Live Update Orchestrator¶

Author:: Pasha Tatashin <pasha.tatashin@soleen.com>

Live Update is a specialized, kexec-based reboot process that allows a running kernel to be updated from one version to another while preserving the state of selected resources and keeping designated hardware devices operational. For these devices, DMA activity may continue throughout the kernel transition.

While the primary use case driving this work is supporting live updates of the Linux kernel when it is used as a hypervisor in cloud environments, the LUO framework itself is designed to be workload-agnostic. Live Update facilitates a full kernel version upgrade for any type of system.

For example, a non-hypervisor system running an in-memory cache like memcached with many gigabytes of data can use LUO. The userspace service can place its cache into a memfd, have its state preserved by LUO, and restore it immediately after the kernel kexec.

Whether the system is running virtual machines, containers, a high-performance database, or networking services, LUO’s primary goal is to enable a full kernel update by preserving critical userspace state and keeping essential devices operational.

The core of LUO is a mechanism that tracks the progress of a live update, along with a callback API that allows other kernel subsystems to participate in the process. Example subsystems that can hook into LUO include: kvm, iommu, interrupts, vfio, participating filesystems, and memory management.

LUO uses Kexec Handover to transfer memory state from the current kernel to the next kernel. For more details see Kexec Handover Concepts.

LUO Sessions¶

LUO Sessions provide the core mechanism for grouping and managing struct file * instances that need to be preserved across a kexec-based live update. Each session acts as a named container for a set of file objects, allowing a userspace agent to manage the lifecycle of resources critical to a workload.

Core Concepts:

Named Containers: Sessions are identified by a unique, user-provided name, which is used for both creation in the current kernel and retrieval in the next kernel.
Userspace Interface: Session management is driven from userspace via ioctls on /dev/liveupdate.
Serialization: Session metadata is preserved using the KHO framework. When a live update is triggered via kexec, an array of struct luo_session_ser is populated and placed in a preserved memory region. An FDT node is also created, containing the count of sessions and the physical address of this array.

Session Lifecycle:

Creation: A userspace agent calls luo_session_create() to create a new, empty session and receives a file descriptor for it.
Serialization: When the reboot(LINUX_REBOOT_CMD_KEXEC) syscall is made, luo_session_serialize() is called. It iterates through all active sessions and writes their metadata into a memory area preserved by KHO.
Deserialization (in new kernel): After kexec, luo_session_deserialize() runs, reading the serialized data and creating a list of struct luo_session objects representing the preserved sessions.
Retrieval: A userspace agent in the new kernel can then call luo_session_retrieve() with a session name to get a new file descriptor and access the preserved state.

LUO Preserving File Descriptors¶

LUO provides the infrastructure to preserve specific, stateful file descriptors across a kexec-based live update. The primary goal is to allow workloads, such as virtual machines using vfio, memfd, or iommufd, to retain access to their essential resources without interruption.

The framework is built around a callback-based handler model and a well- defined lifecycle for each preserved file.

Handler Registration: Kernel modules responsible for a specific file type (e.g., memfd, vfio) register a struct liveupdate_file_handler. This handler provides a set of callbacks that LUO invokes at different stages of the update process, most notably:

can_preserve(): A lightweight check to determine if the handler is compatible with a given ‘struct file’.

preserve(): The heavyweight operation that saves the file’s state and returns an opaque u64 handle. This is typically performed while the workload is still active to minimize the downtime during the actual reboot transition.

unpreserve(): Cleans up any resources allocated by .preserve(), called if the preservation process is aborted before the reboot (i.e. session is closed).

freeze(): A final pre-reboot opportunity to prepare the state for kexec. We are already in reboot syscall, and therefore userspace cannot mutate the file anymore.

unfreeze(): Undoes the actions of .freeze(), called if the live update is aborted after the freeze phase.

retrieve(): Reconstructs the file in the new kernel from the preserved handle.

finish(): Performs final check and cleanup in the new kernel. After succesul finish call, LUO gives up ownership to this file.

File Preservation Lifecycle happy path:

Preserve (Normal Operation): A userspace agent preserves files one by one via an ioctl. For each file, luo_preserve_file() finds a compatible handler, calls its .preserve() operation, and creates an internal struct luo_file to track the live state.
Freeze (Pre-Reboot): Just before the kexec, luo_file_freeze() is called. It iterates through all preserved files, calls their respective .freeze() operation, and serializes their final metadata (compatible string, token, and data handle) into a contiguous memory block for KHO.
Deserialize: After kexec, luo_file_deserialize() runs when session gets deserialized (which is when /dev/liveupdate is first opened). It reads the serialized data from the KHO memory region and reconstructs the in-memory list of struct luo_file instances for the new kernel, linking them to their corresponding handlers.
Retrieve (New Kernel - Userspace Ready): The userspace agent can now restore file descriptors by providing a token. luo_retrieve_file() searches for the matching token, calls the handler’s .retrieve() op to re-create the ‘struct file’, and returns a new FD. Files can be retrieved in ANY order.
Finish (New Kernel - Cleanup): Once a session retrival is complete, luo_file_finish() is called. It iterates through all files, invokes their .finish() operations for final cleanup, and releases all associated kernel resources.

