libbpf: use negative fd to specify missing BTF

0 is a valid FD, so it's better to initialize it to -1, as is done in
other places. Also, technically, BTF type ID 0 is valid (it's a VOID
type), so it's more reliable to check btf_fd, instead of
btf_key_type_id, to determine if there is any BTF associated with a map.

Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -1751,7 +1751,7 @@ bpf_object__create_maps(struct bpf_object *obj)
 		create_attr.key_size = def->key_size;
 		create_attr.value_size = def->value_size;
 		create_attr.max_entries = def->max_entries;
-		create_attr.btf_fd = 0;
+		create_attr.btf_fd = -1;
 		create_attr.btf_key_type_id = 0;
 		create_attr.btf_value_type_id = 0;
 		if (bpf_map_type__is_map_in_map(def->type) &&
@@ -1765,11 +1765,11 @@ bpf_object__create_maps(struct bpf_object *obj)
 		}
 
 		*pfd = bpf_create_map_xattr(&create_attr);
-		if (*pfd < 0 && create_attr.btf_key_type_id) {
+		if (*pfd < 0 && create_attr.btf_fd >= 0) {
 			cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
 			pr_warning("Error in bpf_create_map_xattr(%s):%s(%d). Retrying without BTF.\n",
 				   map->name, cp, errno);
-			create_attr.btf_fd = 0;
+			create_attr.btf_fd = -1;
 			create_attr.btf_key_type_id = 0;
 			create_attr.btf_value_type_id = 0;
 			map->btf_key_type_id = 0;
@@ -2053,6 +2053,9 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 	char *log_buf;
 	int ret;
 
+	if (!insns || !insns_cnt)
+		return -EINVAL;
+
 	memset(&load_attr, 0, sizeof(struct bpf_load_program_attr));
 	load_attr.prog_type = prog->type;
 	load_attr.expected_attach_type = prog->expected_attach_type;
@@ -2063,7 +2066,7 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 	load_attr.license = license;
 	load_attr.kern_version = kern_version;
 	load_attr.prog_ifindex = prog->prog_ifindex;
-	load_attr.prog_btf_fd = prog->btf_fd >= 0 ? prog->btf_fd : 0;
+	load_attr.prog_btf_fd = prog->btf_fd;
 	load_attr.func_info = prog->func_info;
 	load_attr.func_info_rec_size = prog->func_info_rec_size;
 	load_attr.func_info_cnt = prog->func_info_cnt;
@@ -2072,8 +2075,6 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 	load_attr.line_info_cnt = prog->line_info_cnt;
 	load_attr.log_level = prog->log_level;
 	load_attr.prog_flags = prog->prog_flags;
-	if (!load_attr.insns || !load_attr.insns_cnt)
-		return -EINVAL;
 
 retry_load:
 	log_buf = malloc(log_buf_size);