Re: [PATCH bpf-next v3] bpf/docs: Document kfunc lifecycle / stability expectations

Cong Wang
2023-02-05T20:42:08Z

On Fri, Feb 03, 2023 at 09:57:27AM -0600, David Vernet wrote:
> BPF kernel <-> kernel API stability has been discussed at length over
> the last several weeks and months. Now that we've largely aligned over
> kfuncs being the way forward, and BPF helpers being considered
> functionally frozen, it's time to document the expectations for kfunc
> lifecycles and stability so that everyone (BPF users, kfunc developers,
> and maintainers) are all aligned, and have a crystal-clear understanding
> of the expectations surrounding kfuncs.
>
> To do that, this patch adds that documentation to the main kfuncs
> documentation page via a new 'kfunc lifecycle expectations' section. The
> patch describes how decisions are made in the kernel regarding whether
> to include, keep, deprecate, or change / remove a kfunc. As described
> very overtly in the patch itself, but likely worth highlighting here:
>
> "kfunc stability" does not mean, nor ever will mean, "BPF APIs may block
> development elsewhere in the kernel".
>
> Rather, the intention and expectation is for kfuncs to be treated like
> EXPORT_SYMBOL_GPL symbols in the kernel. The goal is for kfuncs to be a
> safe and valuable option for maintainers and kfunc developers to extend
> the kernel, without tying anyone's hands, or imposing any kind of
> restrictions on maintainers in the same way that UAPI changes do.

I think they are still different, kernel modules are still considered as
a part of kernel development, while eBPF code is not that supposed to be
kernel development, at least much further. Treating them alike is
misleading, IMHO.

>
> In addition to the 'kfunc lifecycle expectations' section, this patch
> also adds documentation for a new KF_DEPRECATED kfunc flag which kfunc
> authors or maintainers can choose to add to kfuncs if and when they
> decide to deprecate them. Note that as described in the patch itself, a
> kfunc need not be deprecated before being changed or removed -- this
> flag is simply provided as an available deprecation mechanism for those
> that want to provide a deprecation story / timeline to their users.
> When necessary, kfuncs may be changed or removed to accommodate changes
> elsewhere in the kernel without any deprecation at all.

This fundamentally contradicts with Compile-Once-Run-Everywhere
https://facebookmicrosites.github.io/bpf/blog/2020/02/19/bpf-portability-and-co-re.html
Could you add some clarification for this too? Especically how we could
respect CO-RE meanwhile deprecating kfuncs?

BTW, not related to compatibility, but still kfuncs related confusion,
it also contradicts with Documentation/bpf/bpf_design_QA.rst:

"
Q: Can BPF functionality such as new program or map types, new
helpers, etc be added out of kernel module code?

A: NO.
"

The conntrack kfuncs like bpf_skb_ct_alloc() reside in a kernel module.

Thanks!