File Preservation Lifecycle unhappy paths:

Abort Before Reboot: If the userspace agent aborts the live update process before calling reboot (e.g., by closing the session file descriptor), the session’s release handler calls luo_file_unpreserve_files(). This invokes the .unpreserve() callback on all preserved files, ensuring all allocated resources are cleaned up and returning the system to a clean state.
Freeze Failure: During the reboot() syscall, if any handler’s .freeze() op fails, the .unfreeze() op is invoked on all previously successful freezes to roll back their state. The reboot() syscall then returns an error to userspace, canceling the live update.
Finish Failure: In the new kernel, if a handler’s .finish() op fails, the luo_file_finish() operation is aborted. LUO retains ownership of all files within that session, including those that were not yet processed. The userspace agent can attempt to call the finish operation again later. If the issue cannot be resolved, these resources will be held by LUO until the next live update cycle, at which point they will be discarded.

LUO File Lifecycle Bound Global Data¶

File-Lifecycle-Bound (FLB) objects provide a mechanism for managing global state that is shared across multiple live-updatable files. The lifecycle of this shared state is tied to the preservation of the files that depend on it.

An FLB represents a global resource, such as the IOMMU core state, that is required by multiple file descriptors (e.g., all VFIO fds).

The preservation of the FLB’s state is triggered when the first file depending on it is preserved. The cleanup of this state (unpreserve or finish) is triggered when the last file depending on it is unpreserved or finished.

Handler Dependency: A file handler declares its dependency on one or more FLBs by registering them via liveupdate_register_flb().

Callback Model: Each FLB is defined by a set of operations (struct liveupdate_flb_ops) that LUO invokes at key points:

.preserve(): Called for the first file. Saves global state.

.unpreserve(): Called for the last file (if aborted pre-reboot).

.retrieve(): Called on-demand in the new kernel to restore the state.

.finish(): Called for the last file in the new kernel for cleanup.

This reference-counted approach ensures that shared state is saved exactly once and restored exactly once, regardless of how many files depend on it, and that its lifecycle is correctly managed across the kexec transition.

Live Update Orchestrator ABI¶

This header defines the stable Application Binary Interface used by the Live Update Orchestrator to pass state from a pre-update kernel to a post-update kernel. The ABI is built upon the Kexec HandOver framework and uses a Flattened Device Tree to describe the preserved data.

This interface is a contract. Any modification to the FDT structure, node properties, compatible strings, or the layout of the __packed serialization structures defined here constitutes a breaking change. Such changes require incrementing the version number in the relevant _COMPATIBLE string to prevent a new kernel from misinterpreting data from an old kernel.

Changes are allowed provided the compatibility version is incremented; however, backward/forward compatibility is only guaranteed for kernels supporting the same ABI version.

FDT Structure Overview:

The entire LUO state is encapsulated within a single KHO entry named “LUO”. This entry contains an FDT with the following layout:

/ {
    compatible = "luo-v1";
    liveupdate-number = <...>;

    luo-session {
        compatible = "luo-session-v1";
        luo-session-header = <phys_addr_of_session_header_ser>;
    };

    luo-flb {
        compatible = "luo-flb-v1";
        luo-flb-header = <phys_addr_of_flb_header_ser>;
    };
};

Main LUO Node (/):

compatible: “luo-v1” Identifies the overall LUO ABI version.

liveupdate-number: u64 A counter tracking the number of successful live updates performed.

Session Node (luo-session):

This node describes all preserved user-space sessions.

compatible: “luo-session-v1” Identifies the session ABI version.
luo-session-header: u64 The physical address of a struct luo_session_header_ser. This structure is the header for a contiguous block of memory containing an array of struct luo_session_ser, one for each preserved session.

File-Lifecycle-Bound Node (luo-flb):

This node describes all preserved global objects whose lifecycle is bound to that of the preserved files (e.g., shared IOMMU state).

compatible: “luo-flb-v1” Identifies the FLB ABI version.
luo-flb-header: u64 The physical address of a struct luo_flb_header_ser. This structure is the header for a contiguous block of memory containing an array of struct luo_flb_ser, one for each preserved global object.

