Refactor the verifier by pulling the common logic from
process_timer_func() into a dedicated helper. This allows reusing
process_async_func() helper for verifying bpf_task_work struct in the
next patch.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>

Calculation of the BPF map key, given the pointer to a value is
duplicated in a couple of places in helpers already, in the next patch
another use case is introduced as well.
This patch extracts that functionality into a separate function.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Kumar Kartikeya Dwivedi <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>

Implementation of the new bpf_task_work_schedule kfuncs, that let a BPF
program schedule task_work callbacks for a target task:
 * bpf_task_work_schedule_signal() → schedules with TWA_SIGNAL
 * bpf_task_work_schedule_resume() → schedules with TWA_RESUME

Each map value should embed a struct bpf_task_work, which the kernel
side pairs with struct bpf_task_work_kern, containing a pointer to
struct bpf_task_work_ctx, that maintains metadata relevant for the
concrete callback scheduling.

A small state machine and refcounting scheme ensures safe reuse and
teardown:
 STANDBY -> PENDING -> SCHEDULING -> SCHEDULED -> RUNNING -> STANDBY

A FREED terminal state coordinates with map-value
deletion (bpf_task_work_cancel_and_free()).

Scheduling itself is deferred via irq_work to keep the kfunc callable
from NMI context.

Lifetime is guarded with refcount_t + RCU Tasks Trace.

Main components:
 * struct bpf_task_work_context – Metadata and state management per task
work.
 * enum bpf_task_work_state – A state machine to serialize work
 scheduling and execution.
 * bpf_task_work_schedule() – The central helper that initiates
scheduling.
 * bpf_task_work_acquire_ctx() - Attempts to take ownership of the context,
 pointed by passed struct bpf_task_work, allocates new context if none
 exists yet.
 * bpf_task_work_callback() – Invoked when the actual task_work runs.
 * bpf_task_work_irq() – An intermediate step (runs in softirq context)
to enqueue task work.
 * bpf_task_work_cancel_and_free() – Cleanup for deleted BPF map entries.

Flow of successful task work scheduling
 1) bpf_task_work_schedule_* is called from BPF code.
 2) Transition state from STANDBY to PENDING, marks context is owned by
 this task work scheduler
 3) irq_work_queue() schedules bpf_task_work_irq().
 4) Transition state from PENDING to SCHEDULING.
 4) bpf_task_work_irq() attempts task_work_add(). If successful, state
 transitions to SCHEDULED.
 5) Task work calls bpf_task_work_callback(), which transition state to
 RUNNING.
 6) BPF callback is executed
 7) Context is cleaned up, refcounts released, context state set back to
 STANDBY.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Reviewed-by: Andrii Nakryiko <[email protected]>

Introducing selftests that check BPF task work scheduling mechanism.
Validate that verifier does not accepts incorrect calls to
bpf_task_work_schedule kfunc.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>