[RFC PATCH 2/5] misc: add documentation for FDBox

Pratyush Yadav
2025-03-07T00:58:39Z

With FDBox in place, add documentation that describes what it is and how
it is used, along with its UAPI and in-kernel API.

Since the document refers to KHO, add a reference tag in kho/index.rst.

Signed-off-by: Pratyush Yadav <ptyadav@amazon.de>
---
Documentation/filesystems/locking.rst | 21 +++
Documentation/kho/fdbox.rst | 224 ++++++++++++++++++++++++++
Documentation/kho/index.rst | 3 +
MAINTAINERS | 1 +
4 files changed, 249 insertions(+)
create mode 100644 Documentation/kho/fdbox.rst

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index d20a32b77b60f..5526833faf79a 100644
--- a/Documentation/filesystems/locking.rst
+++ b/Documentation/filesystems/locking.rst
@@ -607,6 +607,27 @@ used. To block changes to file contents via a memory mapping during the
operation, the filesystem must take mapping->invalidate_lock to coordinate
with ->page_mkwrite.

+fdbox_file_ops
+==============
+
+prototypes::
+
+ int (*kho_write)(struct fdbox_fd *box_fd, void *fdt);
+ int (*seal)(struct fdbox *box);
+ int (*unseal)(struct fdbox *box);
+
+
+locking rules:
+ all may block
+
+============== ==================================================
+ops i_rwsem(box_fd->file->f_inode)
+============== ==================================================
+kho_write: exclusive
+seal: no
+unseal: no
+============== ==================================================
+
dquot_operations
================

