[bpf-next,00/18] Introduce bpf_wq

Message

Benjamin Tissoires April 16, 2024, 2:08 p.m. UTC

This is a followup of sleepable bpf_timer[0].

When discussing sleepable bpf_timer, it was thought that we should give
a try to bpf_wq, as the 2 APIs are similar but distinct enough to
justify a new one.

So here it is.

I tried to keep as much as possible common code in kernel/bpf/helpers.c
but I couldn't get away with code duplication in kernel/bpf/verifier.c.

This series introduces a basic bpf_wq support:
- creation is supported
- assignment is supported
- running a simple bpf_wq is also supported.

We will probably need to extend the API further with:
- a full delayed_work API (can be piggy backed on top with a correct
  flag)
- bpf_wq_cancel()
- bpf_wq_cancel_sync() (for sleepable programs)
- documentation

But for now, let's focus on what we currently have to see if it's worth
it compared to sleepable bpf_timer.

FWIW, I still have a couple of concerns with this implementation:
- I'm explicitely declaring the async callback as sleepable or not
  (BPF_F_WQ_SLEEPABLE) through a flag. Is it really worth it?
  Or should I just consider that any wq is running in a sleepable
  context?
- bpf_wq_work() access ->prog without protection, but I think this might
  be racing with bpf_wq_set_callback(): if we have the following:

  CPU 0                                     CPU 1
  bpf_wq_set_callback()
  bpf_start()
                                            bpf_wq_work():
                                              prog = cb->prog;

  bpf_wq_set_callback()
    cb->prog = prog;
    bpf_prog_put(prev)
    rcu_assign_ptr(cb->callback_fn,
                   callback_fn);
                                           callback = READ_ONCE(w->cb.callback_fn);

  As I understand callback_fn is fine, prog might be, but we clearly
  have an inconstency between "prog" and "callback_fn" as they can come
  from 2 different bpf_wq_set_callback() calls.

IMO we should protect this by the async->lock, but I'm not sure if
  it's OK or not.

---

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 <[e-mail]>
To: Daniel Borkmann <[e-mail]>
To: Andrii Nakryiko <[e-mail]>
To: Martin KaFai Lau <[e-mail]>
To: Eduard Zingerman <[e-mail]>
To: Song Liu <[e-mail]>
To: Yonghong Song <[e-mail]>
To: John Fastabend <[e-mail]>
To: KP Singh <[e-mail]>
To: Stanislav Fomichev <[e-mail]>
To: Hao Luo <[e-mail]>
To: Jiri Olsa <[e-mail]>
To: Mykola Lysenko <[e-mail]>
To: Shuah Khan <[e-mail]>
Cc:  <[e-mail]>
Cc:  <[e-mail]>
Cc:  <[e-mail]>
Signed-off-by: Benjamin Tissoires <[e-mail]>

[0] https://lore.kernel.org/all/[e-mail]/

---
Benjamin Tissoires (18):
      bpf: trampoline: export __bpf_prog_enter/exit_recur
      bpf: make timer data struct more generic
      bpf: replace bpf_timer_init with a generic helper
      bpf: replace bpf_timer_set_callback with a generic helper
      bpf: replace bpf_timer_cancel_and_free with a generic helper
      bpf: add support for bpf_wq user type
      tools: sync include/uapi/linux/bpf.h
      bpf: add support for KF_ARG_PTR_TO_WORKQUEUE
      bpf: allow struct bpf_wq to be embedded in arraymaps and hashmaps
      selftests/bpf: add bpf_wq tests
      bpf: wq: add bpf_wq_init
      tools: sync include/uapi/linux/bpf.h
      selftests/bpf: wq: add bpf_wq_init() checks
      bpf/verifier: add is_sleepable argument to push_callback_call
      bpf: wq: add bpf_wq_set_callback_impl
      selftests/bpf: add checks for bpf_wq_set_callback()
      bpf: add bpf_wq_start
      selftests/bpf: wq: add bpf_wq_start() checks

 include/linux/bpf.h                                |  17 +-
 include/linux/bpf_verifier.h                       |   1 +
 include/uapi/linux/bpf.h                           |  13 +
 kernel/bpf/arraymap.c                              |  18 +-
 kernel/bpf/btf.c                                   |  17 +
 kernel/bpf/hashtab.c                               |  55 ++-
 kernel/bpf/helpers.c                               | 371 ++++++++++++++++-----
 kernel/bpf/syscall.c                               |  16 +-
 kernel/bpf/trampoline.c                            |   6 +-
 kernel/bpf/verifier.c                              | 195 ++++++++++-
 tools/include/uapi/linux/bpf.h                     |  13 +
 tools/testing/selftests/bpf/bpf_experimental.h     |   7 +
 .../selftests/bpf/bpf_testmod/bpf_testmod.c        |   5 +
 .../selftests/bpf/bpf_testmod/bpf_testmod_kfunc.h  |   1 +
 tools/testing/selftests/bpf/prog_tests/wq.c        |  41 +++
 tools/testing/selftests/bpf/progs/wq.c             | 192 +++++++++++
 tools/testing/selftests/bpf/progs/wq_failures.c    | 197 +++++++++++
 17 files changed, 1052 insertions(+), 113 deletions(-)
---
base-commit: ffa6b26b4d8a0520b78636ca9373ab842cb3b1a8
change-id: 20240411-bpf_wq-fe24e8d24f5e

Best regards,