linux-kernel - Re: [PATCH bpf] bpf: verifier: support BPF_LOAD_ACQ in insn_def

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Message-ID: <80ef5e2e-c2d9-45b7-9a48-f8c1a4767eae@gmail.com>
Date: Wed, 21 May 2025 12:13:42 -0700
From: Eduard Zingerman <eddyz87@...il.com>
To: Puranjay Mohan <puranjay@...nel.org>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 John Fastabend <john.fastabend@...il.com>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: verifier: support BPF_LOAD_ACQ in
 insn_def_regno()


On 2025-05-21 11:39, Puranjay Mohan wrote:
[...]
> @@ -3643,6 +3643,9 @@ static bool is_reg64(struct bpf_verifier_env *env, struct bpf_insn *insn,
>   /* Return the regno defined by the insn, or -1. */
>   static int insn_def_regno(const struct bpf_insn *insn)
>   {
> +	if (is_atomic_load_insn(insn))
> +		return insn->dst_reg;
> +
>   	switch (BPF_CLASS(insn->code)) {
>   	case BPF_JMP:
>   	case BPF_JMP32:

I'm confused, is_atomic_load_insn() is defined as:

          return BPF_CLASS(insn->code) == BPF_STX &&
                 BPF_MODE(insn->code) == BPF_ATOMIC &&
                 insn->imm == BPF_LOAD_ACQ;

And insn_def_regno() has the following case:

          case BPF_STX:
                  if (BPF_MODE(insn->code) == BPF_ATOMIC ||
                      BPF_MODE(insn->code) == BPF_PROBE_ATOMIC) {
                          if (insn->imm == BPF_CMPXCHG)
                                  return BPF_REG_0;
                          else if (insn->imm == BPF_LOAD_ACQ)
                                  return insn->dst_reg;
                          else if (insn->imm & BPF_FETCH)
                                  return insn->src_reg;
                  }
                  return -1;

Why is it not triggering?

Also, can this be tested with a BPF_F_TEST_RND_HI32 flag?
E.g. see verifier_scalar_ids.c:linked_regs_and_subreg_def() test case.