netdev - Re: [PATCH bpf-next] libbpf: Add support for dynamic program attach target

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Date:   Wed, 12 Feb 2020 09:35:26 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Eelco Chaudron <echaudro@...hat.com>, bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf-next] libbpf: Add support for dynamic program attach target

On Wed, Feb 12, 2020 at 5:05 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Eelco Chaudron <echaudro@...hat.com> writes:
>
> > Currently when you want to attach a trace program to a bpf program
> > the section name needs to match the tracepoint/function semantics.
> >
> > However the addition of the bpf_program__set_attach_target() API
> > allows you to specify the tracepoint/function dynamically.
> >
> > The call flow would look something like this:
> >
> >   xdp_fd = bpf_prog_get_fd_by_id(id);
> >   trace_obj = bpf_object__open_file("func.o", NULL);
> >   prog = bpf_object__find_program_by_title(trace_obj,
> >                                            "fentry/myfunc");
> >   bpf_program__set_attach_target(prog, xdp_fd,
> >                                  "fentry/xdpfilt_blk_all");
>
> I think it would be better to have the attach type as a separate arg
> instead of encoding it in the function name. I.e., rather:
>
>    bpf_program__set_attach_target(prog, xdp_fd,
>                                   "xdpfilt_blk_all", BPF_TRACE_FENTRY);

I agree about not specifying section name prefix (e.g., fentry/). But
disagree that expected attach type (BPF_TRACE_FENTRY) should be part
of this API. We already have bpf_program__set_expected_attach_type()
API, no need to duplicate it here.

>
> -Toke
>