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

[Fuyu Zhao]

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.

>> +	}
>> +
>> +	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.

>> +				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