Re: [PATCH bpf v2] bpf: fix nested bpf tracepoints with per-cpu data

[Daniel Borkmann]

On 06/14/2019 02:55 AM, Alexei Starovoitov wrote:
> On Thu, Jun 13, 2019 at 5:52 PM Matt Mullins <mmullins@xxxxxx> wrote:
>> On Fri, 2019-06-14 at 00:47 +0200, Daniel Borkmann wrote:
>>> On 06/12/2019 07:00 AM, Andrii Nakryiko wrote:
>>>> On Tue, Jun 11, 2019 at 8:48 PM Matt Mullins <mmullins@xxxxxx> wrote:
>>>>>
>>>>> BPF_PROG_TYPE_RAW_TRACEPOINTs can be executed nested on the same CPU, as
>>>>> they do not increment bpf_prog_active while executing.
>>>>>
>>>>> This enables three levels of nesting, to support
>>>>>   - a kprobe or raw tp or perf event,
>>>>>   - another one of the above that irq context happens to call, and
>>>>>   - another one in nmi context
>>>>> (at most one of which may be a kprobe or perf event).
>>>>>
>>>>> Fixes: 20b9d7ac4852 ("bpf: avoid excessive stack usage for perf_sample_data")
>>>
>>> Generally, looks good to me. Two things below:
>>>
>>> Nit, for stable, shouldn't fixes tag be c4f6699dfcb8 ("bpf: introduce BPF_RAW_TRACEPOINT")
>>> instead of the one you currently have?
>>
>> Ah, yeah, that's probably more reasonable; I haven't managed to come up
>> with a scenario where one could hit this without raw tracepoints.  I'll
>> fix up the nits that've accumulated since v2.
>>
>>> One more question / clarification: we have __bpf_trace_run() vs trace_call_bpf().
>>>
>>> Only raw tracepoints can be nested since the rest has the bpf_prog_active per-CPU
>>> counter via trace_call_bpf() and would bail out otherwise, iiuc. And raw ones use
>>> the __bpf_trace_run() added in c4f6699dfcb8 ("bpf: introduce BPF_RAW_TRACEPOINT").
>>>
>>> 1) I tried to recall and find a rationale for mentioned trace_call_bpf() split in
>>> the c4f6699dfcb8 log, but couldn't find any. Is the raison d'Ãtre purely because of
>>> performance overhead (and desire to not miss events as a result of nesting)? (This
>>> also means we're not protected by bpf_prog_active in all the map ops, of course.)
>>> 2) Wouldn't this also mean that we only need to fix the raw tp programs via
>>> get_bpf_raw_tp_regs() / put_bpf_raw_tp_regs() and won't need this duplication for
>>> the rest which relies upon trace_call_bpf()? I'm probably missing something, but
>>> given they have separate pt_regs there, how could they be affected then?
>>
>> For the pt_regs, you're correct: I only used get/put_raw_tp_regs for
>> the _raw_tp variants.  However, consider the following nesting:
>>
>>                                     trace_nest_level raw_tp_nest_level
>>   (kprobe) bpf_perf_event_output            1               0
>>   (raw_tp) bpf_perf_event_output_raw_tp     2               1
>>   (raw_tp) bpf_get_stackid_raw_tp           2               2
>>
>> I need to increment a nest level (and ideally increment it only once)
>> between the kprobe and the first raw_tp, because they would otherwise
>> share the struct perf_sample_data.  But I also need to increment a nest
>> level between the two raw_tps, since they share the pt_regs -- I can't
>> use trace_nest_level for everything because it's not used by
>> get_stackid, and I can't use raw_tp_nest_level for everything because
>> it's not incremented by kprobes.
>>
>> If raw tracepoints were to bump bpf_prog_active, then I could get away
>> with just using that count in these callsites -- I'm reluctant to do
>> that, though, since it would prevent kprobes from ever running inside a
>> raw_tp.  I'd like to retain the ability to (e.g.)
>>   trace.py -K htab_map_update_elem
>> and get some stack traces from at least within raw tracepoints.
>>
>> That said, as I wrote up this example, bpf_trace_nest_level seems to be
>> wildly misnamed; I should name those after the structure they're
>> protecting...
> 
> I still don't get what's wrong with the previous approach.
> Didn't I manage to convince both of you that perf_sample_data
> inside bpf_perf_event_array doesn't have any issue that Daniel brought up?
> I think this refcnting approach is inferior.

Hm, but looking at perf RB handling code, it can deal with nesting situation
just fine. I think this was your main concern with prior email:

  because I suspect that 'struct bpf_event_entry' is not reentrable
  (even w/o issues with 'struct perf_sample_data').

  Even if we always use 'struct perf_sample_data' on stack, I suspect
  the same 'struct bpf_event_entry' cannot be reused in the nested way.

Check the perf_output_{get,put}_handle() and the way it updates head pointer
and does the final propagation to user_page. So if it's designed to handle
these situations, then bailing out doesn't make sense from BPF side.