diff --git a/sys/kern/subr_syscall.c b/sys/kern/subr_syscall.c
--- a/sys/kern/subr_syscall.c
+++ b/sys/kern/subr_syscall.c
@@ -56,7 +56,7 @@
 	struct syscall_args *sa;
 	struct sysent *se;
 	int error;
-	bool sy_thr_static, traced;
+	bool do_set_rv, sy_thr_static, traced;
 
 	VM_CNT_INC(v_syscall);
 	p = td->td_proc;
@@ -98,6 +98,10 @@
 		PROC_UNLOCK(p);
 
 		if ((td->td_dbgflags & TDB_USERWR) != 0) {
+			PROC_LOCK(p);
+			td->td_dbgflags &= ~TDB_USERWR;
+			PROC_UNLOCK(p);
+
 			/*
 			 * Reread syscall number and arguments if debugger
 			 * modified registers or memory.
@@ -203,12 +207,16 @@
 	    (uintptr_t)td, "pid:%d", td->td_proc->p_pid, "error:%d", error,
 	    "retval0:%#lx", td->td_retval[0], "retval1:%#lx",
 	    td->td_retval[1]);
+	do_set_rv = true;
 	if (__predict_false(traced)) {
 		PROC_LOCK(p);
-		td->td_dbgflags &= ~(TDB_SCE | TDB_BOUNDARY);
+		if ((td->td_dbgflags & TDB_USERWR) != 0)
+			do_set_rv = false;
+		td->td_dbgflags &= ~(TDB_SCE | TDB_BOUNDARY | TDB_USERWR);
 		PROC_UNLOCK(p);
 	}
-	(p->p_sysent->sv_set_syscall_retval)(td, error);
+	if (__predict_true(do_set_rv))
+		(p->p_sysent->sv_set_syscall_retval)(td, error);
 	if (error != 0 && (td->td_pflags2 & TDP2_UEXTERR) != 0)
 		exterr_copyout(td);
 }