Re: BPF maps don't work without CONFIG_TRACING/CONFIG_FTRACE

["Toke Høiland-Jørgensen" <toke@kernel.org>]

Michael Zimmermann <sigmaepsilon92@gmail.com> writes:

> On Thu, May 12, 2022 at 9:48 AM Toke Høiland-Jørgensen <toke@kernel.org> wrote:
>>
>> Michael Zimmermann <sigmaepsilon92@gmail.com> writes:
>>
>> > On Thu, May 12, 2022 at 5:21 AM Andrii Nakryiko
>> > <andrii.nakryiko@gmail.com> wrote:
>> >>
>> >> On Mon, May 9, 2022 at 10:12 PM Michael Zimmermann
>> >> <sigmaepsilon92@gmail.com> wrote:
>> >> >
>> >> > Thank you for your answer.
>> >> > What I'm ultimately trying to do is: Use aya-rs to watch egress on a
>> >> > network interface and notify userspace through a map (for certain IPs
>> >> > only).
>> >> >
>> >> > In my actual use case, the userspace is supposed to do more complex
>> >> > stuff but for testing I simply logged the receival of a message
>> >> > through the BPF map on the console. And that is what I expect to
>> >> > happen and which does happen as long as CONFIG_TRACING/CONFIG_FTRACE
>> >> > are active. If not, I simply never receive any messages on any map.
>> >> >
>> >> > I've also tried this using an XDP program which sends a message every
>> >> > time it sees a packet. And while the program seemed to be
>> >> > working(since it did block certain traffic), I never saw any data in
>> >> > the map when those configs were disabled.
>> >> >
>> >> > Also, I'm giving you two configs(tracing and ftrace) since the other
>> >> > one seems to get y-selected automatically if one of them is active.
>> >>
>> >> Please don't top post, reply inline instead.
>> > Sorry for that, GMail does that by default and even hides that it's
>> > quoting at all.
>> >
>> >>
>> >> I don't think we have enough to investigate here, even "receive any
>> >> messages on any map" is so ambiguous that it's hard to even guess what
>> >> you are really trying to do. BPF maps are not sending/receiving
>> >> messages. So please provide some pieces of code and what you are doing
>> >> to check. CONFIG_TRACING and CONFIG_FTRACE shouldn't have any effect
>> >> on functioning of BPF maps, so it's most probably that you are doing
>> >> something besides BPF map update/lookup, but you don't provide enough
>> >> information to check anything.
>> >
>> > An aya project I tested where I don't receive any events:
>> > https://github.com/aya-rs/book/tree/6b52a6fac5fa3e5a1165f98591b2eaff9692048a/examples/myapp-03
>>
>> It's using a PERF_EVENT_ARRAY map to send events to userspace. This
>> requires CONFIG_BPF_EVENTS to work, which depends on CONFIG_PERF_EVENTS.
>> Not sure if this depends on CONFIG_TRACING specifically, but maybe you
>> disabled PERF_EVENTS as well?
>
> PERF_EVENTS is enabled in both my working and my broken config.
> But both directly and through others, BPF_EVENTS depends on FTRACE
> (which then also selects TRACING).
>
> So is this some weird dependency chain or expected behavior?
> If it's expected, are there alternatives to achieve similar
> functionality or do I have to convince my distro to enable tracing
> support?

Andrii can probably answer for sure, but I *think* this is expected.
There's the BPF_MAP_TYPE_RINGBUF which can be used as an alternative;
not sure what the dependencies are for that, but from a cursory glance
it looks like it doesn't depend on anything in TRACING...

-Toke