icaltime: Fails to convert time to time_t before epoch

The time_t of a time before epoch is a negative number, but the code
considered a negative number as an incorrectly entered time.

ReleaseNotes.txt

@@ -9,6 +9,7 @@ Version 3.0.19 (UNRELEASED):
  * Improved libicu discrovery on Mac with homebrew
  * Properly set DYLD_LIBRARY_PATH on Mac for libical-ical tests
  * Resolved known limitation: Negative values are now also supported for `BYMONTHDAY` and `BYYEARDAY`.
+ * Fix time conversion to time_t for times before epoch
 
 Version 3.0.18 (31 March 2024):
 -------------------------------

src/libical/icaltime.c

@@ -80,8 +80,11 @@ static int icaltime_leap_days(int y1, int y2)
  *  in that we don't want the automatic adjustments for
  *  local daylight savings time applied to the result.
  *  This function expects well-formed input.
+ *
+ *  The out_time_t is to store the result, it can be NULL.
+ *  Returns 0 on failure, 1 on success.
  */
-static time_t make_time(const struct tm *tm, int tzm)
+static int make_time(const struct tm *tm, int tzm, time_t *out_time_t)
 {
     time_t tim;
     int febs;
@@ -91,29 +94,29 @@ static time_t make_time(const struct tm *tm, int tzm)
     /* check that month specification within range */
 
     if (tm->tm_mon < 0 || tm->tm_mon > 11)
-        return ((time_t) - 1);
+        return 0;
 
     if (tm->tm_year < 2)
-        return ((time_t)-1);
+        return 0;
 
 #if (SIZEOF_TIME_T == 4)
     /* check that year specification within range */
 
     if (tm->tm_year > 138)
-        return ((icaltime_t)-1);
+        return 0;
 
     /* check for upper bound of Jan 17, 2038 (to avoid possibility of 32-bit arithmetic overflow) */
     if (tm->tm_year == 138) {
         if (tm->tm_mon > 0) {
-            return ((time_t) - 1);
+            return 0;
         } else if (tm->tm_mday > 17) {
-            return ((time_t) - 1);
+            return 0;
         }
     }
 #else
     /* We don't support years >= 10000, because the function has not been tested at this range. */
     if (tm->tm_year >= 8100) {
-        return ((time_t)-1);
+        return 0;
     }
 #endif /* SIZEOF_TIME_T */
 
@@ -162,7 +165,10 @@ static time_t make_time(const struct tm *tm, int tzm)
 
     /* return number of seconds since start of the epoch */
 
-    return (tim);
+    if (out_time_t)
+        *out_time_t = tim;
+
+    return 1;
 }
 
 /*
@@ -177,7 +183,7 @@ static time_t icaltime_timegm(const struct tm *tm)
     time_t seconds;
 
     /* Validate the tm structure by passing it through make_time() */
-    if (make_time(tm, 0) < 0) {
+    if (!make_time(tm, 0, NULL)) {
         /* we have some invalid data in the tm struct */
         return 0;
     }
@@ -249,7 +255,7 @@ struct icaltimetype icaltime_today(void)
 time_t icaltime_as_timet(const struct icaltimetype tt)
 {
     struct tm stm;
-    time_t t;
+    time_t t = (time_t)-1;
 
     /* If the time is the special null time, return 0. */
     if (icaltime_is_null_time(tt)) {
@@ -272,7 +278,8 @@ time_t icaltime_as_timet(const struct icaltimetype tt)
     stm.tm_year = tt.year - 1900;
     stm.tm_isdst = -1;
 
-    t = make_time(&stm, 0);
+    if (!make_time(&stm, 0, &t))
+        t = ((time_t)-1);
 
     return t;
 }

src/test/regression.c

@@ -2012,6 +2012,27 @@ void do_test_time(const char *zone)
     int_is("icaltime_compare(): same UTC and NY",
            icaltime_compare(icttutc, icttny),
            0);
+
+    /* Conversion to time_t around the epoch */
+    ictt = icaltime_from_string("19691231T235958Z");
+    tt = icaltime_as_timet_with_zone(ictt, utczone);
+    int_is("convert to time_t EPOCH-2", (int)tt, -2);
+
+    ictt = icaltime_from_string("19691231T235959Z");
+    tt = icaltime_as_timet_with_zone(ictt, utczone);
+    int_is("convert to time_t EPOCH-1", (int)tt, -1);
+
+    ictt = icaltime_from_string("19700101T000000Z");
+    tt = icaltime_as_timet_with_zone(ictt, utczone);
+    int_is("convert to time_t EPOCH", (int)tt, 0);
+
+    ictt = icaltime_from_string("19700101T000001Z");
+    tt = icaltime_as_timet_with_zone(ictt, utczone);
+    int_is("convert to time_t EPOCH+1", (int)tt, 1);
+
+    ictt = icaltime_from_string("19700101T000002Z");
+    tt = icaltime_as_timet_with_zone(ictt, utczone);
+    int_is("convert to time_t EPOCH+2", (int)tt, 2);
 }
 
 void test_iterators(void)