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: a3820c481112 ("libbpf: Support opting out from autoloading BPF programs declaratively")
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/btf.c

···
 	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;
 
···
 			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);
···
 
 		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;
 }
···
 {
 	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

···
 		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);
···
 				BTF_EXT_ELF_SEC, err);
 			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:
···
 	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;
···
 	}
 
 	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)
···
 	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

···
 	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)					\