Re: [PATCH v3] kernel/bpf/btf.c: reject to register duplicated kfunc

[Alexei Starovoitov]

On Mon, May 25, 2026 at 2:20 AM Kaitao Cheng <kaitao.cheng@xxxxxxxxx> wrote:
>
> 在 2026/5/24 16:29, Song Chen 写道:
> > I had an ebpf program which calls a kfunc defined and
> > implemented in one of my kernel modules, it was working
> > fine in 6.16, but was rejected to run by libbpf in 6.19,
> > the error message was:
> >
> > libbpf: extern (func ksym) 'bpf_strstr': func_proto [5]
> > incompatible with vmlinux [94389]
> >
> > The reason is there is a new added kfunc in kernel 6.19
> > which happens to be the same name with my kfunc. However the
> > error message is not obvious enough to address problem
> > immediately.
> >
> > Therefore, this patches searches duplicated kfunc in
> > both btf_vmlinux and btf_modules list before a kernel module
> > attempts to register kfuncs through register_btf_kfunc_id_set,
> > it prints clear error message and returns error code if same name
> > kfunc has already implemented and registered, then developer
> > knows at the first place.
> >
> > Suggested-by: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
> > Suggested-by: Kaitao Cheng <kaitao.cheng@xxxxxxxxx>
> > Signed-off-by: Song Chen <chensong_2000@xxxxxxx>
> >
> > ---
> > changelog:
> > v1 --- v2:
> > libbpf has already specified which module this kfunc belongs to as
> > ebpf code onwer's expectation, then verifier uses
> > find_kallsyms_symbol_value to search kfunc's addr.
> >
> > v2 --- v3:
> > After v2, I tried a new idea of introducing a namespace in libbpf
> > to specify kfunc owner in an ebpf program suggested by Kaitao Cheng,
> > please see [1]. Alex suggested to go back to report an error during
> > kmod load on conflicting kfunc name for now. What's more, v2 only
> > searched bpf_vmlinux, v3 also traverses btf_modules list.
> >
> > [1]:https://lore.kernel.org/all/CAADnVQ+jYGMjAC9aNygmhyppUO9foWN4z9cjSpwCEXAFHpRVJQ@xxxxxxxxxxxxxx/
> > ---
> >  kernel/bpf/btf.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 43 insertions(+), 1 deletion(-)
> >
>
> btf: reject to register duplicated kfunc
>
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 4872d2a6c42d..a14ad3720872 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -8618,6 +8618,47 @@ static int btf_check_iter_kfuncs(struct btf *btf, const char *func_name,
> >       return 0;
> >  }
> >
> > +static int btf_check_kfunc_name(struct btf *btf, const char *func_name, u32 kind)
> > +{
> > +#ifdef CONFIG_DEBUG_INFO_BTF_MODULES
> > +     struct btf_module *btf_mod, *tmp;
> > +#endif
> > +     s32 id;
> > +     int ret = 0;
>
> This ret variable may be unnecessary.
>
> > +
> > +     if (!btf_is_module(btf))
> > +             goto out;
> > +
> > +     id = btf_find_by_name_kind(bpf_get_btf_vmlinux(),
> > +                                     func_name, kind);
>
> It seems unnecessary to split this call across multiple lines. Also,
> some of the continuation-line indentation elsewhere does not appear
> to follow the kernel coding style.
>
> > +     if (id >= 0) {
> > +             pr_err("kfunc %s (id: %d) is already present in vmlinux.\n",
> > +                                     func_name, id);
> > +             ret = -EINVAL;
> > +             goto out;
>
> return -EINVAL;
>
> > +     }
> > +
> > +#ifdef CONFIG_DEBUG_INFO_BTF_MODULES
> > +     mutex_lock(&btf_module_mutex);
> > +     list_for_each_entry_safe(btf_mod, tmp, &btf_modules, list) {
> > +             if (btf_mod->btf == btf)
> > +                     continue;
> > +             id = btf_find_by_name_kind(btf_mod->btf,
> > +                                     func_name, kind);
> > +             if (id >= 0) {
> > +                     pr_err("kfunc %s (id: %d) is already present in module %s.\n",
> > +                                     func_name, id, btf_mod->module->name);
>
> follow the kernel coding style
>
> > +                     ret = -EINVAL;
> > +                     break;
>
> mutex_unlock(&btf_module_mutex);
> return -EINVAL;
>
> > +             }
> > +     }
> > +     mutex_unlock(&btf_module_mutex);
> > +#endif
> > +
> > +out:
> > +     return ret;
> > +}
> > +
> >  static int btf_check_kfunc_protos(struct btf *btf, u32 func_id, u32 func_flags)
> >  {
> >       const struct btf_type *func;
> > @@ -8631,7 +8672,8 @@ static int btf_check_kfunc_protos(struct btf *btf, u32 func_id, u32 func_flags)
> >
> >       /* sanity check kfunc name */
> >       func_name = btf_name_by_offset(btf, func->name_off);
> > -     if (!func_name || !func_name[0])
> > +     if (!func_name || !func_name[0]
> > +                     || btf_check_kfunc_name(btf, func_name, BTF_INFO_KIND(func->info)))
>
> follow the kernel coding style

yeah. formatting is way off.
pls respin.