netdev - Re: [PATCH bpf-next 1/3] bpf: introduce helper bpf_get_task_stack

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Date:   Tue, 23 Jun 2020 11:41:53 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next 1/3] bpf: introduce helper bpf_get_task_stack_trace()

On Tue, Jun 23, 2020 at 10:00 AM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Jun 23, 2020, at 8:19 AM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> >
> > On Tue, Jun 23, 2020 at 12:08 AM Song Liu <songliubraving@...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.

I actually think it's a good idea, because it makes writing code that
uses variable-sized buffers easier. Remember how we had
bpf_perf_event_output() forcing size > 0? That was a major PITA and
required unnecessary code gymnastics to prove verifier it's OK (even
if zero size was never possible). Yonghong eventually fixed that to be
_OR_ZERO.

So if this is not causing any problems, please leave it as _OR_ZERO.
Thank you from everyone who had to suffer through dealing with
anything variable-sized in BPF!

>
> >
> >> +       .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?
>
> Thanks,
> Song
>