extmod/utime_mphal: Make ticks_add check for overflow of delta.

Work done in collaboration with @jimmo.

Signed-off-by: Damien George <damien@micropython.org>

extmod/utime_mphal.c

@@ -95,6 +95,19 @@ STATIC mp_obj_t time_ticks_add(mp_obj_t ticks_in, mp_obj_t delta_in) {
     // we assume that first argument come from ticks_xx so is small int
     mp_uint_t ticks = MP_OBJ_SMALL_INT_VALUE(ticks_in);
     mp_uint_t delta = mp_obj_get_int(delta_in);
+
+    // Check that delta does not overflow the range that ticks_diff can handle.
+    // This ensures the following:
+    //  - ticks_diff(ticks_add(T, delta), T) == delta
+    //  - ticks_diff(T, ticks_add(T, delta)) == -delta
+    // The latter requires excluding delta=-TICKS_PERIOD/2.
+    //
+    // This unsigned comparison is equivalent to a signed comparison of:
+    //   delta <= TICKS_PERIOD/2 || delta >= TICKS_PERIOD/2
+    if (delta + MICROPY_PY_UTIME_TICKS_PERIOD / 2 - 1 >= MICROPY_PY_UTIME_TICKS_PERIOD - 1) {
+        mp_raise_msg(&mp_type_OverflowError, MP_ERROR_TEXT("ticks interval overflow"));
+    }
+
     return MP_OBJ_NEW_SMALL_INT((ticks + delta) & (MICROPY_PY_UTIME_TICKS_PERIOD - 1));
 }
 MP_DEFINE_CONST_FUN_OBJ_2(mp_utime_ticks_add_obj, time_ticks_add);

tests/extmod/ticks_add.py

@@ -0,0 +1,42 @@
+try:
+    from utime import ticks_diff, ticks_add
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+# Maximum value returned from ticks_add, ticks_ms, etc.
+TICKS_MAX = ticks_add(0, -1)
+# Maximum value returned from ticks_diff.
+TICKS_INTERVAL_MAX = TICKS_MAX // 2
+
+# Invariants:
+# - ticks_diff(ticks_add(T, delta), T) == delta
+# - ticks_diff(T, ticks_add(T, delta)) == -delta
+
+# Check actual values of ticks_add.
+print(ticks_add(20, 12))
+print(ticks_add(20, -12))
+
+# Check invariant.
+print(ticks_diff(ticks_add(100, 123), 100))
+print(ticks_diff(ticks_add(100, -123), 100))
+print(ticks_diff(100, ticks_add(100, 123)))
+print(ticks_diff(100, ticks_add(100, -123)))
+
+# Check limits.
+for T in (0, 10, TICKS_MAX):
+    for delta in (
+        -TICKS_INTERVAL_MAX - 1,
+        -TICKS_INTERVAL_MAX,
+        0,
+        TICKS_INTERVAL_MAX,
+        TICKS_INTERVAL_MAX + 1,
+    ):
+        try:
+            print(ticks_diff(ticks_add(T, delta), T) == delta)
+        except OverflowError:
+            print("OverflowError")
+        try:
+            print(ticks_diff(T, ticks_add(T, delta)) == -delta)
+        except OverflowError:
+            print("OverflowError")

tests/extmod/ticks_add.py.exp

@@ -0,0 +1,36 @@
+32
+8
+123
+-123
+-123
+123
+OverflowError
+OverflowError
+True
+True
+True
+True
+True
+True
+OverflowError
+OverflowError
+OverflowError
+OverflowError
+True
+True
+True
+True
+True
+True
+OverflowError
+OverflowError
+OverflowError
+OverflowError
+True
+True
+True
+True
+True
+True
+OverflowError
+OverflowError