On Wed, Aug 12, 2026 at 8:26 PM Fuyu Zhao <[email protected]> wrote:
>
> Add support for a new ELF section ".kmod_btfs" that allows BPF programs
> to declare which kernel modules need BTF loading. This avoids loading
> all module BTFs and speeds up program load when only a subset of modules
> is needed.
>
> Specifically, this patch introduces two internal functions:
> - bpf_object__collect_kmod_btf_names(): parses the ".kmod_btfs"
> section and collects the declared module names.
> - is_kmod_btf_needed(): determines whether a given module's BTF
> should be loaded, allowing libbpf to skip unneeded modules.
>
> Signed-off-by: Fuyu Zhao <[email protected]>
> ---
> tools/lib/bpf/bpf_helpers.h | 14 +++++
> tools/lib/bpf/libbpf.c | 106 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 120 insertions(+)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index 9d160b5b9c0e..171ea055cd32 100644
> --- a/tools/lib/bpf/bpf_helpers.h
> +++ b/tools/lib/bpf/bpf_helpers.h
> @@ -188,6 +188,20 @@ enum libbpf_tristate {
> TRI_MODULE = 2,
> };
>
> +/* Helper typedef for declaring kernel module names that need BTF loading.
> + *
> + * Usage: define an array in the ".kmod_btfs" ELF section to specify
> + * which modules need BTF loading:
> + *
> + * DEFINE_KMOD_BTFS(_needed_kmods)= { "module1", "module2", ... };
> + *
> + * This avoids unnecessary BTF loading and speeds up the BPF program
> + * load process.
> + */
> +#define KMOD_NAME_LEN 64
> +#define DEFINE_KMOD_BTFS(name) \
> + SEC(".kmod_btfs") char name[][KMOD_NAME_LEN]
> +
I don't really like another special ELF section just to provide a
custom list of modules.
If we have to do a customization of BTF searching behavior of libbpf,
let's do this through bpf_object_open_opts struct and pass NULL or a
empty/non-empty list of kernel modules, or whatever other appropriate
way to specify this.
But in any case, extending ELF format for BPF object file for this
seems like an overkill and overcomplication.
pw-bot: cr
> #define __kconfig __attribute__((section(".kconfig")))
> #define __ksym __attribute__((section(".ksyms")))
> #define __kptr_untrusted __attribute__((btf_type_tag("kptr_untrusted")))
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 514e4e9daa82..9fe8a653e8b6 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -548,6 +548,7 @@ struct bpf_struct_ops {
> #define STRUCT_OPS_SEC ".struct_ops"
> #define STRUCT_OPS_LINK_SEC ".struct_ops.link"
> #define ARENA_SEC ".addr_space.1"
> +#define KMODS_BTFS_SEC ".kmod_btfs"
>
> enum libbpf_map_type {
> LIBBPF_MAP_UNSPEC,
> @@ -703,6 +704,9 @@ enum bpf_object_state {
> OBJ_LOADED,
> };
>
> +/* Should match the definition in bpf_helpers.h */
> +#define KMOD_NAME_LEN 64
> +
> struct bpf_object {
> char name[BPF_OBJ_NAME_LEN];
> char license[64];
> @@ -779,6 +783,14 @@ struct bpf_object {
> char *token_path;
> int token_fd;
>
> + /* kernel module BTFs to load, declared in ".kmod_btfs" ELF section */
> + struct {
> + char (*data)[KMOD_NAME_LEN];
> + size_t nr_names;
> + size_t nr_loaded;
> + struct hashmap *hashmap;
> + } *kmod_btfs;
> +
> char path[];
> };
>
> @@ -901,6 +913,75 @@ bpf_object__init_prog(struct bpf_object *obj, struct
> bpf_program *prog,
> return -ENOMEM;
> }
>
> +static size_t mod_name_hash_fn(long key, void *ctx)
> +{
> + return str_hash((char *)key);
> +}
> +
> +static bool mod_name_equal_fn(long key1, long key2, void *ctx)
> +{
> + return strcmp((char *)key1, (char *)key2) == 0;
> +}
> +
> +static int