diff --git a/Documentation/kho/fdbox.rst b/Documentation/kho/fdbox.rst
new file mode 100644
index 0000000000000..44a3f5cdf1efb
--- /dev/null
+++ b/Documentation/kho/fdbox.rst
@@ -0,0 +1,224 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+===========================
+File Descriptor Box (FDBox)
+===========================
+
+:Author: Pratyush Yadav
+
+Introduction
+============
+
+The File Descriptor Box (FDBox) is a mechanism for userspace to name file
+descriptors and give them over to the kernel to hold. They can later be
+retrieved by passing in the same name.
+
+The primary purpose of FDBox is to be used with :ref:`kho`. There are many kinds
+anonymous file descriptors in the kernel like memfd, guest_memfd, iommufd, etc.
+that would be useful to be preserved using KHO. To be able to do that, there
+needs to be a mechanism to label FDs that allows userspace to set the label
+before doing KHO and to use the label to map them back after KHO. FDBox achieves
+that purpose by exposing a miscdevice which exposes ioctls to label and transfer
+FDs between the kernel and userspace. FDBox is not intended to work with any
+generic file descriptor. Support for each kind of FDs must be explicitly
+enabled.
+
+FDBox can be enabled by setting the ``CONFIG_FDBOX`` option to ``y``. While the
+primary purpose of FDBox is to be used with KHO, it does not explicitly require
+``CONFIG_KEXEC_HANDOVER``, since it can be used without KHO, simply as a way to
+preserve or transfer FDs when userspace exits.
+
+Concepts
+========
+
+Box
+---
+
+The box is a container for FDs. Boxes are identified by their name, which must
+be unique. Userspace can put FDs in the box using the ``FDBOX_PUT_FD``
+operation, and take them out of the box using the ``FDBOX_GET_FD`` operation.
+Once all the required FDs are put into the box, it can be sealed to make it
+ready for shipping. This can be done by the ``FDBOX_SEAL`` operation. The seal
+operation notifies each FD in the box. If any of the FDs have a dependency on
+another, this gives them an opportunity to ensure all dependencies are met, or
+fail the seal if not. Once a box is sealed, no FDs can be added or removed from
+the box until it is unsealed. Only sealed boxes are transported to a new kernel
+via KHO. The box can be unsealed by the ``FDBOX_UNSEAL`` operation. This is the
+opposite of seal. It also notifies each FD in the box to ensure all dependencies
+are met. This can be useful in case some FDs fail to be restored after KHO.
+
+Box FD
+------
+
+The Box FD is a FD that is currently in a box. It is identified by its name,
+which must be unique in the box it belongs to. The Box FD is created when a FD
+is put into a box by using the ``FDBOX_PUT_FD`` operation. This operation
+removes the FD from the calling task. The FD can be restored by passing the
+unique name to the ``FDBOX_GET_FD`` operation.
+
+FDBox control device
+--------------------
+
+This is the ``/dev/fdbox/fdbox`` device. A box can be created using the
+``FDBOX_CREATE_BOX`` operation on the device. A box can be removed using the
+``FDBOX_DELETE_BOX`` operation.
+
+UAPI
+====
+
+FDBOX_NAME_LEN
+--------------
+
+.. code-block:: c
+
+ #define FDBOX_NAME_LEN 256
+
+Maximum length of the name of a Box or Box FD.
+
+Ioctls on /dev/fdbox/fdbox
+--------------------------
+
+FDBOX_CREATE_BOX
+~~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+ #define FDBOX_CREATE_BOX _IO(FDBOX_TYPE, FDBOX_BASE + 0)
+ struct fdbox_create_box {
+ __u64 flags;
+ __u8 name[FDBOX_NAME_LEN];
+ };
+
+Create a box.
+
+After this returns, the box is available at ``/dev/fdbox/<name>``.
+
+``name``
+ The name of the box to be created. Must be unique.
+
+``flags``
+ Flags to the operation. Currently, no flags are defined.
+
+Returns:
+ 0 on success, -1 on error, with errno set.
+
+FDBOX_DELETE_BOX
+~~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+ #define FDBOX_DELETE_BOX _IO(FDBOX_TYPE, FDBOX_BASE + 1)
+ struct fdbox_delete_box {
+ __u64 flags;
+ __u8 name[FDBOX_NAME_LEN];
+ };
+
+Delete a box.
+
+After this returns, the box is no longer available at ``/dev/fdbox/<name>``.
+
+``name``
+ The name of the box to be deleted.
+
+``flags``
+ Flags to the operation. Currently, no flags are defined.
+
+Returns:
+ 0 on success, -1 on error, with errno set.
+
+Ioctls on /dev/fdbox/<boxname>
+------------------------------
+
+These must be performed on the ``/dev/fdbox/<boxname>`` device.
+
+FDBX_PUT_FD
+~~~~~~~~~~~
+
+.. code-block:: c
+
+ #define FDBOX_PUT_FD _IO(FDBOX_TYPE, FDBOX_BASE + 2)
+ struct fdbox_put_fd {
+ __u64 flags;
+ __u32 fd;
+ __u32 pad;
+ __u8 name[FDBOX_NAME_LEN];
+ };
+
+
+Put FD into the box.
+
+After this returns, ``fd`` is removed from the task and can no longer be used by
+it.
+
+``name``
+ The name of the FD.
+
+``fd``
+ The file descriptor number to be
+
+``flags``
+ Flags to the operation. Currently, no flags are defined.
+
+Returns:
+ 0 on success, -1 on error, with errno set.
+
+FDBX_GET_FD
+~~~~~~~~~~~
+
+.. code-block:: c
+
+ #define FDBOX_GET_FD _IO(FDBOX_TYPE, FDBOX_BASE + 3)
+ struct fdbox_get_fd {
+ __u64 flags;
+ __u8 name[FDBOX_NAME_LEN];
+ };
+
+Get an FD from the box.
+
+After this returns, the FD identified by ``name`` is mapped into the task and is
+available for use.
+
+``name``
+ The name of the FD to get.
+
+``flags``
+ Flags to the operation. Currently, no flags are defined.
+
+Returns:
+ FD number on success, -1 on error with errno set.
+
+FDBOX_SEAL
+~~~~~~~~~~
+
+.. code-block:: c
+
+ #define FDBOX_SEAL _IO(FDBOX_TYPE, FDBOX_BASE + 4)
+
+Seal the box.
+
+Gives the kernel an opportunity to ensure all dependencies are met in the box.
+After this returns, the box is sealed and FDs can no longer be added or removed
+from it. A box must be sealed for it to be transported across KHO.
+
+Returns:
+ 0 on success, -1 on error with errno set.
+
+FDBOX_UNSEAL
+~~~~~~~~~~~~
+
+.. code-block:: c
+
+ #define FDBOX_UNSEAL _IO(FDBOX_TYPE, FDBOX_BASE + 5)
+
+Unseal the box.
+
+Gives the kernel an opportunity to ensure all dependencies are met in the box,
+and in case of KHO, no FDs have been lost in transit.
+
+Returns:
+ 0 on success, -1 on error with errno set.
+
+Kernel functions and structures
+===============================
+
+.. kernel-doc:: include/linux/fdbox.h
diff --git a/Documentation/kho/index.rst b/Documentation/kho/index.rst
index 5e7eeeca8520f..051513b956075 100644
--- a/Documentation/kho/index.rst
+++ b/Documentation/kho/index.rst
@@ -1,5 +1,7 @@
.. SPDX-License-Identifier: GPL-2.0-or-later

+.. _kho:
+
========================
Kexec Handover Subsystem
========================
@@ -9,6 +11,7 @@ Kexec Handover Subsystem

concepts
usage
+ fdbox

.. only:: subproject and html

diff --git a/MAINTAINERS b/MAINTAINERS
index d329d3e5514c5..135427582e60f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8866,6 +8866,7 @@ FDBOX
M: Pratyush Yadav <pratyush@kernel.org>
L: linux-fsdevel@vger.kernel.org
S: Maintained
+F: Documentation/kho/fdbox.rst
F: drivers/misc/fdbox.c
F: include/linux/fdbox.h
F: include/uapi/linux/fdbox.h
--
2.47.1