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

[Eduard Zingerman <eddyz87@gmail.com>]

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?

> +	} *kmod_btfs;

Why indirection?

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.

...

> +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?

> +	}
> +
> +	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.

> +				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.

>  	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.

> +	 */
> +	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