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Date:   Wed, 27 May 2020 10:03:56 -0700
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Andrii Nakryiko" <andrii.nakryiko@...il.com>
Cc:     "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>,
        "john fastabend" <john.fastabend@...il.com>,
        "KP Singh" <kpsingh@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        "Jakub Kicinski" <kuba@...nel.org>,
        "Jesper Dangaard Brouer" <hawk@...nel.org>,
        "bpf" <bpf@...r.kernel.org>, "Networking" <netdev@...r.kernel.org>,
        "open list" <linux-kernel@...r.kernel.org>,
        "clang-built-linux" <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH bpf-next] libbpf: Export bpf_object__load_vmlinux_btf

Hi Andrii,

On Tue May 26, 2020 at 3:09 PM PST, Andrii Nakryiko wrote:
> On Tue, May 26, 2020 at 7:09 PM Daniel Xu <dxu@...uu.xyz> wrote:
> >
> > Right now the libbpf model encourages loading the entire object at once.
> > In this model, libbpf handles loading BTF from vmlinux for us. However,
> > it can be useful to selectively load certain maps and programs inside an
> > object without loading everything else.
>
> There is no way to selectively load or not load a map. All maps are
> created, unless they are reusing map FD or pinned instances. See
> below, I'd like to understand the use case better.
>
> >
> > In the latter model, there was perviously no way to load BTF on-demand.
> > This commit exports the bpf_object__load_vmlinux_btf such that we are
> > able to load BTF on demand.
> >
>
> Let's start with the real problem, not a solution. Do you have
> specific use case where you need bpf_object__load_vmlinux_btf()? It
> might not do anything if none of BPF programs in the object requires
> BTF, because it's very much tightly coupled with loading bpf_object as
> a whole model. I'd like to understand what you are after with this,
> before exposing internal implementation details as an API.

If I try loading a program through the following sequence:

    bpf_object__open_file()
    bpf_object__find_program_by_name()
    bpf_program__load()

And the program require BTF (tp_btf), I get an unavoidable (to the best
of my knowledge) segfault in the following code path:

    bpf_program__load()
      libbpf_find_attach_btf_id()    <-- [0]
        __find_vmlinx_btf_id()
          find_btf_by_prefix_kind()
            btf__find_by_name_kind() <-- boom (btf->nr_types)

because [0] passes prog->obj->btf_vmlinux which is still null. So the
solution I'm proposing is exporting bpf_object__load_vmlinux_btf() and
calling that on struct bpf_object before performing prog loads.

[...]

Thanks,
Daniel

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