libbpf: identify maps by section index in addition to offset

To support maps to be defined in multiple sections, it's important to
identify map not just by offset within its section, but section index as
well. This patch adds tracking of section index.

For global data, we record section index of corresponding
.data/.bss/.rodata ELF section for uniformity, and thus don't need
a special value of offset for those maps.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -207,7 +207,8 @@ static const char * const libbpf_type_to_btf_name[] = {
 struct bpf_map {
 	int fd;
 	char *name;
-	size_t offset;
+	int sec_idx;
+	size_t sec_offset;
 	int map_ifindex;
 	int inner_map_fd;
 	struct bpf_map_def def;
@@ -647,7 +648,9 @@ static int compare_bpf_map(const void *_a, const void *_b)
 	const struct bpf_map *a = _a;
 	const struct bpf_map *b = _b;
 
-	return a->offset - b->offset;
+	if (a->sec_idx != b->sec_idx)
+		return a->sec_idx - b->sec_idx;
+	return a->sec_offset - b->sec_offset;
 }
 
 static bool bpf_map_type__is_map_in_map(enum bpf_map_type type)
@@ -800,7 +803,7 @@ static struct bpf_map *bpf_object__add_map(struct bpf_object *obj)
 
 static int
 bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
-			      Elf_Data *data, void **data_buff)
+			      int sec_idx, Elf_Data *data, void **data_buff)
 {
 	char map_name[BPF_OBJ_NAME_LEN];
 	struct bpf_map_def *def;
@@ -811,7 +814,8 @@ bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
 		return PTR_ERR(map);
 
 	map->libbpf_type = type;
-	map->offset = ~(typeof(map->offset))0;
+	map->sec_idx = sec_idx;
+	map->sec_offset = 0;
 	snprintf(map_name, sizeof(map_name), "%.8s%.7s", obj->name,
 		 libbpf_type_to_btf_name[type]);
 	map->name = strdup(map_name);
@@ -819,6 +823,8 @@ bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
 		pr_warning("failed to alloc map name\n");
 		return -ENOMEM;
 	}
+	pr_debug("map '%s' (global data): at sec_idx %d, offset %zu.\n",
+		 map_name, map->sec_idx, map->sec_offset);
 
 	def = &map->def;
 	def->type = BPF_MAP_TYPE_ARRAY;
@@ -851,6 +857,7 @@ static int bpf_object__init_global_data_maps(struct bpf_object *obj)
 	 */
 	if (obj->efile.data_shndx >= 0) {
 		err = bpf_object__init_internal_map(obj, LIBBPF_MAP_DATA,
+						    obj->efile.data_shndx,
 						    obj->efile.data,
 						    &obj->sections.data);
 		if (err)
@@ -858,6 +865,7 @@ static int bpf_object__init_global_data_maps(struct bpf_object *obj)
 	}
 	if (obj->efile.rodata_shndx >= 0) {
 		err = bpf_object__init_internal_map(obj, LIBBPF_MAP_RODATA,
+						    obj->efile.rodata_shndx,
 						    obj->efile.rodata,
 						    &obj->sections.rodata);
 		if (err)
@@ -865,6 +873,7 @@ static int bpf_object__init_global_data_maps(struct bpf_object *obj)
 	}
 	if (obj->efile.bss_shndx >= 0) {
 		err = bpf_object__init_internal_map(obj, LIBBPF_MAP_BSS,
+						    obj->efile.bss_shndx,
 						    obj->efile.bss, NULL);
 		if (err)
 			return err;
@@ -948,7 +957,10 @@ static int bpf_object__init_user_maps(struct bpf_object *obj, bool strict)
 		}
 
 		map->libbpf_type = LIBBPF_MAP_UNSPEC;
-		map->offset = sym.st_value;
+		map->sec_idx = sym.st_shndx;
+		map->sec_offset = sym.st_value;
+		pr_debug("map '%s' (legacy): at sec_idx %d, offset %zu.\n",
+			 map_name, map->sec_idx, map->sec_offset);
 		if (sym.st_value + map_def_sz > data->d_size) {
 			pr_warning("corrupted maps section in %s: last map \"%s\" too small\n",
 				   obj->path, map_name);
@@ -1448,9 +1460,13 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
 				if (maps[map_idx].libbpf_type != type)
 					continue;
 				if (type != LIBBPF_MAP_UNSPEC ||
-				    maps[map_idx].offset == sym.st_value) {
-					pr_debug("relocation: find map %zd (%s) for insn %u\n",
-						 map_idx, maps[map_idx].name, insn_idx);
+				    (maps[map_idx].sec_idx == sym.st_shndx &&
+				     maps[map_idx].sec_offset == sym.st_value)) {
+					pr_debug("relocation: found map %zd (%s, sec_idx %d, offset %zu) for insn %u\n",
+						 map_idx, maps[map_idx].name,
+						 maps[map_idx].sec_idx,
+						 maps[map_idx].sec_offset,
+						 insn_idx);
 					break;
 				}
 			}
@@ -3468,13 +3484,7 @@ bpf_object__find_map_fd_by_name(struct bpf_object *obj, const char *name)
 struct bpf_map *
 bpf_object__find_map_by_offset(struct bpf_object *obj, size_t offset)
 {
-	int i;
-
-	for (i = 0; i < obj->nr_maps; i++) {
-		if (obj->maps[i].offset == offset)
-			return &obj->maps[i];
-	}
-	return ERR_PTR(-ENOENT);
+	return ERR_PTR(-ENOTSUP);
 }
 
 long libbpf_get_error(const void *ptr)