Re: [RFC PATCH bpf-next v3 1/2] libbpf: support selective kernel module BTF loading via bpf_object_open_opts

[Fuyu Zhao]

On 8/22/2026 2:51 AM, Andrii Nakryiko wrote:
> On Thu, Aug 20, 2026 at 8:51 PM Fuyu Zhao <zhaofuyu@xxxxxxxx> wrote:
>>
>> Hi Eduard,
>>
>> Thanks for the suggestions. Please see my responses inline below.
>>
>> On 8/21/2026 7:58 AM, Eduard Zingerman wrote:
>>> On Wed, 2026-08-19 at 17:04 +0800, Fuyu Zhao wrote:
>>>
>>> Overall the logic seem to be fine for me, please find a few comments below.
>>>
>>> ...
>>>
>>>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>>>> index 514e4e9daa82..37934ca49dd7 100644
>>>> --- a/tools/lib/bpf/libbpf.c
>>>> +++ b/tools/lib/bpf/libbpf.c
>>>> @@ -779,6 +779,13 @@ struct bpf_object {
>>>>      char *token_path;
>>>>      int token_fd;
>>>>
>>>> +    /* kernel module BTFs to load, as specified via bpf_object_open_opts */
>>>> +    struct {
>>>> +            char **names;
>>>> +            size_t nr_names;
>>>> +            struct hashmap *names_map;
>>>
>>> We already have a strset type, please use it instead of a direct
>>> hashmap reference. This would remove the need for `names` field
>>> and simplify the bpf_object__init_kmod_btfs() function.
>>>
>>> Also, do you expect `nr_names` to be high?
>>> If not, wouldn't plain array search be simpler/faster here?
>>>
>>
>> I initially used a hashmap to avoid repeatedly walking the requested
>> module name list. But I don't expect nr_names to be high in practice, so
>> a plain array search should be sufficient and simpler. I'll change this
>> in the next version.
>>
>>>> +    } *kmod_btfs;
>>>
>>> Why indirection?
>>>
>>
>> I followed the existing pattern in bpf_object and used an anonymous
>> structure with indirection here. I don't have a strong reason to keep
>> the indirection, though. I'll simplify it.
>>
>>> Also, I agree with the bot here, prior fields use "module" in the name,
>>> so something like "btf_module_names" or similar is a better fit.
>>> Same for publicly visible 'opts' name.
>>>
>>
>> I think `btf_module_names` is a much better name than my original choice.
>> I'll change that in `opts`, too.
>>
>>> ...
>>>
>>>> +static int bpf_object__init_kmod_btfs(struct bpf_object *obj,
>>>> +                                  const struct bpf_object_open_opts *opts)
>>>> +{
>>>> +    const char **kmod_btf_names;
>>>> +    size_t i, kmod_btf_names_cnt;
>>>> +    int err;
>>>> +
>>>> +    kmod_btf_names = OPTS_GET(opts, kmod_btf_names, NULL);
>>>> +    if (!kmod_btf_names)
>>>> +            return 0;
>>>> +
>>>> +    kmod_btf_names_cnt = OPTS_GET(opts, kmod_btf_names_cnt, 0);
>>>> +    if (!kmod_btf_names_cnt) {
>>>> +            pr_warn("kmod_btf_names_cnt must be set when kmod_btf_names is provided\n");
>>>> +            return -EINVAL;
>>>
>>> I kinda agree with the bot here, why disallow an empty filter here?
>>>
>>
>> I understand the point now. I hadn't considered the use case of
>> explicitly skipping all module BTFs. A non-NULL `kmod_btf_names` with a
>> zero count is a way to represent this case, so I'll support it in the
>> next version.
>>
> 
> besides all Eduard's suggestions, one naming nit: don't use double
> underscore naming, those are reserved for extern method-like APIs,
> while you are adding internal helpers
> 

Got it. I'll rename it and keep this in mind going forward. Thanks.

