diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c
index 42cdebcc8c8..3561f6ff465 100644
--- a/sys/kern/sys_process.c
+++ b/sys/kern/sys_process.c
@@ -332,11 +332,13 @@ ptrace(struct thread *td, struct ptrace_args *uap)
 		struct fpreg fpreg;
 		struct reg reg;
 	} r;
-	struct proc *p;
+	struct proc *curp, *p, *pp;
 	struct thread *td2;
 	int error, write;
 	int proctree_locked = 0;
 
+	curp = td->td_proc;
+
 	/*
 	 * Do copyin() early before getting locks and lock proctree before
 	 * locking the process.
@@ -422,6 +424,17 @@ ptrace(struct thread *td, struct ptrace_args *uap)
 			goto fail;
 		}
 
+		/* Can't trace an ancestor if you're being traced. */
+		if (curp->p_flag & P_TRACED) {
+			for (pp = curp->p_pptr; pp != NULL; pp = pp->p_pptr) {
+				if (pp == p) {
+					error = EINVAL;
+					goto fail;
+				}
+			}
+		}
+
+
 		/* OK */
 		break;