sleepable bpf_timer (was: allow HID-BPF to do device IOs)

New version of the sleepable bpf_timer code.

I'm posting this as this is the result of the previous review, so we can
have a baseline to compare to.

The plan is now to introduce a new user API struct bpf_wq, as the timer
API working on softIRQ seems to be quite far away from a wq.

For reference, the use cases I have in mind:

---

Basically, I need to be able to defer a HID-BPF program for the
following reasons (from the aforementioned patch):
1. defer an event:
   Sometimes we receive an out of proximity event, but the device can not
   be trusted enough, and we need to ensure that we won't receive another
   one in the following n milliseconds. So we need to wait those n
   milliseconds, and eventually re-inject that event in the stack.

2. inject new events in reaction to one given event:
   We might want to transform one given event into several. This is the
   case for macro keys where a single key press is supposed to send
   a sequence of key presses. But this could also be used to patch a
   faulty behavior, if a device forgets to send a release event.

3. communicate with the device in reaction to one event:
   We might want to communicate back to the device after a given event.
   For example a device might send us an event saying that it came back
   from sleeping state and needs to be re-initialized.

Currently we can achieve that by keeping a userspace program around,
raise a bpf event, and let that userspace program inject the events and
commands.
However, we are just keeping that program alive as a daemon for just
scheduling commands. There is no logic in it, so it doesn't really justify
an actual userspace wakeup. So a kernel workqueue seems simpler to handle.

bpf_timers are currently running in a soft IRQ context, this patch
series implements a sleppable context for them.

Cheers,
Benjamin

To: Alexei Starovoitov <ast@kernel.org>
To: Daniel Borkmann <daniel@iogearbox.net>
To: Andrii Nakryiko <andrii@kernel.org>
To: Martin KaFai Lau <martin.lau@linux.dev>
To: Eduard Zingerman <eddyz87@gmail.com>
To: Song Liu <song@kernel.org>
To: Yonghong Song <yonghong.song@linux.dev>
To: John Fastabend <john.fastabend@gmail.com>
To: KP Singh <kpsingh@kernel.org>
To: Stanislav Fomichev <sdf@google.com>
To: Hao Luo <haoluo@google.com>
To: Jiri Olsa <jolsa@kernel.org>
To: Mykola Lysenko <mykolal@fb.com>
To: Shuah Khan <shuah@kernel.org>
Cc: Benjamin Tissoires <bentiss@kernel.org>
Cc: <bpf@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>
Cc: <linux-kselftest@vger.kernel.org>

---
Changes in v6:
- Use of a workqueue to clean up sleepable timers
- integrated Kumar's patch instead of mine
- Link to v5: https://lore.kernel.org/r/20240322-hid-bpf-sleepable-v5-0...

Changes in v5:
- took various reviews into account
- rewrote the tests to be separated to not have a uggly include
- Link to v4: https://lore.kernel.org/r/20240315-hid-bpf-sleepable-v4-0...

Changes in v4:
- dropped the HID changes, they can go independently from bpf-core
- addressed Alexei's and Eduard's remarks
- added selftests
- Link to v3: https://lore.kernel.org/r/20240221-hid-bpf-sleepable-v3-0...

Changes in v3:
- fixed the crash from v2
- changed the API to have only BPF_F_TIMER_SLEEPABLE for
  bpf_timer_start()
- split the new kfuncs/verifier patch into several sub-patches, for
  easier reviews
- Link to v2: https://lore.kernel.org/r/20240214-hid-bpf-sleepable-v2-0...

Changes in v2:
- make use of bpf_timer (and dropped the custom HID handling)
- implemented bpf_timer_set_sleepable_cb as a kfunc
- still not implemented global subprogs
- no sleepable bpf_timer selftests yet
- Link to v1: https://lore.kernel.org/r/20240209-hid-bpf-sleepable-v1-0...

---
Benjamin Tissoires (5):
      bpf/helpers: introduce sleepable bpf_timers
      bpf/helpers: introduce bpf_timer_set_sleepable_cb() kfunc
      bpf/helpers: mark the callback of bpf_timer_set_sleepable_cb() as sleepable
      tools: sync include/uapi/linux/bpf.h
      selftests/bpf: add sleepable timer tests

Kumar Kartikeya Dwivedi (1):
      bpf: Add support for KF_ARG_PTR_TO_TIMER

 include/linux/bpf_verifier.h                       |   1 +
 include/uapi/linux/bpf.h                           |  13 ++
 kernel/bpf/helpers.c                               | 202 ++++++++++++++++---
 kernel/bpf/verifier.c                              |  98 +++++++++-
 tools/include/uapi/linux/bpf.h                     |  20 +-
 tools/testing/selftests/bpf/bpf_experimental.h     |   5 +
 .../selftests/bpf/bpf_testmod/bpf_testmod.c        |   5 +
 .../selftests/bpf/bpf_testmod/bpf_testmod_kfunc.h  |   1 +
 tools/testing/selftests/bpf/prog_tests/timer.c     |  34 ++++
 .../testing/selftests/bpf/progs/timer_sleepable.c  | 213 +++++++++++++++++++++
 10 files changed, 553 insertions(+), 39 deletions(-)
---
base-commit: 61df575632d6b39213f47810c441bddbd87c3606
change-id: 20240205-hid-bpf-sleepable-c01260fd91c4

Best regards,
-- 
Benjamin Tissoires <bentiss@kernel.org>