Re: [PATCH bpf-next 1/3] bpf: introduce helper bpf_get_task_stack_trace()

From: Song Liu <songliubraving@fb.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf <bpf@vger.kernel.org>,
	Network Development <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <Kernel-team@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>
Subject: Re: [PATCH bpf-next 1/3] bpf: introduce helper bpf_get_task_stack_trace()
Date: Tue, 23 Jun 2020 17:40:36 +0000	[thread overview]
Message-ID: <C0EBD4AF-C9D4-41AE-9F18-57D4097B7DE2@fb.com> (raw)
In-Reply-To: <FF92494E-D1EB-4B84-9D2F-8CD43FEAB164@fb.com>

> On Jun 23, 2020, at 9:59 AM, Song Liu <songliubraving@fb.com> wrote:
> 
> 
> 
>> On Jun 23, 2020, at 8:19 AM, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
>> 
>> On Tue, Jun 23, 2020 at 12:08 AM Song Liu <songliubraving@fb.com> wrote:
>>> 
> 
> [...]
> 
>>> 
>>> +BPF_CALL_3(bpf_get_task_stack_trace, struct task_struct *, task,
>>> +          void *, entries, u32, size)
>>> +{
>>> +       return stack_trace_save_tsk(task, (unsigned long *)entries, size, 0);
>>> +}
>>> +
>>> +static int bpf_get_task_stack_trace_btf_ids[5];
>>> +static const struct bpf_func_proto bpf_get_task_stack_trace_proto = {
>>> +       .func           = bpf_get_task_stack_trace,
>>> +       .gpl_only       = true,
>> 
>> why?
> 
> Actually, I am not sure when we should use gpl_only = true. 
> 
>> 
>>> +       .ret_type       = RET_INTEGER,
>>> +       .arg1_type      = ARG_PTR_TO_BTF_ID,
>>> +       .arg2_type      = ARG_PTR_TO_MEM,
>>> +       .arg3_type      = ARG_CONST_SIZE_OR_ZERO,
>> 
>> OR_ZERO ? why?
> 
> Will fix. 
> 
>> 
>>> +       .btf_id         = bpf_get_task_stack_trace_btf_ids,
>>> +};
>>> +
>>> static const struct bpf_func_proto *
>>> raw_tp_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>>> {
>>> @@ -1521,6 +1538,10 @@ tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>>>               return prog->expected_attach_type == BPF_TRACE_ITER ?
>>>                      &bpf_seq_write_proto :
>>>                      NULL;
>>> +       case BPF_FUNC_get_task_stack_trace:
>>> +               return prog->expected_attach_type == BPF_TRACE_ITER ?
>>> +                       &bpf_get_task_stack_trace_proto :
>> 
>> why limit to iter only?
> 
> I guess it is also useful for other types. Maybe move to bpf_tracing_func_proto()?
> 
>> 
>>> + *
>>> + * int bpf_get_task_stack_trace(struct task_struct *task, void *entries, u32 size)
>>> + *     Description
>>> + *             Save a task stack trace into array *entries*. This is a wrapper
>>> + *             over stack_trace_save_tsk().
>> 
>> size is not documented and looks wrong.
>> the verifier checks it in bytes, but it's consumed as number of u32s.
> 
> I am not 100% sure, but verifier seems check it correctly. And I think it is consumed
> as u64s?

I was wrong. Verifier checks as bytes. Will fix. 

Song