Serialization Structures:

The FDT properties point to memory regions containing arrays of simple, __packed structures. These structures contain the actual preserved state.

struct luo_session_header_ser: Header for the session array. Contains the total page count of the preserved memory block and the number of struct luo_session_ser entries that follow.
struct luo_session_ser: Metadata for a single session, including its name and a physical pointer to another preserved memory block containing an array of struct luo_file_ser for all files in that session.
struct luo_file_ser: Metadata for a single preserved file. Contains the compatible string to find the correct handler in the new kernel, a user-provided token for identification, and an opaque data handle for the handler to use.
struct luo_flb_header_ser: Header for the FLB array. Contains the total page count of the preserved memory block and the number of struct luo_flb_ser entries that follow.
struct luo_flb_ser: Metadata for a single preserved global object. Contains its name (compatible string), an opaque data handle, and the count number of files depending on it.

The following types of file descriptors can be preserved

Memfd Preservation via LUO

Public API¶

struct liveupdate_file_op_args¶: Arguments for file operation callbacks.

Definition:

struct liveupdate_file_op_args {
    struct liveupdate_file_handler *handler;
    bool retrieved;
    struct file *file;
    u64 serialized_data;
    void *private_data;
};

Members

handler: The file handler being called.
retrieved: The retrieve status for the ‘can_finish / finish’ operation.
file: The file object. For retrieve: [OUT] The callback sets this to the new file. For other ops: [IN] The caller sets this to the file being operated on.
serialized_data: The opaque u64 handle, preserve/prepare/freeze may update this field.
private_data: Private data for the file used to hold runtime state that is not preserved. Set by the handler’s .preserve() callback, and must be freed in the handler’s .unpreserve() callback.

Description

This structure bundles all parameters for the file operation callbacks. The ‘data’ and ‘file’ fields are used for both input and output.

struct liveupdate_file_ops¶: Callbacks for live-updatable files.

Definition:

struct liveupdate_file_ops {
    bool (*can_preserve)(struct liveupdate_file_handler *handler, struct file *file);
    int (*preserve)(struct liveupdate_file_op_args *args);
    void (*unpreserve)(struct liveupdate_file_op_args *args);
    int (*freeze)(struct liveupdate_file_op_args *args);
    void (*unfreeze)(struct liveupdate_file_op_args *args);
    int (*retrieve)(struct liveupdate_file_op_args *args);
    bool (*can_finish)(struct liveupdate_file_op_args *args);
    void (*finish)(struct liveupdate_file_op_args *args);
    struct module *owner;
};

Members

can_preserve: Required. Lightweight check to see if this handler is compatible with the given file.
preserve: Required. Performs state-saving for the file.
unpreserve: Required. Cleans up any resources allocated by preserve.
freeze: Optional. Final actions just before kernel transition.
unfreeze: Optional. Undo freeze operations.
retrieve: Required. Restores the file in the new kernel.
can_finish: Optional. Check if this FD can finish, i.e. all restoration pre-requirements for this FD are satisfied. Called prior to finish, in order to do successful finish calls for all resources in the session.
finish: Required. Final cleanup in the new kernel.
owner: Module reference

Description

All operations (except can_preserve) receive a pointer to a ‘struct liveupdate_file_op_args’ containing the necessary context.

struct liveupdate_file_handler¶: Represents a handler for a live-updatable file type.

Definition:

struct liveupdate_file_handler {
    const struct liveupdate_file_ops *ops;
    const char compatible[LIVEUPDATE_HNDL_COMPAT_LENGTH];
};

Members

ops: Callback functions
compatible: The compatibility string (e.g., “memfd-v1”, “vfiofd-v1”) that uniquely identifies the file type this handler supports. This is matched against the compatible string associated with individual struct file instances.

Description

Modules that want to support live update for specific file types should register an instance of this structure. LUO uses this registration to determine if a given file can be preserved and to find the appropriate operations to manage its state across the update.

struct liveupdate_flb_op_args¶: Arguments for FLB operation callbacks.

Definition:

struct liveupdate_flb_op_args {
    struct liveupdate_flb *flb;
    u64 data;
    void *obj;
};

Members

flb: The global FLB instance for which this call is performed.
data: For .preserve(): [OUT] The callback sets this field. For .unpreserve(): [IN] The handle from .preserve(). For .retrieve(): [IN] The handle from .preserve().
obj: For .preserve(): [OUT] Sets this to the live object. For .retrieve(): [OUT] Sets this to the live object. For .finish(): [IN] The live object from .retrieve().

Description

This structure bundles all parameters for the FLB operation callbacks.

struct liveupdate_flb_ops¶: Callbacks for global File-Lifecycle-Bound data.

