Re: [PATCH dwarves] pahole: Inject kfunc decl tags into BTF

[Jiri Olsa <olsajiri@gmail.com>]

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@dxuuu.xyz>

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
>