Re: [PATCH v2 bpf-next 3/4] libbpf: deprecate legacy BPF map definitions

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Jan 21, 2022 at 2:04 PM David Ahern <dsahern@gmail.com> wrote:
>
> On 1/21/22 1:43 PM, Toke Høiland-Jørgensen wrote:
> > Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
> >
> >> On Thu, Jan 20, 2022 at 3:44 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >>>
> >>> Andrii Nakryiko <andrii@kernel.org> writes:
> >>>
> >>>> Enact deprecation of legacy BPF map definition in SEC("maps") ([0]). For
> >>>> the definitions themselves introduce LIBBPF_STRICT_MAP_DEFINITIONS flag
> >>>> for libbpf strict mode. If it is set, error out on any struct
> >>>> bpf_map_def-based map definition. If not set, libbpf will print out
> >>>> a warning for each legacy BPF map to raise awareness that it goes
> >>>> away.
> >>>
> >>> We've touched upon this subject before, but I (still) don't think it's a
> >>> good idea to remove this support entirely: It makes it impossible to
> >>> write a loader that can handle both new and old BPF objects.
> >>>
> >>> So discourage the use of the old map definitions, sure, but please don't
> >>> make it completely impossible to load such objects.
> >>
> >> BTF-defined maps have been around for quite a long time now and only
> >> have benefits on top of the bpf_map_def way. The source code
> >> translation is also very straightforward. If someone didn't get around
> >> to update their BPF program in 2 years, I don't think we can do much
> >> about that.
> >>
> >> Maybe instead of trying to please everyone (especially those that
> >> refuse to do anything to their BPF programs), let's work together to
> >> nudge laggards to actually modernize their source code a little bit
> >> and gain some benefits from that along the way?
> >
> > I'm completely fine with nudging people towards the newer features, and
> > I think the compile-time deprecation warning when someone is using the
> > old-style map definitions in their BPF programs is an excellent way to
> > do that.
> >
> > I'm also fine with libbpf *by default* refusing to load programs that
> > use the old-style map definitions, but if the code is removed completely
> > it becomes impossible to write general-purpose loaders that can handle
> > both old and new programs. The obvious example of such a loader is
> > iproute2, the loader in xdp-tools is another.
> >
>
> I agree with Toke's response.
>
> 2 years is a very small amount of time when it comes to OS and kernel
> versions. Many companies base products on enterprise distributions and
> run them for 10+ years. During that time there will be needs to update
> some components - like kernel version or some tool but that is done with
> the least amount of churn possible. Every update has the potential to
> bring in unknown behavior changes. Requiring updates to entire tool
> chains, multiple tool sets and libraries to accommodate some deprecation
> will only hinder being able to update anything.
>
> Further, programs (e.g., debugging as just one example) can and will
> need to be used across many OS and kernel versions.

Which is why all the things that are being deprecated have better
alternatives that work *right now* with libbpf v0.x and will keep
working with v1.x.