Bug 1891690: Return GMT when RFPTarget::JSDateTimeUTC is enabled. r=timhuang

Differential Revision: https://phabricator.services.mozilla.com/D216411

browser/components/resistfingerprinting/test/browser/browser.toml

@@ -196,3 +196,5 @@ lineno = "172"
 
 ["browser_timezone.js"]
 lineno = "176"
+
+["browser_exslt_timezone_load.js"]

browser/components/resistfingerprinting/test/browser/browser_exslt_timezone_load.js

@@ -0,0 +1,62 @@
+/**
+ * Bug 1891690 - A test case for verifying EXSLT date will use Atlantic/Reykjavik
+ *               timezone (GMT and "real" equivalent to UTC) after fingerprinting
+ *               resistance is enabled.
+ */
+
+function getTimeZone(tab) {
+  const extractTime = function () {
+    const xslText = `
+    <xsl:stylesheet version="1.0"
+                    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
+                    xmlns:date="http://exslt.org/dates-and-times"
+                    extension-element-prefixes="date">
+      <xsl:output method="text" />
+      <xsl:template match="/">
+        <xsl:value-of select="date:date-time()" />
+      </xsl:template>
+    </xsl:stylesheet>`;
+
+    const parser = new DOMParser();
+    const xsltProcessor = new XSLTProcessor();
+    const xslStylesheet = parser.parseFromString(xslText, "application/xml");
+    xsltProcessor.importStylesheet(xslStylesheet);
+    const xmlDoc = parser.parseFromString("<test />", "application/xml");
+    const styledDoc = xsltProcessor.transformToDocument(xmlDoc);
+    const time = styledDoc.firstChild.textContent;
+
+    return time;
+  };
+
+  const extractTimeExpr = `(${extractTime.toString()})();`;
+
+  return SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [extractTimeExpr],
+    async funccode => content.eval(funccode)
+  );
+}
+
+add_task(async function test_new_window() {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["privacy.fingerprintingProtection", true],
+      ["privacy.fingerprintingProtection.overrides", "+JSDateTimeUTC"],
+    ],
+  });
+
+  // Open a tab for extracting the time zone from XSLT.
+  const tab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: TEST_PATH + "file_dummy.html",
+    forceNewProcess: true,
+  });
+
+  SpecialPowers.Cu.getJSTestingFunctions().setTimeZone("America/Toronto");
+  const timeZone = await getTimeZone(tab);
+
+  ok(timeZone.endsWith("+00:00"), "Timezone was spoofed.");
+
+  BrowserTestUtils.removeTab(tab);
+  await SpecialPowers.popPrefEnv();
+});

dom/xslt/xslt/txEXSLTFunctions.cpp

@@ -590,7 +590,15 @@ nsresult txEXSLTFunctionCall::evaluate(txIEvalContext* aContext,
       // http://exslt.org/date/functions/date-time/
 
       PRExplodedTime prtime;
-      PR_ExplodeTime(PR_Now(), PR_LocalTimeParameters, &prtime);
+      Document* sourceDoc = getSourceDocument(aContext);
+      NS_ENSURE_STATE(sourceDoc);
+
+      PR_ExplodeTime(
+          PR_Now(),
+          sourceDoc->ShouldResistFingerprinting(RFPTarget::JSDateTimeUTC)
+              ? PR_GMTParameters
+              : PR_LocalTimeParameters,
+          &prtime);
 
       int32_t offset =
           (prtime.tm_params.tp_gmt_offset + prtime.tm_params.tp_dst_offset) /
@@ -634,7 +642,7 @@ Expr::ResultType txEXSLTFunctionCall::getReturnType() {
 
 bool txEXSLTFunctionCall::isSensitiveTo(ContextSensitivity aContext) {
   if (mType == txEXSLTType::NODE_SET || mType == txEXSLTType::SPLIT ||
-      mType == txEXSLTType::TOKENIZE) {
+      mType == txEXSLTType::TOKENIZE || mType == txEXSLTType::DATE_TIME) {
     return (aContext & PRIVATE_CONTEXT) || argsSensitiveTo(aContext);
   }
   return argsSensitiveTo(aContext);