Re: [PATCH v4 14/30] liveupdate: luo_session: Add ioctls for file preservation and state management

[Pratyush Yadav <pratyush@kernel.org>]

Hi Pasha,

On Mon, Sep 29 2025, Pasha Tatashin wrote:

> Introducing the userspace interface and internal logic required to
> manage the lifecycle of file descriptors within a session. Previously, a
> session was merely a container; this change makes it a functional
> management unit.
>
> The following capabilities are added:
>
> A new set of ioctl commands are added, which operate on the file
> descriptor returned by CREATE_SESSION. This allows userspace to:
> - LIVEUPDATE_SESSION_PRESERVE_FD: Add a file descriptor to a session
>   to be preserved across the live update.
> - LIVEUPDATE_SESSION_UNPRESERVE_FD: Remove a previously added file
>   descriptor from the session.
> - LIVEUPDATE_SESSION_RESTORE_FD: Retrieve a preserved file in the
>   new kernel using its unique token.
>
> A state machine for each individual session, distinct from the global
> LUO state. This enables more granular control, allowing userspace to
> prepare or freeze specific sessions independently. This is managed via:
> - LIVEUPDATE_SESSION_SET_EVENT: An ioctl to send PREPARE, FREEZE,
>   CANCEL, or FINISH events to a single session.
> - LIVEUPDATE_SESSION_GET_STATE: An ioctl to query the current state
>   of a single session.
>
> The global subsystem callbacks (luo_session_prepare, luo_session_freeze)
> are updated to iterate through all existing sessions. They now trigger
> the appropriate per-session state transitions for any sessions that
> haven't already been transitioned individually by userspace.
>
> The session's .release handler is enhanced to be state-aware. When a
> session's file descriptor is closed, it now correctly cancels or
> finishes the session based on its current state before freeing all
> associated file resources, preventing resource leaks.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
[...]
> +/**
> + * struct liveupdate_session_get_state - ioctl(LIVEUPDATE_SESSION_GET_STATE)
> + * @size:     Input; sizeof(struct liveupdate_session_get_state)
> + * @incoming: Input; If 1, query the state of a restored file from the incoming
> + *            (previous kernel's) set. If 0, query a file being prepared for
> + *            preservation in the current set.

Spotted this when working on updating my test suite for LUO. This seems
to be a leftover from a previous version. I don't see it being used
anywhere in the code.

Also, I think the model we should have is to only allow new sessions in
normal state. Currently luo_session_create() allows creating a new
session in updated state. This would end up mixing sessions from a
previous boot and sessions from current boot. I don't really see a
reason for that and I think the userspace should first call finish
before starting new serialization. Keeps things simpler.

> + * @reserved: Must be zero.
> + * @state:    Output; The live update state of this FD.
> + *
> + * Query the current live update state of a specific preserved file descriptor.
> + *
> + * - %LIVEUPDATE_STATE_NORMAL:   Default state
> + * - %LIVEUPDATE_STATE_PREPARED: Prepare callback has been performed on this FD.
> + * - %LIVEUPDATE_STATE_FROZEN:   Freeze callback ahs been performed on this FD.
> + * - %LIVEUPDATE_STATE_UPDATED:  The system has successfully rebooted into the
> + *                               new kernel.
> + *
> + * See the definition of &enum liveupdate_state for more details on each state.
> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +struct liveupdate_session_get_state {
> +	__u32		size;
> +	__u8		incoming;
> +	__u8		reserved[3];
> +	__u32		state;
> +};
> +
> +#define LIVEUPDATE_SESSION_GET_STATE					\
> +	_IO(LIVEUPDATE_IOCTL_TYPE, LIVEUPDATE_CMD_SESSION_GET_STATE)
[...]

-- 
Regards,
Pratyush Yadav