Definition:

struct liveupdate_flb_ops {
    int (*preserve)(struct liveupdate_flb_op_args *argp);
    void (*unpreserve)(struct liveupdate_flb_op_args *argp);
    int (*retrieve)(struct liveupdate_flb_op_args *argp);
    void (*finish)(struct liveupdate_flb_op_args *argp);
    struct module *owner;
};

Members

preserve: Called when the first file using this FLB is preserved. The callback must save its state and return a single, self-contained u64 handle by setting the ‘argp->data’ field and ‘argp->obj’.
unpreserve: Called when the last file using this FLB is unpreserved (aborted before reboot). Receives the handle via ‘argp->data’ and live object via ‘argp->obj’.
retrieve: Called on-demand in the new kernel, the first time a component requests access to the shared object. It receives the preserved handle via ‘argp->data’ and must reconstruct the live object, returning it by setting the ‘argp->obj’ field.
finish: Called in the new kernel when the last file using this FLB is finished. Receives the live object via ‘argp->obj’ for cleanup.
owner: Module reference

Description

Operations that manage global shared data with file bound lifecycle, triggered by the first file that uses it and concluded by the last file that uses it, across all sessions.

struct liveupdate_flb¶: A global definition for a shared data object.

Definition:

struct liveupdate_flb {
    const struct liveupdate_flb_ops *ops;
    const char compatible[LIVEUPDATE_FLB_COMPAT_LENGTH];
};

Members

ops: Callback functions
compatible: The compatibility string (e.g., “iommu-core-v1” that uniquely identifies the FLB type this handler supports. This is matched against the compatible string associated with individual struct liveupdate_flb instances.

Description

This struct is the “template” that a driver registers to define a shared, file-lifecycle-bound object. The actual runtime state (the live object, refcount, etc.) is managed privately by the LUO core.

struct luo_file_ser¶: Represents the serialized preserves files.

Definition:

struct luo_file_ser {
    char compatible[LIVEUPDATE_HNDL_COMPAT_LENGTH];
    u64 data;
    u64 token;
};

Members

compatible: File handler compatible string.
data: Private data
token: User provided token for this file

Description

If this structure is modified, LUO_SESSION_COMPATIBLE must be updated.

struct luo_file_set_ser¶: Represents the serialized metadata for file set

Definition:

struct luo_file_set_ser {
    u64 files;
    u64 count;
};

Members

files: The physical address of a contiguous memory block that holds the serialized state of files (array of luo_file_ser) in this file set.
count: The total number of files that were part of this session during serialization. Used for iteration and validation during restoration.

struct luo_session_header_ser¶: Header for the serialized session data block.

Definition:

struct luo_session_header_ser {
    u64 count;
};

Members

count: The number of struct luo_session_ser entries that immediately follow this header in the memory block.

Description

This structure is located at the beginning of a contiguous block of physical memory preserved across the kexec. It provides the necessary metadata to interpret the array of session entries that follow.

If this structure is modified, LUO_FDT_SESSION_COMPATIBLE must be updated.

struct luo_session_ser¶: Represents the serialized metadata for a LUO session.

Definition:

struct luo_session_ser {
    char name[LIVEUPDATE_SESSION_NAME_LENGTH];
    struct luo_file_set_ser file_set_ser;
};

Members

name: The unique name of the session, provided by the userspace at the time of session creation.
file_set_ser: Serialized files belonging to this session,

Description

This structure is used to package session-specific metadata for transfer between kernels via Kexec Handover. An array of these structures (one per session) is created and passed to the new kernel, allowing it to reconstruct the session context.

If this structure is modified, LUO_FDT_SESSION_COMPATIBLE must be updated.

struct luo_flb_header_ser¶: Header for the serialized FLB data block.

Definition:

struct luo_flb_header_ser {
    u64 pgcnt;
    u64 count;
};

Members

pgcnt: The total number of pages occupied by the entire preserved memory region, including this header and the subsequent array of struct luo_flb_ser entries.
count: The number of struct luo_flb_ser entries that follow this header in the memory block.

Description

This structure is located at the physical address specified by the LUO_FDT_FLB_HEADER FDT property. It provides the new kernel with the necessary information to find and iterate over the array of preserved File-Lifecycle-Bound objects and to manage the underlying memory.

If this structure is modified, LUO_FDT_FLB_COMPATIBLE must be updated.

struct luo_flb_ser¶: Represents the serialized state of a single FLB object.

Definition:

struct luo_flb_ser {
    char name[LIVEUPDATE_FLB_COMPAT_LENGTH];
    u64 data;
    u64 count;
};

Members

name: The unique compatibility string of the FLB object, used to find the corresponding struct liveupdate_flb handler in the new kernel.
data: The opaque u64 handle returned by the FLB’s .preserve() operation in the old kernel. This handle encapsulates the entire state needed for restoration.
count: The reference count at the time of serialization; i.e., the number of preserved files that depended on this FLB. This is used by the new kernel to correctly manage the FLB’s lifecycle.

