Re: [PATCH V2 1/2] tools/lib/bpf/libbpf: Prioritize module kfuncs over vmlinux kfuncs

[Alexei Starovoitov]

On Thu, May 14, 2026 at 5:56 AM Song Chen <chensong_2000@xxxxxxx> wrote:
>
> Hi;
>
> On 5/13/26 05:41, Alexei Starovoitov wrote:
> > On Mon, May 11, 2026 at 8:19 PM Kaitao Cheng <kaitao.cheng@xxxxxxxxx> wrote:
> >>
> >> It seems that there are two problems to solve here:
> >>
> >> 1. Allowing kernel modules to override vmlinux kfuncs with the same
> >>     name.
> >>
> >>     This should be relatively straightforward to implement by matching
> >>     names in kernel modules first, and then falling back to vmlinux.
> >>
> >> 2. Handling the case where different kernel modules have kfuncs with
> >>     the same name.
> >>
> >>     Could we use a new libbpf API to establish the mapping manually?
> >>     for example:
> >>         bpf_object__set_ksym_btf(obj, "kfunc_name", "module_name")
> >>
> >>     Or define a new attribute, for example:
> >>         extern void kfunc_name(void) __ksym __module("module_name");
> >>
> >>     We probably need to hear suggestions from other developers on this.
> >
> > I'm not excited about 2.
> > Two different modules are not allowed to have two different functions
> > with the same name.
> > The kernel has only one global namespace.
> > Hence I don't think we should be introducing such namespacing concept
> > into kfuncs.
> >
> > But 1, I feel, is useful for experiments and mitigations,
> > but it probably should be done transparently to libbpf.
> >
> > register_btf_kfunc_id_set() should be changed to return an error
> > when a module attempts to register a kfunc with a name that already
> > exists in the kernel.
> > We also introduce override_btf_kfunc_set() that would register
> > a replacement kfunc.
> > override_btf_kfunc_set() would check that all overriding kfuncs
> > have the exact same kfunc already in vmlinux BTF: proto and name should match.
> > Then the kernel will just remember that the address of that kfunc
> > is in the module.
> > libbpf doesn't need to be aware of this substitution.
> > It will find kfunc in the vmlinux BTF, but the verifier will supply
> > implementation from the module to bpf prog.
> > 2nd module should be able to override_btf_kfunc_set() from vmlinux
> > and from 1st module that replaced some kfuncs.
> > All that only for bpf progs that are loaded after override.
> > Already loaded progs don't need to be live patched.
> > Thoughts?
>
> If i understand you correctly, you suggest instead of introduce
> namespace in libbpf, we should replace kfunc in
> vmlinux(btf->types[type_id]) with kfunc in modules by introducing
> override_btf_kfunc_set.
>
> In [1], I simply returned an error, your suggestion is a little further
> than that, correct me if i'm wrong.
>
> [1]:https://lore.kernel.org/all/20260122090404.15553-1-chensong_2000@xxxxxx/

Right. The error on name conflict and new api to override
if there is a full match.