Re: [PATCH bpf-next v1 04/19] bpf: allow loading of a bpf_iter program

Yonghong Song
2020-04-29T01:27:52Z

On 4/28/20 5:54 PM, Martin KaFai Lau wrote:
> On Mon, Apr 27, 2020 at 01:12:39PM -0700, Yonghong Song wrote:
>> A bpf_iter program is a tracing program with attach type
>> BPF_TRACE_ITER. The load attribute
>> attach_btf_id
>> is used by the verifier against a particular kernel function,
>> e.g., __bpf_iter__bpf_map in our previous bpf_map iterator.
>>
>> The program return value must be 0 for now. In the
>> future, other return values may be used for filtering or
>> teminating the iterator.
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>> include/uapi/linux/bpf.h | 1 +
>> kernel/bpf/verifier.c | 20 ++++++++++++++++++++
>> tools/include/uapi/linux/bpf.h | 1 +
>> 3 files changed, 22 insertions(+)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 4a6c47f3febe..f39b9fec37ab 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -215,6 +215,7 @@ enum bpf_attach_type {
>> BPF_TRACE_FEXIT,
>> BPF_MODIFY_RETURN,
>> BPF_LSM_MAC,
>> + BPF_TRACE_ITER,
>> __MAX_BPF_ATTACH_TYPE
>> };
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 91728e0f27eb..fd36c22685d9 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -7074,6 +7074,11 @@ static int check_return_code(struct bpf_verifier_env *env)
>> return 0;
>> range = tnum_const(0);
>> break;
>> + case BPF_PROG_TYPE_TRACING:
>> + if (env->prog->expected_attach_type != BPF_TRACE_ITER)
>> + return 0;
>> + range = tnum_const(0);
>> + break;
>> default:
>> return 0;
>> }
>> @@ -10454,6 +10459,7 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>> struct bpf_prog *tgt_prog = prog->aux->linked_prog;
>> u32 btf_id = prog->aux->attach_btf_id;
>> const char prefix[] = "btf_trace_";
>> + struct btf_func_model fmodel;
>> int ret = 0, subprog = -1, i;
>> struct bpf_trampoline *tr;
>> const struct btf_type *t;
>> @@ -10595,6 +10601,20 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>> prog->aux->attach_func_proto = t;
>> prog->aux->attach_btf_trace = true;
>> return 0;
>> + case BPF_TRACE_ITER:
>> + if (!btf_type_is_func(t)) {
>> + verbose(env, "attach_btf_id %u is not a function\n",
>> + btf_id);
>> + return -EINVAL;
>> + }
>> + t = btf_type_by_id(btf, t->type);
>> + if (!btf_type_is_func_proto(t))
> Other than the type tests,
> to ensure the attach_btf_id is a supported bpf_iter target,
> should the prog be checked against the target list
> ("struct list_head targets") here also during the prog load time?

This is a good question. In my RFC v2, I did this, checking against
registered targets (essentially, program loading + attaching to the target).

In this version, program loading and attaching are separated.
- program loading: against btf_id
- attaching: linking bpf program to target
current linking parameter only bpf_program, but later on
there may be additional parameters like map_id, pid, cgroup_id
etc. for tailoring the iterator behavior.

This seems having a better separation. Agreed that checking
at load time may return error earlier instead at link_create
time. Let me think about this.

>
>> + return -EINVAL;
>> + prog->aux->attach_func_name = tname;
>> + prog->aux->attach_func_proto = t;
>> + ret = btf_distill_func_proto(&env->log, btf, t,
>> + tname, &fmodel);
>> + return ret;
>> default:
>> if (!prog_extension)
>> return -EINVAL;