Description

An array of these structures is created in a preserved memory region and passed to the new kernel. Each entry allows the LUO core to restore one global, shared object.

If this structure is modified, LUO_FDT_FLB_COMPATIBLE must be updated.

Internal API¶

int liveupdate_reboot(void)¶: Kernel reboot notifier for live update final serialization.

Parameters

void: no arguments

Description

This function is invoked directly from the reboot() syscall pathway if kexec is in progress.

If any callback fails, this function aborts KHO, undoes the freeze() callbacks, and returns an error.

bool liveupdate_enabled(void)¶: Check if the live update feature is enabled.

Parameters

void: no arguments

Description

This function returns the state of the live update feature flag, which can be controlled via the liveupdate kernel command-line parameter.

return true if live update is enabled, false otherwise.

int luo_flb_file_preserve(struct liveupdate_file_handler *fh)¶: Notifies FLBs that a file is about to be preserved.

Parameters

struct liveupdate_file_handler *fh: The file handler for the preserved file.

Description

This function iterates through all FLBs associated with the given file handler. It increments the reference count for each FLB. If the count becomes 1, it triggers the FLB’s .preserve() callback to save the global state.

This operation is atomic. If any FLB’s .preserve() op fails, it will roll back by calling .unpreserve() on any FLBs that were successfully preserved during this call.

Context

Called from luo_preserve_file()

Return

0 on success, or a negative errno on failure.

void luo_flb_file_unpreserve(struct liveupdate_file_handler *fh)¶: Notifies FLBs that a dependent file was unpreserved.

Parameters

struct liveupdate_file_handler *fh: The file handler for the unpreserved file.

Description

This function iterates through all FLBs associated with the given file handler, in reverse order of registration. It decrements the reference count for each FLB. If the count becomes 0, it triggers the FLB’s .unpreserve() callback to clean up the global state.

Context

Called when a preserved file is being cleaned up before reboot (e.g., from luo_file_unpreserve_files()).

void luo_flb_file_finish(struct liveupdate_file_handler *fh)¶: Notifies FLBs that a dependent file has been finished.

Parameters

struct liveupdate_file_handler *fh: The file handler for the finished file.

Description

This function iterates through all FLBs associated with the given file handler, in reverse order of registration. It decrements the incoming reference count for each FLB. If the count becomes 0, it triggers the FLB’s .finish() callback for final cleanup in the new kernel.

Context

Called from luo_file_finish() for each file being finished.

int liveupdate_register_flb(struct liveupdate_file_handler *fh, struct liveupdate_flb *flb)¶: Associate an FLB with a file handler and register it globally.

Parameters

struct liveupdate_file_handler *fh: The file handler that will now depend on the FLB.
struct liveupdate_flb *flb: The File-Lifecycle-Bound object to associate.

Description

Establishes a dependency, informing the LUO core that whenever a file of type fh is preserved, the state of flb must also be managed.

On the first registration of a given flb object, it is added to a global registry. This function checks for duplicate registrations, both for a specific handler and globally, and ensures the total number of unique FLBs does not exceed the system limit.

Context

Typically called from a subsystem’s module init function after both the handler and the FLB have been defined and initialized.

Return

0 on success. Returns a negative errno on failure: -EINVAL if arguments are NULL or not initialized. -ENOMEM on memory allocation failure. -EEXIST if this FLB is already registered with this handler. -ENOSPC if the maximum number of global FLBs has been reached. -EOPNOTSUPP if live update is disabled or not configured.

int liveupdate_unregister_flb(struct liveupdate_file_handler *fh, struct liveupdate_flb *flb)¶: Remove an FLB dependency from a file handler.

Parameters

struct liveupdate_file_handler *fh: The file handler that is currently depending on the FLB.
struct liveupdate_flb *flb: The File-Lifecycle-Bound object to remove.

Description

Removes the association between the specified file handler and the FLB previously established by liveupdate_register_flb().

This function manages the global lifecycle of the FLB. It decrements the FLB’s usage count. If this was the last file handler referencing this FLB, the FLB is removed from the global registry and the reference to its owner module (acquired during registration) is released.

Context

This function ensures the session is quiesced (no active FDs being created) during the update. It is typically called from a subsystem’s module exit function.

Return

0 on success. -EOPNOTSUPP if live update is disabled. -EBUSY if the live update session is active and cannot be quiesced. -ENOENT if the FLB was not found in the file handler’s list.

int liveupdate_flb_get_incoming(struct liveupdate_flb *flb, void **objp)¶: Retrieve the incoming FLB object.

Parameters

struct liveupdate_flb *flb: The FLB definition.
void **objp: Output parameter; will be populated with the live shared object.

Description

Returns a pointer to its shared live object for the incoming (post-reboot) path.

