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

[David Vernet <void@manifault.com>]

On Sun, Feb 05, 2023 at 12:42:03PM -0800, Cong Wang wrote:
> 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.

I'm not following. How is a BPF program not kernel development?

> > 
> > 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

Sorry, but again, I'm not following your point. What exactly about this
"fundamentally contradicts" with CO-RE? Please elaborate if you're going
to claim that something is a fundamental contradiction.

> Could you add some clarification for this too? Especically how we could
> respect CO-RE meanwhile deprecating kfuncs?

I don't know what you mean by "respecting CO-RE". You can compile a BPF
program that calls a kfunc and invoke it on differents, as long as
whatever kernel you're running on provides that kfunc with the same BTF
encoding. This is no different than e.g. accessing a struct element on
two kernel versions.

Also, CO-RE doesn't provide any ironclad guarantees either. If you
access a struct element in a BPF program, and then a kernel version
removes that element from the struct, that BPF program will fail to load
on that kernel.

> 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.

Agreed, we should improve the QA to mention that you can load kfuncs
from a module -- thanks for pointing that out!

> "
> 
> The conntrack kfuncs like bpf_skb_ct_alloc() reside in a kernel module.
> 
> Thanks!