Bug 1912129: Reduce time precision for EXSLT date time function. r=timhuang

Differential Revision: https://phabricator.services.mozilla.com/D218783

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

@@ -198,3 +198,5 @@ lineno = "172"
 lineno = "176"
 
 ["browser_exslt_timezone_load.js"]
+
+["browser_exslt_time_precision.js"]

browser/components/resistfingerprinting/test/browser/browser_exslt_time_precision.js

@@ -0,0 +1,71 @@
+/**
+ * Bug 1912129 - A test case for verifying EXSLT date will report second-precise
+ *               time fingerprinting resistance is enabled.
+ */
+
+function getTime(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", "+ReduceTimerPrecision"],
+    ],
+  });
+
+  // Open a tab for extracting the time from XSLT.
+  const tab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: TEST_PATH + "file_dummy.html",
+    forceNewProcess: true,
+  });
+
+  for (let i = 0; i < 10; i++) {
+    // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
+    await new Promise(res => setTimeout(res, 25));
+
+    // The regex could be a lot shorter (e.g. /\.(\d{3})/) but I wrote the whole
+    // thing to make sure the time is in the expected format and to allow us
+    // to re-use this regex in the future if we need to.
+    // Note: Date format is not locale dependent.
+    const regex = /\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.(\d{3})[-+]\d{2}:\d{2}/;
+    const time = await getTime(tab);
+    const [, milliseconds] = time.match(regex);
+
+    is(milliseconds, "000", "Date's precision was reduced to seconds.");
+  }
+
+  BrowserTestUtils.removeTab(tab);
+  await SpecialPowers.popPrefEnv();
+});

dom/xslt/xslt/txEXSLTFunctions.cpp

@@ -593,12 +593,19 @@ nsresult txEXSLTFunctionCall::evaluate(txIEvalContext* aContext,
       Document* sourceDoc = getSourceDocument(aContext);
       NS_ENSURE_STATE(sourceDoc);
 
-      PR_ExplodeTime(
-          PR_Now(),
+      PRTimeParamFn timezone =
           sourceDoc->ShouldResistFingerprinting(RFPTarget::JSDateTimeUTC)
               ? PR_GMTParameters
-              : PR_LocalTimeParameters,
-          &prtime);
+              : PR_LocalTimeParameters;
+
+      PRTime time =
+          sourceDoc->ShouldResistFingerprinting(RFPTarget::ReduceTimerPrecision)
+              ? (PRTime)nsRFPService::ReduceTimePrecisionAsSecs(
+                    (double)PR_Now() / PR_USEC_PER_SEC, 0,
+                    RTPCallerType::ResistFingerprinting) *
+                    PR_USEC_PER_SEC
+              : PR_Now();
+      PR_ExplodeTime(time, timezone, &prtime);
 
       int32_t offset =
           (prtime.tm_params.tp_gmt_offset + prtime.tm_params.tp_dst_offset) /