If this is the first time the object is requested in the new kernel, this function will trigger the FLB’s .retrieve() callback to reconstruct the object from its preserved state. Subsequent calls will return the same cached object.

Return

0 on success, or a negative errno on failure. -ENODATA means no incoming FLB data, -ENOENT means specific flb not found in the incoming data, and -EOPNOTSUPP when live update is disabled or not configured.

int liveupdate_flb_get_outgoing(struct liveupdate_flb *flb, void **objp)¶: Retrieve the outgoing FLB object.

Parameters

struct liveupdate_flb *flb: The FLB definition.
void **objp: Output parameter; will be populated with the live shared object.

Description

Returns a pointer to its shared live object for the outgoing (pre-reboot) path.

This function assumes the object has already been created by the FLB’s .preserve() callback, which is triggered when the first dependent file is preserved.

Return

0 on success, or a negative errno on failure.

void luo_flb_serialize(void)¶: Serializes all active FLB objects for KHO.

Parameters

void: no arguments

Description

This function is called from the reboot path. It iterates through all registered File-Lifecycle-Bound (FLB) objects. For each FLB that has been preserved (i.e., its reference count is greater than zero), it writes its metadata into the memory region designated for Kexec Handover.

The serialized data includes the FLB’s compatibility string, its opaque data handle, and the final reference count. This allows the new kernel to find the appropriate handler and reconstruct the FLB’s state.

Context

Called from liveupdate_reboot() just before kho_finalize().

struct luo_session_header¶: Header struct for managing LUO sessions.

Definition:

struct luo_session_header {
    long count;
    struct list_head list;
    struct rw_semaphore rwsem;
    struct luo_session_header_ser *header_ser;
    struct luo_session_ser *ser;
    bool active;
};

Members

count: The number of sessions currently tracked in the list.
list: The head of the linked list of struct luo_session instances.
rwsem: A read-write semaphore providing synchronized access to the session list and other fields in this structure.
header_ser: The header data of serialization array.
ser: The serialized session data (an array of struct luo_session_ser).
active: Set to true when first initialized. If previous kernel did not send session data, active stays false for incoming.

struct luo_session_global¶: Global container for managing LUO sessions.

Definition:

struct luo_session_global {
    struct luo_session_header incoming;
    struct luo_session_header outgoing;
};

Members

incoming: The sessions passed from the previous kernel.
outgoing: The sessions that are going to be passed to the next kernel.

bool luo_session_quiesce(void)¶: Ensure no active sessions exist and lock session lists.

Parameters

void: no arguments

Description

Acquires exclusive write locks on both incoming and outgoing session lists. It then validates no sessions exist in either list.

This mechanism is used during file handler un/registration to ensure that no sessions are currently using the handler, and no new sessions can be created while un/registration is in progress.

This prevents registering new handlers while sessions are active or while deserialization is in progress.

Return

true - System is quiescent (0 sessions) and locked. false - Active sessions exist. The locks are released internally.

void luo_session_resume(void)¶: Unlock session lists and resume normal activity.

Parameters

void: no arguments

Description

Releases the exclusive locks acquired by a successful call to luo_session_quiesce().

struct luo_file¶: Represents a single preserved file instance.

Definition:

struct luo_file {
    struct liveupdate_file_handler *fh;
    struct file *file;
    u64 serialized_data;
    void *private_data;
    bool retrieved;
    struct mutex mutex;
    struct list_head list;
    u64 token;
};

Members

fh: Pointer to the struct liveupdate_file_handler that manages this type of file.
file: Pointer to the kernel’s struct file that is being preserved. This is NULL in the new kernel until the file is successfully retrieved.
serialized_data: The opaque u64 handle to the serialized state of the file. This handle is passed back to the handler’s .freeze(), .retrieve(), and .finish() callbacks, allowing it to track and update its serialized state across phases.
private_data: Pointer to the private data for the file used to hold runtime state that is not preserved. Set by the handler’s .preserve() callback, and must be freed in the handler’s .unpreserve() callback.
retrieved: A flag indicating whether a user/kernel in the new kernel has successfully called retrieve() on this file. This prevents multiple retrieval attempts.
mutex: A mutex that protects the fields of this specific instance (e.g., retrieved, file), ensuring that operations like retrieving or finishing a file are atomic.
list: The list_head linking this instance into its parent file_set’s list of preserved files.
token: The user-provided unique token used to identify this file.

Description

This structure is the core in-kernel representation of a single file being managed through a live update. An instance is created by luo_preserve_file() to link a ‘struct file’ to its corresponding handler, a user-provided token, and the serialized state handle returned by the handler’s .preserve() operation.

