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

Randy Dunlap
2025-03-07T02:20:13Z

On March 6, 2025 4:57:36 PM PST, Pratyush Yadav <ptyadav@amazon.de> wrote:
>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

many kinds of

>+anonymous file descriptors in the kernel like memfd, guest_memfd, iommufd, etc.

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.

Is this ioctl range documented is ioctl-number.rst?
I didn't notice a patch for that.

>+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

What if KHO is not being used?

>+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
>+------

I can't tell in my email font, but is each underlinoat least as long as the title above it?

>+
>+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

~Randy