[PATCH bpf-next v2 0/3] bpf: Allow selected kfuncs under bpf_spin_lock

[Kaitao Cheng]

The verifier currently has a hard-coded allowlist of kfuncs that may be
called while a BPF program holds a bpf_spin_lock. This works for the
small set of built-in kfuncs known to the verifier, but it does not give
kfunc providers a registration-time way to declare that a kfunc is safe
in such a region. In particular, module kfuncs cannot be added to that
allowlist without changing verifier code.

This series adds a new KF_SPINLOCK_SAFE kfunc flag and teaches the
verifier to use kfunc registration metadata when deciding whether a
kfunc call is allowed while a bpf_spin_lock is held.

The built-in kfuncs that are currently accepted by the verifier's
lock-held allowlist are annotated with the new flag. This preserves the
existing behavior while removing the verifier-side category checks and
uses the same mechanism for built-in and module kfuncs.

The selftest coverage marks one bpf_testmod kfunc as KF_SPINLOCK_SAFE
and verifies that it can be called under a bpf_spin_lock. It also calls
another registered but unmarked bpf_testmod kfunc under the lock and
checks that the verifier rejects it.

Changes in v2:
- Rename KF_SPIN_LOCK to KF_SPINLOCK_SAFE. (Kumar Kartikeya Dwivedi,
  Leon Hwang)
- Deprecate the verifier's lock-held allowlist mechanism and annotate the
  relevant kfuncs uniformly with KF_SPINLOCK_SAFE (Kumar Kartikeya Dwivedi)
- Add selftests. (Leon Hwang)

Link to v1:
https://lore.kernel.org/bpf/DKG0YUDSTBUY.1X220287HT9V3@xxxxxxxxx/

Kaitao Cheng (3):
  bpf: Add KF_SPINLOCK_SAFE flag for kfuncs under bpf_spin_lock
  bpf: Mark existing lock-safe kfuncs with KF_SPINLOCK_SAFE
  selftests/bpf: Test module kfunc calls under spin lock

 include/linux/btf.h                           |  1 +
 kernel/bpf/arena.c                            |  6 +-
 kernel/bpf/helpers.c                          | 56 ++++++++++---------
 kernel/bpf/rqspinlock.c                       |  8 +--
 kernel/bpf/verifier.c                         | 38 +++----------
 .../selftests/bpf/prog_tests/kfunc_call.c     |  2 +
 .../selftests/bpf/progs/kfunc_call_fail.c     | 12 ++++
 .../selftests/bpf/progs/kfunc_call_test.c     | 12 ++++
 .../selftests/bpf/test_kmods/bpf_testmod.c    |  2 +-
 9 files changed, 72 insertions(+), 65 deletions(-)

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