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Mon, 27 Jul 2026 09:48:32 -0700 (PDT) Message-ID: Subject: Re: [PATCH bpf-next v2 03/18] bpf: Split kfunc map argument into __const_map and __map From: Eduard Zingerman To: Amery Hung , bpf@vger.kernel.org Cc: alexei.starovoitov@gmail.com, andrii@kernel.org, daniel@iogearbox.net, memxor@gmail.com, kernel-team@meta.com Date: Mon, 27 Jul 2026 09:48:28 -0700 In-Reply-To: <015164cfd5ac0c3bae007313c3859fef6a583233.camel@gmail.com> References: <20260724190813.1458271-1-ameryhung@gmail.com> <20260724190813.1458271-4-ameryhung@gmail.com> <015164cfd5ac0c3bae007313c3859fef6a583233.camel@gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2-10 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2026-07-24 at 12:07 -0700, Amery Hung wrote: > Kfuncs used a single '__map' suffix (KF_ARG_PTR_TO_MAP) for two different > things: a verifier-known map matched by map_uid against a bound > timer/wq/task_work object (bpf_wq_init, bpf_task_work_schedule*), and an > opaque 'struct bpf_map *' used only at runtime (bpf_arena_*), which may b= e a > map fd or a PTR_TO_BTF_ID struct bpf_map (e.g. a bpf_map iterator's ctx->= map). >=20 > That combined path only accepted the btf map form due to type confusion. = The > 'if (!reg->map_ptr)' check reads reg->map_ptr, which aliases reg->btf in = the > bpf_reg_state union. A PTR_TO_BTF_ID register always has a non-NULL reg->= btf, > so the guard silently passed and validation fell through to > process_kf_arg_ptr_to_btf_id(). It also recorded PTR_TO_BTF_ID info in > meta->map, which would be meaningless. What a bug, I wonder if we have more places like that. In LLVM code base they use the following pattern: class foo { int tag; union { struct bar_info { ... } bar; struct buz_info { ... } buz; }; }; struct bar_info *get_bar(class foo *f) { if (f->tag !=3D BAR) fatal_error(); // the error is compiled away in release builds return &f->bar; } And similar pattern with checks in getter for just mutually exclusive fields. I wonder if we should adopt the same approach. > Split the annotation to avoid such type confusion and to align with > helper: >=20 > - '__const_map' -> KF_ARG_CONST_MAP_PTR: verifier-known map, handled by > =C2=A0 process_map_ptr_arg() like helper ARG_CONST_MAP_PTR. >=20 > - '__map' -> KF_ARG_PTR_TO_BTF_ID: opaque struct bpf_map, validated by > =C2=A0 process_kf_arg_ptr_to_btf_id(). A map fd still matches via > =C2=A0 reg2btf_ids[CONST_PTR_TO_MAP], so bpf_arena_alloc_pages(&map) keep= s > =C2=A0 working. >=20 > Signed-off-by: Amery Hung > --- Reviewed-by: Eduard Zingerman