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([2620:10d:c090:500::4:432]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-267e9372e86sm19435835ad.136.2025.09.16.16.26.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 16 Sep 2025 16:26:31 -0700 (PDT) Message-ID: Subject: Re: [PATCH bpf-next] bpf: support nested rcu critical sections From: Eduard Zingerman To: Puranjay Mohan , bpf@vger.kernel.org Cc: kkd@meta.com, Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Puranjay Mohan , kernel-team@fb.com Date: Tue, 16 Sep 2025 16:26:30 -0700 In-Reply-To: <20250916113622.19540-1-puranjay@kernel.org> References: <20250916113622.19540-1-puranjay@kernel.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2 (3.56.2-1.fc42) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2025-09-16 at 11:36 +0000, Puranjay Mohan wrote: > Currently, nested rcu critical sections are rejected by the verifier and > rcu_lock state is managed by a boolean variable. Add support for nested > rcu critical sections by make active_rcu_locks a counter similar to > active_preempt_locks. bpf_rcu_read_lock() increments this counter and > bpf_rcu_read_unlock() decrements it, MEM_RCU -> PTR_UNTRUSTED transition > happens when active_rcu_locks drops to 0. >=20 > Signed-off-by: Puranjay Mohan > --- [...] > @@ -13874,22 +13874,22 @@ static int check_kfunc_call(struct bpf_verifier= _env *env, struct bpf_insn *insn, > } > =20 > if (rcu_lock) { > - verbose(env, "nested rcu read lock (kernel function %s)\n", func_name= ); > - return -EINVAL; > + env->cur_state->active_rcu_locks++; > } else if (rcu_unlock) { > - bpf_for_each_reg_in_vstate_mask(env->cur_state, state, reg, clear_mas= k, ({ > - if (reg->type & MEM_RCU) { > - reg->type &=3D ~(MEM_RCU | PTR_MAYBE_NULL); > - reg->type |=3D PTR_UNTRUSTED; > - } > - })); > - env->cur_state->active_rcu_lock =3D false; > + if (--env->cur_state->active_rcu_locks =3D=3D 0) { > + bpf_for_each_reg_in_vstate_mask(env->cur_state, state, reg, clear_ma= sk, ({ > + if (reg->type & MEM_RCU) { > + reg->type &=3D ~(MEM_RCU | PTR_MAYBE_NULL); > + reg->type |=3D PTR_UNTRUSTED; > + } > + })); > + } > } else if (sleepable) { > verbose(env, "kernel func %s is sleepable within rcu_read_lock region= \n", func_name); > return -EACCES; > } > } else if (rcu_lock) { > - env->cur_state->active_rcu_lock =3D true; > + env->cur_state->active_rcu_locks++; > } else if (rcu_unlock) { > verbose(env, "unmatched rcu read unlock (kernel function %s)\n", func_= name); > return -EINVAL; Nit: active_rcu_locks increment in two places can be avoided e.g. as follow= s: if (rcu_lock) { env->cur_state->active_rcu_locks++; } else if (rcu_unlock) { struct bpf_func_state *state; struct bpf_reg_state *reg; u32 clear_mask =3D (1 << STACK_SPILL) | (1 << STACK_ITER); if (env->cur_state->active_rcu_locks =3D=3D 0) { verbose(private_data: env, fmt: "unmatched rcu read= unlock (kernel function %s)\n", func_name); return -EINVAL; } if (--env->cur_state->active_rcu_locks =3D=3D 0) { bpf_for_each_reg_in_vstate_mask(env->cur_state, sta= te, reg, clear_mask, ({ if (reg->type & MEM_RCU) { reg->type &=3D ~(MEM_RCU | PTR_MAYB= E_NULL); reg->type |=3D PTR_UNTRUSTED; } })); } } else if (sleepable) { verbose(private_data: env, fmt: "kernel func %s is sleepabl= e within rcu_read_lock region\n", func_name); return -EACCES; } if (in_rbtree_lock_required_cb(env) && (rcu_lock || rcu_unlock)) { verbose(private_data: env, fmt: "Calling bpf_rcu_read_{lock= ,unlock} in unnecessary rbtree callback\n"); return -EACCES; } [...]