[settings] Allow system time to be modified via builtin/unixtime

Allow the system time offset to be modified by writing a new time
value to builtin/unixtime, e.g.:

  set builtin/unixtime 0x10d1a884

As with the NTP client, this does not attempt to write to the
underlying clock source (e.g. the RTC clock).  Only the internal
system time offset is updated.

Any system time offset may be reset by clearing the setting:

  clear builtin/unixtime

This will reset the system time offset to zero and so can be used to
undo the effect of a previous "set builtin/unixtime" or "ntp" command.

Requested-by: Christian I. Nilsson <ChristianN@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/settings.c

@@ -2630,13 +2630,42 @@ static int unixtime_fetch ( void *data, size_t len ) {
 	uint32_t content;
 
 	/* Return current time */
-	content = htonl ( time(NULL) );
+	content = htonl ( time ( NULL ) );
 	if ( len > sizeof ( content ) )
 		len = sizeof ( content );
 	memcpy ( data, &content, len );
 	return sizeof ( content );
 }
 
+/**
+ * Store current time setting
+ *
+ * @v data		Setting data, or NULL to clear setting
+ * @v len		Length of setting data
+ * @ret rc		Return status code
+ */
+static int unixtime_store ( const void *data, size_t len ) {
+	unsigned long value;
+	int check_len;
+	int rc;
+
+	/* Get intended current time */
+	check_len = numeric_setting_value ( 0, data, len, &value );
+	if ( check_len < 0 ) {
+		rc = check_len;
+		return rc;
+	}
+
+	/* Adjust or restore system clock, as applicable */
+	if ( len ) {
+		time_adjust ( value - time ( NULL ) );
+	} else {
+		time_restore();
+	}
+
+	return 0;
+}
+
 /** Current time setting */
 const struct setting unixtime_setting __setting ( SETTING_MISC, unixtime ) = {
 	.name = "unixtime",
@@ -2648,6 +2677,7 @@ const struct setting unixtime_setting __setting ( SETTING_MISC, unixtime ) = {
 /** Current time built-in setting */
 struct builtin_setting unixtime_builtin_setting __builtin_setting = {
 	.setting = &unixtime_setting,
+	.store = unixtime_store,
 	.fetch = unixtime_fetch,
 };
 

src/include/ipxe/time.h

@@ -71,4 +71,13 @@ time_adjust ( signed long delta ) {
 	time_offset += delta;
 }
 
+/**
+ * Clear system clock adjustment
+ *
+ */
+static inline __attribute__ (( always_inline )) void time_restore ( void ) {
+
+	time_offset = 0;
+}
+
 #endif /* _IPXE_TIME_H */