Commit 11d5daa8 authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Alexei Starovoitov
Browse files

libbpf: Avoid joining .BTF.ext data with BPF programs by section name 



Instead of using ELF section names as a joining key between .BTF.ext and
corresponding BPF programs, pre-build .BTF.ext section number to ELF
section index mapping during bpf_object__open() and use it later for
matching .BTF.ext information (func/line info or CO-RE relocations) to
their respective BPF programs and subprograms.

This simplifies corresponding joining logic and let's libbpf do
manipulations with BPF program's ELF sections like dropping leading '?'
character for non-autoloaded programs. Original joining logic in
bpf_object__relocate_core() (see relevant comment that's now removed)
was never elegant, so it's a good improvement regardless. But it also
avoids unnecessary internal assumptions about preserving original ELF
section name as BPF program's section name (which was broken when
SEC("?abc") support was added).

Fixes: a3820c48 ("libbpf: Support opting out from autoloading BPF programs declaratively")
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220426004511.2691730-5-andrii@kernel.org
parent 966a7509

tools/lib/bpf/btf.c

+7 −2
Original line number Diff line number Diff line
@@ -2626,6 +2626,7 @@ static int btf_ext_setup_info(struct btf_ext *btf_ext, 
	const struct btf_ext_info_sec *sinfo;

	struct btf_ext_info *ext_info;

	__u32 info_left, record_size;

	size_t sec_cnt = 0;

	/* The start of the info sec (including the __u32 record_size). */

	void *info;
@@ -2689,8 +2690,7 @@ static int btf_ext_setup_info(struct btf_ext *btf_ext, 
			return -EINVAL;

		}



		total_record_size = sec_hdrlen +

				    (__u64)num_records * record_size;

		total_record_size = sec_hdrlen + (__u64)num_records * record_size;

		if (info_left < total_record_size) {

			pr_debug("%s section has incorrect num_records in .BTF.ext\n",

			     ext_sec->desc);
@@ -2699,12 +2699,14 @@ static int btf_ext_setup_info(struct btf_ext *btf_ext, 


		info_left -= total_record_size;

		sinfo = (void *)sinfo + total_record_size;

		sec_cnt++;

	}



	ext_info = ext_sec->ext_info;

	ext_info->len = ext_sec->len - sizeof(__u32);

	ext_info->rec_size = record_size;

	ext_info->info = info + sizeof(__u32);

	ext_info->sec_cnt = sec_cnt;



	return 0;

}
@@ -2788,6 +2790,9 @@ void btf_ext__free(struct btf_ext *btf_ext) 
{

	if (IS_ERR_OR_NULL(btf_ext))

		return;

	free(btf_ext->func_info.sec_idxs);

	free(btf_ext->line_info.sec_idxs);

	free(btf_ext->core_relo_info.sec_idxs);

	free(btf_ext->data);

	free(btf_ext);

}

tools/lib/bpf/libbpf.c

+51 −27
Original line number Diff line number Diff line
@@ -2765,6 +2765,9 @@ static int bpf_object__init_btf(struct bpf_object *obj, 
		btf__set_pointer_size(obj->btf, 8);

	}

	if (btf_ext_data) {

		struct btf_ext_info *ext_segs[3];

		int seg_num, sec_num;



		if (!obj->btf) {

			pr_debug("Ignore ELF section %s because its depending ELF section %s is not found.\n",

				 BTF_EXT_ELF_SEC, BTF_ELF_SEC);
@@ -2778,6 +2781,43 @@ static int bpf_object__init_btf(struct bpf_object *obj, 
			obj->btf_ext = NULL;

			goto out;

		}



		/* setup .BTF.ext to ELF section mapping */

		ext_segs[0] = &obj->btf_ext->func_info;

		ext_segs[1] = &obj->btf_ext->line_info;

		ext_segs[2] = &obj->btf_ext->core_relo_info;

		for (seg_num = 0; seg_num < ARRAY_SIZE(ext_segs); seg_num++) {

			struct btf_ext_info *seg = ext_segs[seg_num];

			const struct btf_ext_info_sec *sec;

			const char *sec_name;

