Re: [RFC bpf-next 0/5] Convert iproute2 to use libbpf (WIP)

On Tue, Aug 20, 2019 at 01:47:01PM +0200, Toke Høiland-Jørgensen wrote:

iproute2 uses its own bpf loader to load eBPF programs, which has
evolved separately from libbpf. Since we are now standardising on
libbpf, this becomes a problem as iproute2 is slowly accumulating
feature incompatibilities with libbpf-based loaders. In particular,
iproute2 has its own (expanded) version of the map definition struct,
which makes it difficult to write programs that can be loaded with both
custom loaders and iproute2.

This series seeks to address this by converting iproute2 to using libbpf
for all its bpf needs. This version is an early proof-of-concept RFC, to
get some feedback on whether people think this is the right direction.

What this series does is the following:

- Updates the libbpf map definition struct to match that of iproute2
  (patch 1).
- Adds functionality to libbpf to support automatic pinning of maps when
  loading an eBPF program, while re-using pinned maps if they already
  exist (patches 2-3).
- Modifies iproute2 to make it possible to compile it against libbpf
  without affecting any existing functionality (patch 4).
- Changes the iproute2 eBPF loader to use libbpf for loading XDP
  programs (patch 5).


As this is an early PoC, there are still a few missing pieces before
this can be merged. Including (but probably not limited to):

- Consolidate the map definition struct in the bpf_helpers.h file in the
  kernel tree. This contains a different, and incompatible, update to
  the struct. Since the iproute2 version has actually been released for
  use outside the kernel tree (and thus is subject to API stability
  constraints), I think it makes the most sense to keep that, and port
  the selftests to use it.

It sounds like you're implying that existing libbpf format is not uapi.
It is and we cannot break it.
If patch 1 means breakage for existing pre-compiled .o that won't load
with new libbpf then we cannot use this method.
Recompiling .o with new libbpf definition of bpf_map_def isn't an option.
libbpf has to be smart before/after and recognize both old and iproute2 format.