[PATCH bpf-next v2 0/2] Introduce bpf_preempt_{disable,enable}

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

This set introduces two kfuncs, bpf_preempt_disable and
bpf_preempt_enable, which are wrappers around preempt_disable and
preempt_enable in the kernel. These functions allow a BPF program to
have code sections where preemption is disabled. There are multiple use
cases that are served by such a feature, a few are listed below:

1. Writing safe per-CPU alogrithms/data structures that work correctly
   across different contexts.
2. Writing safe per-CPU allocators similar to bpf_memalloc on top of
   array/arena memory blobs.
3. Writing locking algorithms in BPF programs natively.

Note that local_irq_disable/enable equivalent is also needed for proper
IRQ context protection, but that is a more involved change and will be
sent later.

While bpf_preempt_{disable,enable} is not sufficient for all of these
usage scenarios on its own, it is still necessary.

The same effect as these kfuncs can in some sense be already achieved
using the bpf_spin_lock or rcu_read_lock APIs, therefore from the
standpoint of kernel functionality exposure in the verifier, this is
well understood territory.

Note that these helpers do allow calling kernel helpers and kfuncs from
within the non-preemptible region (unless sleepable). Otherwise, any
locks built using the preemption helpers will be as limited as
existing bpf_spin_lock.

Nesting is allowed by keeping a counter for tracking remaining enables
required to be performed. Similar approach can be applied to
rcu_read_locks in a follow up.

Changelog
=========
v1: https://lore.kernel.org/bpf/20240423061922.2295517-1-memxor@gmail.com

 * Move kfunc BTF ID declerations above css task kfunc for
   !CONFIG_CGROUPS config (Alexei)
 * Add test case for global function call in non-preemptible region
   (Jiri)

Kumar Kartikeya Dwivedi (2):
  bpf: Introduce bpf_preempt_[disable,enable] kfuncs
  selftests/bpf: Add tests for preempt kfuncs

 include/linux/bpf_verifier.h                  |   1 +
 kernel/bpf/helpers.c                          |  12 ++
 kernel/bpf/verifier.c                         |  71 ++++++++-
 .../selftests/bpf/prog_tests/preempt_lock.c   |   9 ++
 .../selftests/bpf/progs/preempt_lock.c        | 135 ++++++++++++++++++
 5 files changed, 226 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/preempt_lock.c
 create mode 100644 tools/testing/selftests/bpf/progs/preempt_lock.c


base-commit: 6e10b6350a67d398c795ac0b93a7bb7103633fe4
-- 
2.43.0