[PATCH v4 bpf-next 0/4] bpf: Introduce may_goto and cond_break

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: bpf@vger.kernel.org
Cc: daniel@iogearbox.net, andrii@kernel.org, martin.lau@kernel.org,
	memxor@gmail.com, eddyz87@gmail.com, john.fastabend@gmail.com,
	kernel-team@fb.com
Subject: [PATCH v4 bpf-next 0/4] bpf: Introduce may_goto and cond_break
Date: Fri,  1 Mar 2024 18:00:06 -0800	[thread overview]
Message-ID: <20240302020010.95393-1-alexei.starovoitov@gmail.com> (raw)

From: Alexei Starovoitov <ast@kernel.org>

v3 -> v4:
- fix drained issue reported by John.
  may_goto insn could be implemented with sticky state (once
  reaches 0 it stays 0), but the verifier shouldn't assume that.
  It has to explore both branches.
  Arguably drained iterator state shouldn't be there at all.
  bpf_iter_css_next() is not sticky. Can be fixed, but auditing all
  iterators for stickiness. That's an orthogonal discussion.
- explained JMA name reasons in patch 1
- fixed test_progs-no_alu32 issue and added another test

v2 -> v3: Major change
- drop bpf_can_loop() kfunc and introduce may_goto instruction instead
  kfunc is a function call while may_goto doesn't consume any registers
  and LLVM can produce much better code due to less register pressure.
- instead of counting from zero to BPF_MAX_LOOPS start from it instead
  and break out of the loop when count reaches zero
- use may_goto instruction in cond_break macro
- recognize that 'exact' state comparison doesn't need to be truly exact.
  regsafe() should ignore precision and liveness marks, but range_within
  logic is safe to use while evaluating open coded iterators.

Alexei Starovoitov (4):
  bpf: Introduce may_goto instruction
  bpf: Recognize that two registers are safe when their ranges match
  bpf: Add cond_break macro
  selftests/bpf: Test may_goto

 include/linux/bpf_verifier.h                  |   2 +
 include/uapi/linux/bpf.h                      |   1 +
 kernel/bpf/core.c                             |   1 +
 kernel/bpf/disasm.c                           |   3 +
 kernel/bpf/verifier.c                         | 280 +++++++++++++-----
 tools/include/uapi/linux/bpf.h                |   1 +
 tools/testing/selftests/bpf/DENYLIST.s390x    |   1 +
 .../testing/selftests/bpf/bpf_experimental.h  |  12 +
 .../bpf/progs/verifier_iterating_callbacks.c  | 103 ++++++-
 9 files changed, 330 insertions(+), 74 deletions(-)

-- 
2.34.1