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

[Song Chen <chensong_2000@126.com>]

Hi;

On 5/13/26 05:41, Alexei Starovoitov wrote:
> On Mon, May 11, 2026 at 8:19 PM Kaitao Cheng <kaitao.cheng@linux.dev> 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@189.cn/

best regards,

Song