On Wed, May 27, 2020 at 10:12 AM Daniel Xu <dxu@xxxxxxxxx> wrote: > > 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: > > bpf_object__open_file() > bpf_object__find_program_by_name() > bpf_program__load() > bpf_program__load() is just broken and shouldn't have been ever exposed. It **might** work for trivial BPF programs not using maps, Kconfig and global variables, etc, but more by accident. I think the right fix for your use-case is to allow more control of which programs are auto-loaded. There was a patch by Eric Sage previously adding bpf_program__set_autoload(), but it never landed. We should actually do that approach instead. > 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
