diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 54a0347ca812..4b51d9cbcf06 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -502,7 +502,10 @@ out:
  *
  * Caller must be holding current->sighand->siglock lock.
  *
- * Returns 0 on success, -ve on error.
+ * Returns 0 on success, -ve on error, or
+ *   - in TSYNC mode: the pid of a thread which was either not in the correct
+ *     seccomp mode or did not have an ancestral seccomp filter
+ *   - in NEW_LISTENER mode: the fd of the new listener
  */
 static long seccomp_attach_filter(unsigned int flags,
 				  struct seccomp_filter *filter)
@@ -1258,6 +1261,16 @@ static long seccomp_set_mode_filter(unsigned int flags,
 	if (flags & ~SECCOMP_FILTER_FLAG_MASK)
 		return -EINVAL;
 
+	/*
+	 * In the successful case, NEW_LISTENER returns the new listener fd.
+	 * But in the failure case, TSYNC returns the thread that died. If you
+	 * combine these two flags, there's no way to tell whether something
+	 * succeeded or failed. So, let's disallow this combination.
+	 */
+	if ((flags & SECCOMP_FILTER_FLAG_TSYNC) &&
+	    (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER))
+		return -EINVAL;
+
 	/* Prepare the new filter before holding any locks. */
 	prepared = seccomp_prepare_user_filter(filter);
 	if (IS_ERR(prepared))
@@ -1304,7 +1317,7 @@ out:
 		mutex_unlock(&current->signal->cred_guard_mutex);
 out_put_fd:
 	if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
-		if (ret < 0) {
+		if (ret) {
 			listener_f->private_data = NULL;
 			fput(listener_f);
 			put_unused_fd(listener);