> +bpf_object__collect_kmod_btf_names(struct bpf_object *obj, Elf_Data
> *sec_data,
> + const char *sec_name)
> +{
> + int module_cnt, i, err = 0;
> +
> + if (obj->kmod_btfs) {
> + pr_warn("sec '%s': duplicate sections detected\n", sec_name);
> + return -EEXIST;
> + }
> +
> + if (sec_data->d_size % KMOD_NAME_LEN != 0) {
> + pr_warn("sec '%s': size %zu should be multiple of %d\n",
> + sec_name, sec_data->d_size, KMOD_NAME_LEN);
> + return -EINVAL;
> + }
> +
> + module_cnt = sec_data->d_size / KMOD_NAME_LEN;
> + obj->kmod_btfs = calloc(1, sizeof(*obj->kmod_btfs));
> + if (!obj->kmod_btfs)
> + return -ENOMEM;
> +
> + obj->kmod_btfs->data = calloc(module_cnt, KMOD_NAME_LEN);
> + if (!obj->kmod_btfs->data) {
> + err = -ENOMEM;
> + goto err_out;
> + }
> + memcpy(obj->kmod_btfs->data, sec_data->d_buf, sec_data->d_size);
> +
> + obj->kmod_btfs->hashmap = hashmap__new(mod_name_hash_fn,
> + mod_name_equal_fn, NULL);
> + if (IS_ERR(obj->kmod_btfs->hashmap)) {
> + err = PTR_ERR(obj->kmod_btfs->hashmap);
> + goto err_out;
> + }
> +
> + for (i = 0; i < module_cnt; i++) {
> + obj->kmod_btfs->data[i][KMOD_NAME_LEN - 1] = '\0';
> + if (hashmap__find(obj->kmod_btfs->hashmap,
> + obj->kmod_btfs->data[i], NULL)) {
> + pr_warn("sec '%s': ignored duplicate module '%s'\n",
> + sec_name, obj->kmod_btfs->data[i]);
> + continue;
> + }
> + err = hashmap__set(obj->kmod_btfs->hashmap,
> obj->kmod_btfs->data[i],
> + 0, NULL, NULL);
> + if (err)
> + goto err_out;
> + obj->kmod_btfs->nr_names++;
> + }
> + return 0;
> +
> +err_out:
> + hashmap__free(obj->kmod_btfs->hashmap);
> + zfree(&obj->kmod_btfs->data);
> + zfree(&obj->kmod_btfs);
> + return err;
> +}
> +
> static int
> bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
> const char *sec_name, int sec_idx)
> @@ -4034,6 +4115,10 @@ static int bpf_object__elf_collect(struct bpf_object
> *obj)
> memcpy(obj->jumptables_data, data->d_buf,
> data->d_size);
> obj->jumptables_data_sz = data->d_size;
> obj->efile.jumptables_data_shndx = idx;
> + } else if (strcmp(name, KMODS_BTFS_SEC) == 0) {
> + err = bpf_object__collect_kmod_btf_names(obj,
> data, name);
> + if (err)
> + return err;
> } else {
> pr_info("elf: skipping unrecognized data
> section(%d) %s\n",
> idx, name);
> @@ -5803,6 +5888,11 @@ int bpf_core_add_cands(struct bpf_core_cand
> *local_cand,
> return 0;
> }
>
> +static bool is_kmod_btf_needed(struct bpf_object *obj, const char *name)
> +{
> + return hashmap_find(obj->kmod_btfs->hashmap, (long)name, NULL);
> +}
> +
> static int load_module_btfs(struct bpf_object *obj)
> {
> struct bpf_btf_info info;
> @@ -5867,6 +5957,12 @@ static int load_module_btfs(struct bpf_object *obj)
> continue;
> }
>
> + if (obj->kmod_btfs && obj->kmod_btfs->hashmap &&
> + !is_kmod_btf_needed(obj, name)) {
> + close(fd);
> + continue;
> + }
> +
> btf = btf_get_from_fd(fd, obj->btf_vmlinux);
> err = libbpf_get_error(btf);
> if (err) {
> @@ -5891,6 +5987,10 @@ static int load_module_btfs(struct bpf_object *obj)
> break;
> }
> obj->btf_module_cnt++;
> +
> + if (obj->kmod_btfs &&
> + obj->kmod_btfs->nr_names == obj->kmod_btfs->nr_loaded)
> + break;
> }
>
> if (err) {
> @@ -9030,6 +9130,12 @@ static void bpf_object_cleanup_btf(struct bpf_object
> *obj)
> /* clean up vmlinux BTF */
> btf__free(obj->btf_vmlinux);
> obj->btf_vmlinux = NULL;
> +
> + if (obj->kmod_btfs) {
> + hashmap__free(obj->kmod_btfs->hashmap);
> + zfree(&obj->kmod_btfs->data);
> + zfree(&obj->kmod_btfs);
> + }
> }
>
> static void bpf_object_post_load_cleanup(struct bpf_object *obj)
> --
> 2.34.1
>