> 
>>>> +    }
>>>> +
>>>> +    obj->kmod_btfs = calloc(1, sizeof(*obj->kmod_btfs));
>>>> +    if (!obj->kmod_btfs)
>>>> +            return -ENOMEM;
>>>> +
>>>> +    obj->kmod_btfs->names = calloc(kmod_btf_names_cnt, sizeof(char *));
>>>> +    if (!obj->kmod_btfs->names) {
>>>> +            err = -ENOMEM;
>>>> +            goto err_out;
>>>> +    }
>>>> +
>>>> +    obj->kmod_btfs->names_map = hashmap__new(mod_name_hash_fn,
>>>> +                                             mod_name_equal_fn, NULL);
>>>> +    if (IS_ERR(obj->kmod_btfs->names_map)) {
>>>> +            err = PTR_ERR(obj->kmod_btfs->names_map);
>>>> +            obj->kmod_btfs->names_map = NULL;
>>>> +            goto err_out;
>>>> +    }
>>>> +
>>>> +    for (i = 0; i < kmod_btf_names_cnt; i++) {
>>>> +            size_t idx = obj->kmod_btfs->nr_names;
>>>> +
>>>> +            if (!kmod_btf_names[i] || !kmod_btf_names[i][0]) {
>>>> +                    pr_warn("invalid kernel module BTF name at index %zu\n", i);
>>>> +                    err = -EINVAL;
>>>> +                    goto err_out;
>>>> +            }
>>>> +
>>>> +            obj->kmod_btfs->names[idx] = strdup(kmod_btf_names[i]);
>>>> +            if (!obj->kmod_btfs->names[idx]) {
>>>> +                    err = -ENOMEM;
>>>> +                    goto err_out;
>>>> +            }
>>>> +
>>>> +            err = hashmap__add(obj->kmod_btfs->names_map,
>>>> +                               obj->kmod_btfs->names[idx], 0);
>>>> +            if (err) {
>>>> +                    zfree(&obj->kmod_btfs->names[idx]);
>>>> +                    if (err == -EEXIST) {
>>>> +                            pr_warn("duplicate kmod BTF name '%s' ignored\n",
>>>> +                                    kmod_btf_names[i]);
>>>
>>> Nit: I'd downgrade this to debug level, if at all.
>>>
>>
>> Agreed. I'll downgrade this to debug level in the next version.
>>
> 
> when you copy over a list of kernel module names, sort it, and do a
> pass to detect duplicates. if there are any -- just error out, specify
> in the documentation that the list of kernel modules names should not
> contain duplicates. It's not a particularly limiting restriction, is
> it?
> 

I agree. I'll simplify this and reject duplicate module names, and update
the documentation accordingly. Thanks for the suggestion.

>>>> +                            continue;
>>>> +                    }
>>>> +                    goto err_out;
>>>> +            }
>>>> +            obj->kmod_btfs->nr_names++;
>>>> +    }
>>>> +    return 0;
>>>> +
>>>> +err_out:
>>>> +    bpf_object__free_kmod_btfs(obj);
>>>> +    return err;
>>>> +}
>>>
>>> ...
>>>
>>>> @@ -8515,6 +8645,7 @@ static struct bpf_object *bpf_object_open(const char *path, const void *obj_buf,
>>>>      err = err ? : bpf_object__init_maps(obj, opts);
>>>>      err = err ? : bpf_object_init_progs(obj, opts);
>>>>      err = err ? : bpf_object__collect_relos(obj);
>>>> +    err = err ? : bpf_object__init_kmod_btfs(obj, opts);
>>>
>>> Nit: all other options are collected before bpf_object__elf_init()
>>>      call just above.
>>>
>>
>> Agreed. I'll change it in the next version.
>>
>>>>      if (err)
>>>>              goto out;
>>>>
>>>
>>> ...
>>>
>>>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>>>> index b965ad571540..2f8ff6d2d3df 100644
>>>> --- a/tools/lib/bpf/libbpf.h
>>>> +++ b/tools/lib/bpf/libbpf.h
>>>> @@ -224,10 +224,24 @@ struct bpf_object_open_opts {
>>>>       * point (/sys/fs/bpf), in case this default behavior is undesirable.
>>>>       */
>>>>      const char *bpf_token_path;
>>>> +    /*
>>>> +     * Optional list of kernel module names whose BTFs should be loaded.
>>>> +     * kmod_btf_names_cnt specifies the number of entries in
>>>> +     * kmod_btf_names.
>>>> +     *
>>>> +     * If kmod_btf_names is NULL, all module BTFs are loaded, preserving
>>>> +     * the default behavior. Otherwise, only the specified module BTFs
>>>> +     * are loaded.
>>>> +     *
>>>> +     * kmod_btf_names_cnt must be non-zero when kmod_btf_names is
>>>> +     * non-NULL; otherwise -EINVAL is returned.
>>>
>>> Nit: please add a complete list of behaviors this affects, e.g. CO-RE,
>>>      fentry/fexit resolution, etc.
>>>
>>
>> I'll expand the comment to cover the affected behaviors in the next version.
>>
>>>> +     */
>>>> +    const char **kmod_btf_names;
>>>> +    size_t kmod_btf_names_cnt;
>>>>
>>>>      size_t :0;
>>>>  };
>>>> -#define bpf_object_open_opts__last_field bpf_token_path
>>>> +#define bpf_object_open_opts__last_field kmod_btf_names_cnt
>>>>
>>>>  /**
>>>>   * @brief **bpf_object__open()** creates a bpf_object by opening
>>