These instances are tracked in a per-file_set list. The serialized_data field, which holds a handle to the file’s serialized state, may be updated during the .freeze() callback before being serialized for the next kernel. After reboot, these structures are recreated by luo_file_deserialize() and are finally cleaned up by luo_file_finish().

int luo_preserve_file(struct luo_file_set *file_set, u64 token, int fd)¶: Initiate the preservation of a file descriptor.

Parameters

struct luo_file_set *file_set: The file_set to which the preserved file will be added.
u64 token: A unique, user-provided identifier for the file.
int fd: The file descriptor to be preserved.

Description

This function orchestrates the first phase of preserving a file. Upon entry, it takes a reference to the ‘struct file’ via fget(), effectively making LUO a co-owner of the file. This reference is held until the file is either unpreserved or successfully finished in the next kernel, preventing the file from being prematurely destroyed.

This function orchestrates the first phase of preserving a file. It performs the following steps:

Validates that the token is not already in use within the file_set.
Ensures the file_set’s memory for files serialization is allocated (allocates if needed).
Iterates through registered handlers, calling can_preserve() to find one compatible with the given fd.
Calls the handler’s .preserve() operation, which saves the file’s state and returns an opaque private data handle.
Adds the new instance to the file_set’s internal list.

On success, LUO takes a reference to the ‘struct file’ and considers it under its management until it is unpreserved or finished.

In case of any failure, all intermediate allocations (file reference, memory for the ‘luo_file’ struct, etc.) are cleaned up before returning an error.

Context

Can be called from an ioctl handler during normal system operation.

Return

0 on success. Returns a negative errno on failure: -EEXIST if the token is already used. -EBADF if the file descriptor is invalid. -ENOSPC if the file_set is full. -ENOENT if no compatible handler is found. -ENOMEM on memory allocation failure. Other erros might be returned by .preserve().

void luo_file_unpreserve_files(struct luo_file_set *file_set)¶: Unpreserves all files from a file_set.

Parameters

struct luo_file_set *file_set: The files to be cleaned up.

Description

This function serves as the primary cleanup path for a file_set. It is invoked when the userspace agent closes the file_set’s file descriptor.

For each file, it performs the following cleanup actions:

Calls the handler’s .unpreserve() callback to allow the handler to release any resources it allocated.
Removes the file from the file_set’s internal tracking list.
Releases the reference to the ‘struct file’ that was taken by luo_preserve_file() via fput(), returning ownership.
Frees the memory associated with the internal ‘struct luo_file’.

After all individual files are unpreserved, it frees the contiguous memory block that was allocated to hold their serialization data.

int luo_file_freeze(struct luo_file_set *file_set, struct luo_file_set_ser *file_set_ser)¶: Freezes all preserved files and serializes their metadata.

Parameters

struct luo_file_set *file_set: The file_set whose files are to be frozen.
struct luo_file_set_ser *file_set_ser: Where to put the serialized file_set.

Description

This function is called from the reboot() syscall path, just before the kernel transitions to the new image via kexec. Its purpose is to perform the final preparation and serialization of all preserved files in the file_set.

It iterates through each preserved file in FIFO order (the order of preservation) and performs two main actions:

Freezes the File: It calls the handler’s .freeze() callback for each file. This gives the handler a final opportunity to quiesce the device or prepare its state for the upcoming reboot. The handler may update its private data handle during this step.
Serializes Metadata: After a successful freeze, it copies the final file metadata—the handler’s compatible string, the user token, and the final private data handle—into the pre-allocated contiguous memory buffer (file_set->files) that will be handed over to the next kernel via KHO.

Error Handling (Rollback): This function is atomic. If any handler’s .freeze() operation fails, the entire live update is aborted. The __luo_file_unfreeze() helper is immediately called to invoke the .unfreeze() op on all files that were successfully frozen before the point of failure, rolling them back to a running state. The function then returns an error, causing the reboot() syscall to fail.

Context

Called only from the liveupdate_reboot() path.

Return

0 on success, or a negative errno on failure.

void luo_file_unfreeze(struct luo_file_set *file_set, struct luo_file_set_ser *file_set_ser)¶: Unfreezes all files in a file_set and clear serialization

Parameters

struct luo_file_set *file_set: The file_set whose files are to be unfrozen.
struct luo_file_set_ser *file_set_ser: Serialized file_set.

Description

This function rolls back the state of all files in a file_set after the freeze phase has begun but must be aborted. It is the counterpart to luo_file_freeze().

It invokes the __luo_file_unfreeze() helper with a NULL argument, which signals the helper to iterate through all files in the file_set and call their respective .unfreeze() handler callbacks.

Context

This is called when the live update is aborted during the reboot() syscall, after luo_file_freeze() has been called.

int luo_retrieve_file(struct luo_file_set *file_set, u64 token, struct file **filep)¶: Restores a preserved file from a file_set by its token.

