Documentation/filesystems/fuse/fuse-passthrough.rst

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Documentation/filesystems/fuse/fuse-passthrough.rst
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================
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

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