Andrii Nakryiko [off-list ref] writes:

On Thu, Oct 22, 2020 at 8:39 PM Hangbin Liu [off-list ref] wrote:

quoted

This patch converts iproute2 to use libbpf for loading and attaching
BPF programs when it is available, which is started by Toke's
implementation[1]. With libbpf iproute2 could correctly process BTF
information and support the new-style BTF-defined maps, while keeping
compatibility with the old internal map definition syntax.

The old iproute2 bpf code is kept and will be used if no suitable libbpf
is available. When using libbpf, wrapper code in bpf_legacy.c ensures that
iproute2 will still understand the old map definition format, including
populating map-in-map and tail call maps before load.

In bpf_libbpf.c, we init iproute2 ctx and elf info first to check the
legacy bytes. When handling the legacy maps, for map-in-maps, we create
them manually and re-use the fd as they are associated with id/inner_id.
For pin maps, we only set the pin path and let libbp load to handle it.
For tail calls, we find it first and update the element after prog load.

I never implemented tail call map initialization using the same
approach as declarative map-in-map support in libbpf, because no one
asked and/or showed a use case. But all the pieces are there, and if
there's interest, we should probably support that in libbpf as well.

Yeah, that's what we figured; and since this series maintains
compatibility with the old map definition format for declarative
tail-calls, this doesn't have to hold up the conversion: iproute2 will
just magically gain this when/if it lands in libbpf :)

-Toke