From 7b9a6850a3fc11b22d202229edd6a9af8df9d3b3 Mon Sep 17 00:00:00 2001
From: Kaspar Schleiser <kaspar@schleiser.de>
Date: Thu, 9 Dec 2021 13:40:21 +0100
Subject: [PATCH 1/2] sys/ztimer64: make _del_entry_from_list() safe for uninit
 ztimer64_t

---
 sys/ztimer64/ztimer64.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/sys/ztimer64/ztimer64.c b/sys/ztimer64/ztimer64.c
index 71baea29c8..4e1021c9e5 100644
--- a/sys/ztimer64/ztimer64.c
+++ b/sys/ztimer64/ztimer64.c
@@ -128,6 +128,9 @@ static int _add_entry_to_list(ztimer64_clock_t *clock, ztimer64_base_t *entry)
 static int _del_entry_from_list(ztimer64_clock_t *clock, ztimer64_base_t *entry)
 {
     DEBUG("_del_entry_from_list()\n");
+    if (!clock->first) {
+        return 0;
+    }
     assert(_is_set((ztimer64_t *)entry));
     if (clock->first == entry) {
         /* special case: removing first entry */

From 835db7d97f072e1e8b5e5ae2b612ae8a0c012390 Mon Sep 17 00:00:00 2001
From: Kaspar Schleiser <kaspar@schleiser.de>
Date: Thu, 9 Dec 2021 14:10:16 +0100
Subject: [PATCH 2/2] tests/unittests: add ztimer64 regression test

---
 .../tests-ztimer64/tests-ztimer64-core.c      | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/tests/unittests/tests-ztimer64/tests-ztimer64-core.c b/tests/unittests/tests-ztimer64/tests-ztimer64-core.c
index f5fd837085..a494bdbd4b 100644
--- a/tests/unittests/tests-ztimer64/tests-ztimer64-core.c
+++ b/tests/unittests/tests-ztimer64/tests-ztimer64-core.c
@@ -198,6 +198,24 @@ static void test_ztimer64_checkpoint(void)
     TEST_ASSERT_EQUAL_INT(2 * ZTIMER64_CHECKPOINT_INTERVAL + UINT32_MAX, now64);
 }
 
+static void test_ztimer64_set_uninitialized(void)
+{
+    /* regression test for setting an uninitialized ztimer64 on an empty clock */
+    ztimer_mock_t zmock;
+    ztimer_clock_t *z = &zmock.super;
+    ztimer64_clock_t z64mock;
+    ztimer64_clock_t *z64 = &z64mock;
+
+    memset(&zmock, '\0', sizeof(ztimer_mock_t));
+    memset(&z64mock, '\0', sizeof(ztimer64_clock_t));
+    /* ztimer base clock is already extended to 32bit */
+    ztimer_mock_init(&zmock, 32);
+    ztimer64_clock_init(z64, z);
+
+    ztimer64_t timer = { .base.target = 1 };
+    ztimer64_set(z64, &timer, 0);
+}
+
 Test *tests_ztimer64_tests(void)
 {
     EMB_UNIT_TESTFIXTURES(fixtures) {
@@ -206,6 +224,7 @@ Test *tests_ztimer64_tests(void)
         new_TestFixture(test_ztimer64_set_0),
         new_TestFixture(test_ztimer64_set_at),
         new_TestFixture(test_ztimer64_checkpoint),
+        new_TestFixture(test_ztimer64_set_uninitialized),
     };
 
     EMB_UNIT_TESTCALLER(ztimer64_tests, setup, NULL, fixtures);