utils: add warning message when ptrace_attach failed

attaching to already running process require permission for use ptrace
functionality that supports by linux kernel. recently kernel disable
ptrace at default. so, user who want use attaching must enable ptrace.
added message guide user how to do this.

Signed-off-by: Hanbum Park <kese111@gmail.com>

utils/ptrace.c

@@ -14,8 +14,13 @@ void ptrace_attach(pid_t target)
 {
 	int status;
 
-	if (ptrace(PTRACE_ATTACH, target, NULL, NULL) == -1)
+	if (ptrace(PTRACE_ATTACH, target, NULL, NULL) == -1) {
+		pr_warn("attaching requires ptrace permission.\n"
+			"please, enable ptrace permission like following.\n"
+			"[example]\n"
+			"$ echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope\n");
 		pr_err("ptrace(PTRACE_ATTACH) failed");
+	}
 
 	if (waitpid(target, &status, WUNTRACED) != target)
 		pr_err("waitpid(%d) failed", target);