Re: [PATCH bpf-next v2 10/25] bpf: Introduce local kptrs

Dave Marchevsky
2022-10-25T16:33:13Z

On 10/13/22 2:22 AM, Kumar Kartikeya Dwivedi wrote:
> Introduce the idea of local kptrs, i.e. PTR_TO_BTF_ID that point to a
> type in program BTF. This is indicated by the presence of MEM_TYPE_LOCAL
> type tag in reg->type to avoid having to check btf_is_kernel when trying
> to match argument types in helpers.
>
> For now, these local kptrs will always be referenced in verifier
> context, hence ref_obj_id == 0 for them is a bug. It is allowed to write
> to such objects, as long fields that are special are not touched
> (support for which will be added in subsequent patches).
>
> No PROBE_MEM handling is hence done since they can never be in an
> undefined state, and their lifetime will always be valid.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

One nit unrelated to the other thread we have going for this patch.

[...]

> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 066984d73a8b..65f444405d9c 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6019,11 +6019,13 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
> return -EINVAL;
> }
>
> -int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,
> +int btf_struct_access(struct bpf_verifier_log *log,
> + const struct bpf_reg_state *reg, const struct btf *btf,
> const struct btf_type *t, int off, int size,
> enum bpf_access_type atype __maybe_unused,
> u32 *next_btf_id, enum bpf_type_flag *flag)
> {
> + bool local_type = reg && (type_flag(reg->type) & MEM_TYPE_LOCAL);

Can you add a type_is_local_kptr helper (or similar name) to reduce this
type_flag(reg->type) & MEM_TYPE_LOCAL repetition here and elsewhere in the patch?
Some examples of repetition in verifier.c below.

> enum bpf_type_flag tmp_flag = 0;
> int err;
> u32 id;
> @@ -6033,6 +6035,11 @@ int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,
>
> switch (err) {
> case WALK_PTR:
> + /* For local types, the destination register cannot
> + * become a pointer again.
> + */
> + if (local_type)
> + return SCALAR_VALUE;
> /* If we found the pointer or scalar on t+off,
> * we're done.
> */
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 3c47cecda302..6ee8c06c2080 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4522,16 +4522,20 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
> return -EACCES;
> }
>
> - if (env->ops->btf_struct_access) {
> - ret = env->ops->btf_struct_access(&env->log, reg->btf, t,
> + if (env->ops->btf_struct_access && !(type_flag(reg->type) & MEM_TYPE_LOCAL)) {
> + WARN_ON_ONCE(!btf_is_kernel(reg->btf));
> + ret = env->ops->btf_struct_access(&env->log, reg, reg->btf, t,
> off, size, atype, &btf_id, &flag);
> } else {
> - if (atype != BPF_READ) {
> + if (atype != BPF_READ && !(type_flag(reg->type) & MEM_TYPE_LOCAL)) {
> verbose(env, "only read is supported\n");
> return -EACCES;
> }
>
> - ret = btf_struct_access(&env->log, reg->btf, t, off, size,
> + if (reg->type & MEM_TYPE_LOCAL)
> + WARN_ON_ONCE(!reg->ref_obj_id);
> +
> + ret = btf_struct_access(&env->log, reg, reg->btf, t, off, size,
> atype, &btf_id, &flag);
> }
>
> @@ -4596,7 +4600,7 @@ static int check_ptr_to_map_access(struct bpf_verifier_env *env,
> return -EACCES;
> }
>
> - ret = btf_struct_access(&env->log, btf_vmlinux, t, off, size, atype, &btf_id, &flag);
> + ret = btf_struct_access(&env->log, NULL, btf_vmlinux, t, off, size, atype, &btf_id, &flag);
> if (ret < 0)
> return ret;
>
> @@ -5816,6 +5820,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
> * fixed offset.
> */
> case PTR_TO_BTF_ID:
> + case PTR_TO_BTF_ID | MEM_TYPE_LOCAL:
> /* When referenced PTR_TO_BTF_ID is passed to release function,
> * it's fixed offset must be 0. In the other cases, fixed offset
> * can be non-zero. …