[BPF CO-RE clarification] Use CO-RE on older kernel versions.

[Vamsi Kodavanty <vamsi@araalinetworks.com>]

Had a few questions on CO-RE dependencies and usage. From what I read
CO-RE needs a supported kernel version and be compiled with
`CONFIG_DEBUG_INFO_BTF=y`.

I also understand there are three pieces to enable CO-RE
functionality. (1) The BTF format. For efficient/compressed kernel
symbol table. (2) clang changes to emit the BTF relocations. (3)
`libbpf` changes to locate a BTF file and fix-up relocations. Once
these 3 steps are done the resulting byte code is no different from
non-CO-RE byte code.

Given this I am hoping the knowledgeable folks on this mailer correct
and guide me if I am stating something incorrectly.

(1) Is the kernel support requirement ONLY for the purposes of
generating and exposing the BTF file information on
`/sys/kernel/btf/vmlinux`? So that the eBPF CO-RE applications
`libbpf` can find the BTF information at a standard location?.

(2) If the answer to the above question is YES. Could the below
mechanism be used so that it works on all kernels whether they support
the `CONFIG_DEBUG_INFO_BTF` flag or not?.
       (a) Extract BTF generation process outside of the kernel build.
Use this to generate the equivalent BTF file for it.
       (b) Make changes to `libbpf` to look for BTF not only at the
standard locations but also at a user specified location. The BTF file
generated in (a) can be presented here.

This should provide us a way to enable CO-RE functionality on older
kernel versions as well. I tried to make the above changes and tried
against a 4.14 kernel and it did not work. Either I am not doing
something right or my assumptions are wrong.

Thanks in advance for your time. And I hope someone here can guide me
in the right direction.

Regards
Vamsi.