diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 8b4d765c422b9..d71631a019267 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4690,7 +4690,7 @@ int libbpf_prog_type_by_name(const char *name, enum bpf_prog_type *prog_type,
 			}
 			/* prepend "btf_trace_" prefix per kernel convention */
 			strncat(dst, name + section_names[i].len,
-				sizeof(raw_tp_btf_name) - (dst - raw_tp_btf_name));
+				sizeof(raw_tp_btf_name) - sizeof("btf_trace_"));
 			ret = btf__find_by_name(btf, raw_tp_btf_name);
 			btf__free(btf);
 			if (ret <= 0) {