issues
+``IORING_OP_READ_FIXED`` / ``IORING_OP_WRITE_FIXED`` against that index to
+transfer the data directly to/from the client's pages.
+
+For such a request, ``payload_sz`` includes the zero-copied page bytes
+(transferred via the registered buffer at ``ent_zero_copy_buf_index``). Any
+non-page-backed args (e.g. op headers) are still copied through the pool
+payload buffer at ``offset``.
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