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

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Date:   Wed, 12 Feb 2020 18:28:43 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.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>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next] libbpf: Add support for dynamic program attach
 target



> On Feb 12, 2020, at 10:14 AM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> 
> On Wed, Feb 12, 2020 at 10:07 AM Song Liu <songliubraving@...com> wrote:
>> 
>> 
>> 
>>> On Feb 12, 2020, at 9:34 AM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
>>> 
>>> On Wed, Feb 12, 2020 at 4:32 AM Eelco Chaudron <echaudro@...hat.com> wrote:
>>>> 
>>>> 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");
>>>> bpf_object__load(trace_obj)
>>>> 
>>>> Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
>> 
>> 
>> I am trying to solve the same problem with slightly different approach.
>> 
>> It works as the following (with skeleton):
>> 
>>        obj = myobject_bpf__open_opts(&opts);
>>        bpf_object__for_each_program(prog, obj->obj)
>>                bpf_program__overwrite_section_name(prog, new_names[id++]);
>>        err = myobject_bpf__load(obj);
>> 
>> I don't have very strong preference. But I think my approach is simpler?
> 
> I prefer bpf_program__set_attach_target() approach. Section name is a
> program identifier and a *hint* for libbpf to determine program type,
> attach type, and whatever else makes sense. But there still should be
> an API to set all that manually at runtime, thus
> bpf_program__set_attach_target(). Doing same by overriding section
> name feels like a hack, plus it doesn't handle overriding
> attach_program_fd at all.

We already have bpf_object_open_opts to handle different attach_program_fd. 
Can we depreciate bpf_object_open_opts.attach_prog_fd with the 
bpf_program__set_attach_target() approach?

Thanks,
Song