fix: account for DST in XCCDF result timestamp offset

CMakeLists.txt

@@ -213,6 +213,12 @@ check_function_exists(fts_open HAVE_FTS_OPEN)
 check_function_exists(strsep HAVE_STRSEP)
 check_function_exists(strptime HAVE_STRPTIME)
 
+# struct tm.tm_gmtoff is a BSD/GNU extension (present on glibc, musl, the BSDs
+# and macOS, absent on the MSVC runtime). When available it carries the UTC
+# offset including daylight saving time, unlike the global timezone variable.
+include(CheckStructHasMember)
+check_struct_has_member("struct tm" tm_gmtoff time.h HAVE_STRUCT_TM_TM_GMTOFF)
+
 check_include_file(syslog.h HAVE_SYSLOG_H)
 check_include_file(stdio_ext.h HAVE_STDIO_EXT_H)
 check_include_file(shadow.h HAVE_SHADOW_H)

config.h.in

@@ -85,6 +85,7 @@
 #cmakedefine HAVE_STRSEP
 #cmakedefine HAVE_FLOCK
 #cmakedefine HAVE_STRPTIME
+#cmakedefine HAVE_STRUCT_TM_TM_GMTOFF
 
 #cmakedefine OPENSCAP_PROBE_INDEPENDENT_ENVIRONMENTVARIABLE
 #cmakedefine OPENSCAP_PROBE_INDEPENDENT_ENVIRONMENTVARIABLE58

src/XCCDF/result.c

@@ -1842,7 +1842,10 @@ static inline const char *_get_timestamp(void)
 	if (!lt)
 		return NULL;
 
-#if defined(OS_FREEBSD)
+#if defined(HAVE_STRUCT_TM_TM_GMTOFF)
+	/* tm_gmtoff is the offset east of UTC in seconds and already accounts
+	 * for daylight saving time, so it reflects the actual offset in effect
+	 * at the represented local time. */
 	tz_diff = lt->tm_gmtoff;
 
 	if (tz_diff < 0) {
@@ -1851,20 +1854,19 @@ static inline const char *_get_timestamp(void)
 	} else {
 		tz_sign = '+';
 	}
-
-        /*  glibc's timezone offset does not account for daylight savings time.
-         *  So we match that behavior here by adding 3600 seconds
-         */
-        if (lt->tm_isdst)
-		tz_diff += 3600;
 #else
-	/* timezone is a global variable set by localtime(3) */
-	if (timezone <= 0) {
+	/* Fallback for platforms without tm_gmtoff (e.g. the MSVC runtime).
+	 * timezone is a global variable set by localtime(3) holding the offset
+	 * of standard time; add an hour when daylight saving time is in effect. */
+	long std_diff = timezone;
+	if (lt->tm_isdst > 0)
+		std_diff -= 3600;
+	if (std_diff <= 0) {
 		tz_sign = '+';
-		tz_diff = -timezone;
+		tz_diff = -std_diff;
 	} else {
 		tz_sign = '-';
-		tz_diff = timezone;
+		tz_diff = std_diff;
 	}
 #endif
 	tz_diff /= 60;