Document minimum-known linux kernel version requirement about bpfilter

[qdeslandes]

Thanks for raising this issue!

I haven't tested all the various kernel versions, but my expectations are the following:

• Kernel 5.19+
• libbpf 1.0+

I use Fedora 38 with Linux 6.4.10 and libbpf 1.1 without any issue. For Ubuntu:

• 22.04: build failure due to missing struct bpf_dynptr. While Linux 5.19 is available in the updates repositories, the userspace headers package is limited to 5.15.
• 22.10: both userspace headers and Linux version are at least 5.19, but libbpf is only 0.8. bpfilter builds but fails at runtime as libbpf can't find bpf_dynptr_from_skb BTF ID.
• 23.10: userspace headers and Linux are recent enough (6.2), as is libbpf (1.1). bpfilter builds but I have the same runtime failure as with Ubuntu 22.10.

I won't do much about Ubuntu 22.04 due to the old userspace headers, but 23.10 should work fine. For 22.10, even though it doesn't meet the libbpf 1.0 requirements, it might work, but I probably won't dedicate development time to support it.

I will update the readme with more details about my configuration. Hopefully, I'll be able to investigate the Ubuntu runtime issue soon and list supported Ubuntu versions in the readme.

[kevinworth]

This is great info, thank you! Will close this issue since there's at least some base information for folks who are getting started.

[kevinworth]

A little more digging into the in-tree versions of libbpf:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/bpf/libbpf_version.h?h=v5.18 has libbpf version 0.8
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/bpf/libbpf_version.h?h=v5.19 has libbpf version 1.0

So if the kernel needs to be in-sync with at least libbpf 1.0 level of support, perhaps the minimum linux kernel version for bpfilter is 5.19. However, I'm unsure how much of libbpf's "transparent handling of older kernel versions" should be helping reduce dependency here.

[qdeslandes]

Also, with support for BPF_PROG_TYPE_NETFILTER in progress, the minimal kernel version will soon be bumped.