libbpf: Reject legacy 'maps' ELF section

Add explicit error message if BPF object file is still using legacy BPF
map definitions in SEC("maps"). Before this change, if BPF object file
is still using legacy map definition user will see a bit confusing:

  libbpf: elf: skipping unrecognized data section(4) maps
  libbpf: prog 'handler': bad map relo against 'server_map' in section 'maps'

Now libbpf will be explicit about rejecting "maps" ELF section:

  libbpf: elf: legacy map definitions in 'maps' section are not supported by libbpf v1.0+
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220803214202.23750-1-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -591,7 +591,6 @@ struct elf_state {
 	size_t strtabidx;
 	struct elf_sec_desc *secs;
 	int sec_cnt;
-	int maps_shndx;
 	int btf_maps_shndx;
 	__u32 btf_maps_sec_btf_id;
 	int text_shndx;
@@ -1272,7 +1271,6 @@ static struct bpf_object *bpf_object__new(const char *path,
 	 */
 	obj->efile.obj_buf = obj_buf;
 	obj->efile.obj_buf_sz = obj_buf_sz;
-	obj->efile.maps_shndx = -1;
 	obj->efile.btf_maps_shndx = -1;
 	obj->efile.st_ops_shndx = -1;
 	obj->kconfig_map_idx = -1;
@@ -3363,7 +3361,8 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 			if (err)
 				return err;
 		} else if (strcmp(name, "maps") == 0) {
-			obj->efile.maps_shndx = idx;
+			pr_warn("elf: legacy map definitions in 'maps' section are not supported by libbpf v1.0+\n");
+			return -ENOTSUP;
 		} else if (strcmp(name, MAPS_ELF_SEC) == 0) {
 			obj->efile.btf_maps_shndx = idx;
 		} else if (strcmp(name, BTF_ELF_SEC) == 0) {
@@ -3895,8 +3894,7 @@ static bool bpf_object__shndx_is_data(const struct bpf_object *obj,
 static bool bpf_object__shndx_is_maps(const struct bpf_object *obj,
 				      int shndx)
 {
-	return shndx == obj->efile.maps_shndx ||
-	       shndx == obj->efile.btf_maps_shndx;
+	return shndx == obj->efile.btf_maps_shndx;
 }
 
 static enum libbpf_map_type