Re: [PATCH bpf-next] libbpf: Export bpf_object__load_vmlinux_btf

From: Daniel Xu
Date: Wed May 27 2020 - 13:12:36 EST

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@xxxxxxxxx> 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:


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

libbpf_find_attach_btf_id() <-- [0]
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.