Re: [PATCH bpf-next 09/29] bpf: Add support to load multi func tracing program

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>
Subject: Re: [PATCH bpf-next 09/29] bpf: Add support to load multi func tracing program
Date: Tue, 30 Nov 2021 23:17:44 -0800	[thread overview]
Message-ID: <CAEf4Bza5ceiQS0jLAsoCvMSb4CbS7vJ-Qym=wn7CzOGsU2g_DQ@mail.gmail.com> (raw)
In-Reply-To: <YaO/O4bNeg2JRrbU@krava>

On Sun, Nov 28, 2021 at 9:41 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> On Wed, Nov 24, 2021 at 01:51:36PM -0800, Andrii Nakryiko wrote:
> > On Mon, Nov 22, 2021 at 12:15 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > >
> > > On Thu, Nov 18, 2021 at 08:11:59PM -0800, Alexei Starovoitov wrote:
> > > > On Thu, Nov 18, 2021 at 12:24:35PM +0100, Jiri Olsa wrote:
> > > > > +
> > > > > +DEFINE_BPF_MULTI_FUNC(unsigned long a1, unsigned long a2,
> > > > > +                 unsigned long a3, unsigned long a4,
> > > > > +                 unsigned long a5, unsigned long a6)
> > > >
> > > > This is probably a bit too x86 specific. May be make add all 12 args?
> > > > Or other places would need to be tweaked?
> > >
> > > I think si, I'll check
> > >
> > > >
> > > > > +BTF_ID_LIST_SINGLE(bpf_multi_func_btf_id, func, bpf_multi_func)
> > > > ...
> > > > > -   prog->aux->attach_btf_id = attr->attach_btf_id;
> > > > > +   prog->aux->attach_btf_id = multi_func ? bpf_multi_func_btf_id[0] : attr->attach_btf_id;
> > > >
> > > > Just ignoring that was passed in uattr?
> > > > Maybe instead of ignoring dopr BPF_F_MULTI_FUNC and make libbpf
> > > > point to that btf_id instead?
> > > > Then multi or not can be checked with if (attr->attach_btf_id == bpf_multi_func_btf_id[0]).
> > > >
> > >
> > > nice idea, it might fit better than the flag
> >
> > Instead of a flag we can also use a different expected_attach_type
> > (FENTRY vs FENTRY_MULTI, etc).
>
> right, you already asked for that - https://lore.kernel.org/bpf/YS9k26rRcUJVS%2Fvx@krava/
>
> I still think it'd mean more code while this way we just use
> current fentry/fexit code paths with few special handling
> for multi programs
>

I don't see how it makes much difference for kernel implementation.
Checking expected_attach_type vs checking prog_flags is about the same
amount of code. The big advantage of new expected_attach_type (or
prog_type) is that it will be very obvious in all sorts of diagnostics
tooling (think bpftool prog show output, etc). prog_flags are almost
invisible and it will be the last thing that users will think about.
I'd try to minimize the usage of prog_flags overall.

> > As for attach_btf_id, why can't we just
> > enforce it as 0?
>
> there's prog->aux->attach_func_proto that needs to be set based
> on attach_btf_id, and is checked later in btf_ctx_access

right:

if (attach_btf_id == 0)
    prog->aux->attach_func_proto =
&special_func_model_or_proto_or_whatever_that_does_not_have_to_be_known_to_libbpf_and_outside_world_ever;

;) let's keep implementation details as internal implementation
details, instead of dumping all that to UAPI

>
> jirka
>
> >
> > >
> > > thanks,
> > > jirka
> > >
> >
>