			Elf_Scn *scn;



			if (seg->sec_cnt == 0)

				continue;



			seg->sec_idxs = calloc(seg->sec_cnt, sizeof(*seg->sec_idxs));

			if (!seg->sec_idxs) {

				err = -ENOMEM;

				goto out;

			}



			sec_num = 0;

			for_each_btf_ext_sec(seg, sec) {

				/* preventively increment index to avoid doing

				 * this before every continue below

				 */

				sec_num++;



				sec_name = btf__name_by_offset(obj->btf, sec->sec_name_off);

				if (str_is_empty(sec_name))

					continue;

				scn = elf_sec_by_name(obj, sec_name);

				if (!scn)

					continue;



				seg->sec_idxs[sec_num - 1] = elf_ndxscn(scn);

			}

		}

	}

out:

	if (err && libbpf_needs_btf(obj)) {
@@ -5642,7 +5682,7 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path) 
	struct bpf_program *prog;

	struct bpf_insn *insn;

	const char *sec_name;

	int i, err = 0, insn_idx, sec_idx;

	int i, err = 0, insn_idx, sec_idx, sec_num;



	if (obj->btf_ext->core_relo_info.len == 0)

		return 0;
@@ -5663,33 +5703,18 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path) 
	}



	seg = &obj->btf_ext->core_relo_info;

	sec_num = 0;

	for_each_btf_ext_sec(seg, sec) {

		sec_idx = seg->sec_idxs[sec_num];

		sec_num++;



		sec_name = btf__name_by_offset(obj->btf, sec->sec_name_off);

		if (str_is_empty(sec_name)) {

			err = -EINVAL;

			goto out;

		}

		/* bpf_object's ELF is gone by now so it's not easy to find

		 * section index by section name, but we can find *any*

		 * bpf_program within desired section name and use it's

		 * prog->sec_idx to do a proper search by section index and

		 * instruction offset

		 */

		prog = NULL;

		for (i = 0; i < obj->nr_programs; i++) {

			if (strcmp(obj->programs[i].sec_name, sec_name) == 0) {

				prog = &obj->programs[i];

				break;

			}

		}

		if (!prog) {

			pr_warn("sec '%s': failed to find a BPF program\n", sec_name);

			return -ENOENT;

		}

		sec_idx = prog->sec_idx;



		pr_debug("sec '%s': found %d CO-RE relocations\n",

			 sec_name, sec->num_info);

		pr_debug("sec '%s': found %d CO-RE relocations\n", sec_name, sec->num_info);



		for_each_btf_ext_rec(seg, sec, i, rec) {

			if (rec->insn_off % BPF_INSN_SZ)
@@ -5873,14 +5898,13 @@ static int adjust_prog_btf_ext_info(const struct bpf_object *obj, 
	void *rec, *rec_end, *new_prog_info;

	const struct btf_ext_info_sec *sec;

	size_t old_sz, new_sz;

	const char *sec_name;

	int i, off_adj;

	int i, sec_num, sec_idx, off_adj;



	sec_num = 0;

	for_each_btf_ext_sec(ext_info, sec) {

		sec_name = btf__name_by_offset(obj->btf, sec->sec_name_off);

		if (!sec_name)

			return -EINVAL;

		if (strcmp(sec_name, prog->sec_name) != 0)

		sec_idx = ext_info->sec_idxs[sec_num];

		sec_num++;

		if (prog->sec_idx != sec_idx)

			continue;



		for_each_btf_ext_rec(ext_info, sec, i, rec) {

tools/lib/bpf/libbpf_internal.h

+7 −0
Original line number Diff line number Diff line
@@ -376,6 +376,13 @@ struct btf_ext_info { 
	void *info;

	__u32 rec_size;

	__u32 len;

	/* optional (maintained internally by libbpf) mapping between .BTF.ext

	 * section and corresponding ELF section. This is used to join

	 * information like CO-RE relocation records with corresponding BPF

	 * programs defined in ELF sections

	 */

	__u32 *sec_idxs;

	int sec_cnt;

};



#define for_each_btf_ext_sec(seg, sec)					\