Parameters

struct luo_file_set *file_set: The file_set from which to retrieve the file.
u64 token: The unique token identifying the file to be restored.
struct file **filep: Output parameter; on success, this is populated with a pointer to the newly retrieved ‘struct file’.

Description

This function is the primary mechanism for recreating a file in the new kernel after a live update. It searches the file_set’s list of deserialized files for an entry matching the provided token.

The operation is idempotent: if a file has already been successfully retrieved, this function will simply return a pointer to the existing ‘struct file’ and report success without re-executing the retrieve operation. This is handled by checking the ‘retrieved’ flag under a lock.

File retrieval can happen in any order; it is not bound by the order of preservation.

Context

Can be called from an ioctl or other in-kernel code in the new kernel.

Return

0 on success. Returns a negative errno on failure: -ENOENT if no file with the matching token is found. Any error code returned by the handler’s .retrieve() op.

int luo_file_finish(struct luo_file_set *file_set)¶: Completes the lifecycle for all files in a file_set.

Parameters

struct luo_file_set *file_set: The file_set to be finalized.

Description

This function orchestrates the final teardown of a live update file_set in the new kernel. It should be called after all necessary files have been retrieved and the userspace agent is ready to release the preserved state.

The function iterates through all tracked files. For each file, it performs the following sequence of cleanup actions:

If file is not yet retrieved, retrieves it, and calls can_finish() on every file in the file_set. If all can_finish return true, continue to finish.
Calls the handler’s .finish() callback (via luo_file_finish_one) to allow for final resource cleanup within the handler.
Releases LUO’s ownership reference on the ‘struct file’ via fput(). This is the counterpart to the get_file() call in luo_retrieve_file().
Removes the ‘struct luo_file’ from the file_set’s internal list.
Frees the memory for the ‘struct luo_file’ instance itself.

After successfully finishing all individual files, it frees the contiguous memory block that was used to transfer the serialized metadata from the previous kernel.

Error Handling (Atomic Failure): This operation is atomic. If any handler’s .can_finish() op fails, the entire function aborts immediately and returns an error.

Context

Can be called from an ioctl handler in the new kernel.

Return

0 on success, or a negative errno on failure.

int luo_file_deserialize(struct luo_file_set *file_set, struct luo_file_set_ser *file_set_ser)¶: Reconstructs the list of preserved files in the new kernel.

Parameters

struct luo_file_set *file_set: The incoming file_set to fill with deserialized data.
struct luo_file_set_ser *file_set_ser: Serialized KHO file_set data from the previous kernel.

Description

This function is called during the early boot process of the new kernel. It takes the raw, contiguous memory block of ‘struct luo_file_ser’ entries, provided by the previous kernel, and transforms it back into a live, in-memory linked list of ‘struct luo_file’ instances.

For each serialized entry, it performs the following steps:

Reads the ‘compatible’ string.
Searches the global list of registered file handlers for one that matches the compatible string.
Allocates a new ‘struct luo_file’.
Populates the new structure with the deserialized data (token, private data handle) and links it to the found handler. The ‘file’ pointer is initialized to NULL, as the file has not been retrieved yet.
Adds the new ‘struct luo_file’ to the file_set’s files_list.

This prepares the file_set for userspace, which can later call luo_retrieve_file() to restore the actual file descriptors.

Context

Called from session deserialization.

int liveupdate_register_file_handler(struct liveupdate_file_handler *fh)¶: Register a file handler with LUO.

Parameters

struct liveupdate_file_handler *fh: Pointer to a caller-allocated struct liveupdate_file_handler. The caller must initialize this structure, including a unique ‘compatible’ string and a valid ‘fh’ callbacks. This function adds the handler to the global list of supported file handlers.

Context

Typically called during module initialization for file types that support live update preservation.

Return

0 on success. Negative errno on failure.

int liveupdate_unregister_file_handler(struct liveupdate_file_handler *fh)¶: Unregister a liveupdate file handler

Parameters

struct liveupdate_file_handler *fh: The file handler to unregister

Description

Unregisters the file handler from the liveupdate core. This function reverses the operations of liveupdate_register_file_handler().

It ensures safe removal by checking that: No live update session is currently in progress. No FLB registered with this file handler.

If the unregistration fails, the internal test state is reverted.

Return

0 Success. -EOPNOTSUPP when live update is not enabled. -EBUSY A live update is in progress, can’t quiesce live update or FLB is registred with this file handler.

The Linux Kernel

Contents

This Page

Live Update Orchestrator¶

LUO Sessions¶

LUO Preserving File Descriptors¶

LUO File Lifecycle Bound Global Data¶

Live Update Orchestrator ABI¶

Public API¶

Internal API¶

See Also¶