bpf: Introduce deferred task context execution

From: Mykyta Yatsenko <yatsenko@meta.com>

This patch introduces a new mechanism for BPF programs to schedule
deferred execution in the context of a specific task using the kernel’s
task_work infrastructure.

The new bpf_task_work interface enables BPF use cases that
require sleepable subprogram execution within task context, for example,
scheduling sleepable function from the context that does not
allow sleepable, such as NMI.

Introduced kfuncs bpf_task_work_schedule_signal() and
bpf_task_work_schedule_resume() for scheduling BPF callbacks correspond
to different modes used by task_work (TWA_SIGNAL or
TWA_RESUME/TWA_NMI_CURRENT).

The implementation leverages BPF maps for storing callback metadata.
Indirectly call task_work_add() via irq_work to avoid locking in
potentially NMI context. State transitions are managed via an atomic
state machine (bpf_task_work_state) to ensure correctness under
concurrent usage and deletion.

Mykyta Yatsenko (4):
  bpf: bpf task work plumbing
  bpf: extract map key pointer calculation
  bpf: task work scheduling kfuncs
  selftests/bpf: BPF task work scheduling tests

 include/linux/bpf.h                           |  11 +
 include/uapi/linux/bpf.h                      |   4 +
 kernel/bpf/arraymap.c                         |   8 +-
 kernel/bpf/btf.c                              |  15 +
 kernel/bpf/hashtab.c                          |  22 +-
 kernel/bpf/helpers.c                          | 260 ++++++++++++++++--
 kernel/bpf/syscall.c                          |  23 +-
 kernel/bpf/verifier.c                         | 131 ++++++++-
 tools/include/uapi/linux/bpf.h                |   4 +
 .../selftests/bpf/prog_tests/test_task_work.c | 149 ++++++++++
 tools/testing/selftests/bpf/progs/task_work.c | 108 ++++++++
 .../selftests/bpf/progs/task_work_fail.c      |  98 +++++++
 12 files changed, 800 insertions(+), 33 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/test_task_work.c
 create mode 100644 tools/testing/selftests/bpf/progs/task_work.c
 create mode 100644 tools/testing/selftests/bpf/progs/task_work_fail.c

-- 
2.50.1