On Wed, Dec 20, 2023 at 03:19:52PM -0700, Daniel Xu wrote:
> This commit teaches pahole to parse symbols in .BTF_ids section in
> vmlinux and discover exported kfuncs. Pahole then takes the list of
> kfuncs and injects a BTF_KIND_DECL_TAG for each kfunc.
>
> This enables downstream users and tools to dynamically discover which
> kfuncs are available on a system by parsing vmlinux or module BTF, both
> available in /sys/kernel/btf.
>
> Example of encoding:
>
> $ bpftool btf dump file .tmp_vmlinux.btf | rg DECL_TAG | wc -l
> 388
>
> $ bpftool btf dump file .tmp_vmlinux.btf | rg 68940
> [68940] FUNC 'bpf_xdp_get_xfrm_state' type_id=68939 linkage=static
> [128124] DECL_TAG 'kfunc' type_id=68940 component_idx=-1
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
SNIP
> +
> +static int btf_encoder__tag_kfunc(struct btf_encoder *encoder, const char *kfunc)
> +{
> + int nr_types, type_id, err = -1;
> + struct btf *btf = encoder->btf;
could we store the kuncs in sorted array (by name) and iterate all IDs
just once while doing the bsearch for the name over the kfuncs array
> +
> + nr_types = btf__type_cnt(btf);
> + for (type_id = 1; type_id < nr_types; type_id++) {
> + const struct btf_type *type;
> + const char *name;
> +
> + type = btf__type_by_id(btf, type_id);
> + if (!type) {
> + fprintf(stderr, "%s: malformed BTF, can't resolve type for ID %d\n",
> + __func__, type_id);
> + goto out;
> + }
> +
> + if (!btf_is_func(type))
> + continue;
> +
> + name = btf__name_by_offset(btf, type->name_off);
> + if (!name) {
> + fprintf(stderr, "%s: malformed BTF, can't resolve name for ID %d\n",
> + __func__, type_id);
> + goto out;
> + }
> +
> + if (strcmp(name, kfunc))
> + continue;
> +
> + err = btf__add_decl_tag(btf, BTF_KFUNC_TYPE_TAG, type_id, -1);
> + if (err < 0) {
> + fprintf(stderr, "%s: failed to insert kfunc decl tag for '%s': %d\n",
> + __func__, kfunc, err);
> + goto out;
> + }
> +
> + err = 0;
> + break;
> + }
> +
> +out:
> + return err;
> +}
> +
> +static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
> +{
> + const char *filename = encoder->filename;
> + GElf_Shdr shdr_mem, *shdr;
> + int symbols_shndx = -1;
> + int idlist_shndx = -1;
> + Elf_Scn *scn = NULL;
> + Elf_Data *symbols;
> + int fd, err = -1;
> + size_t strtabidx;
> + Elf *elf = NULL;
> + size_t strndx;
> + char *secname;
> + int nr_syms;
> + int i = 0;
> +
> + fd = open(filename, O_RDONLY);
> + if (fd < 0) {
> + fprintf(stderr, "Cannot open %s\n", filename);
> + goto out;
> + }
> +
> + if (elf_version(EV_CURRENT) == EV_NONE) {
> + elf_error("Cannot set libelf version");
> + goto out;
> + }
> +
> + elf = elf_begin(fd, ELF_C_READ, NULL);
> + if (elf == NULL) {
> + elf_error("Cannot update ELF file");
> + goto out;
> + }
SNIP
> + }
> + free(kfunc);
> + }
> +
> + err = 0;
> +out:
leaking fd and elf object (elf_end)
jirka
> + return err;
> +}
> +
> int btf_encoder__encode(struct btf_encoder *encoder)
> {
> int err;
> @@ -1366,6 +1563,11 @@ int btf_encoder__encode(struct btf_encoder *encoder)
> if (btf__type_cnt(encoder->btf) == 1)
> return 0;
>
> + if (btf_encoder__tag_kfuncs(encoder)) {
> + fprintf(stderr, "%s: failed to tag kfuncs!\n", __func__);
> + return -1;
> + }
> +
> if (btf__dedup(encoder->btf, NULL)) {
> fprintf(stderr, "%s: btf__dedup failed!\n", __func__);
> return -1;
> --
> 2.42.1
>
