path: strip extended-length path prefix in C++ PathResolve and JS win32.resolve

Author: jazelly

lib/fs.js

@@ -132,14 +132,6 @@ const {
 const {
   CHAR_FORWARD_SLASH,
   CHAR_BACKWARD_SLASH,
-  CHAR_COLON,
-  CHAR_QUESTION_MARK,
-  CHAR_UPPERCASE_A,
-  CHAR_UPPERCASE_C,
-  CHAR_UPPERCASE_Z,
-  CHAR_LOWERCASE_A,
-  CHAR_LOWERCASE_N,
-  CHAR_LOWERCASE_Z,
 } = require('internal/constants');
 const {
   isInt32,
@@ -2636,43 +2628,6 @@ function unwatchFile(filename, listener) {
 }
 
 
-// Strips the Windows extended-length path prefix (\\?\) from a resolved path.
-// Extended-length paths (\\?\C:\... or \\?\UNC\...) are a Win32 API mechanism
-// to bypass MAX_PATH limits. Node.js should handle them transparently by
-// converting to standard paths for internal processing. The \\?\ prefix is
-// re-added when needed via path.toNamespacedPath() before system calls.
-// See: https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file
-function stripExtendedPathPrefix(p) {
-  // Check for \\?\ prefix
-  if (p.length >= 4 &&
-      StringPrototypeCharCodeAt(p, 0) === CHAR_BACKWARD_SLASH &&
-      StringPrototypeCharCodeAt(p, 1) === CHAR_BACKWARD_SLASH &&
-      StringPrototypeCharCodeAt(p, 2) === CHAR_QUESTION_MARK &&
-      StringPrototypeCharCodeAt(p, 3) === CHAR_BACKWARD_SLASH) {
-    // \\?\C:\ -> C:\  (extended drive path)
-    if (p.length >= 6) {
-      const drive = StringPrototypeCharCodeAt(p, 4);
-      if (((drive >= CHAR_UPPERCASE_A && drive <= CHAR_UPPERCASE_Z) ||
-           (drive >= CHAR_LOWERCASE_A && drive <= CHAR_LOWERCASE_Z)) &&
-          StringPrototypeCharCodeAt(p, 5) === CHAR_COLON) {
-        return StringPrototypeSlice(p, 4);
-      }
-    }
-    // \\?\UNC\server\share -> \\server\share  (extended UNC path)
-    if (p.length >= 8 &&
-        (StringPrototypeCharCodeAt(p, 4) === 85 /* U */ ||
-         StringPrototypeCharCodeAt(p, 4) === 117 /* u */) &&
-        (StringPrototypeCharCodeAt(p, 5) === 78 /* N */ ||
-         StringPrototypeCharCodeAt(p, 5) === CHAR_LOWERCASE_N) &&
-        (StringPrototypeCharCodeAt(p, 6) === CHAR_UPPERCASE_C ||
-         StringPrototypeCharCodeAt(p, 6) === 99 /* c */) &&
-        StringPrototypeCharCodeAt(p, 7) === CHAR_BACKWARD_SLASH) {
-      return '\\\\' + StringPrototypeSlice(p, 8);
-    }
-  }
-  return p;
-}
-
 let splitRoot;
 if (isWindows) {
   // Regex to find the device root on Windows (e.g. 'c:\\'), including trailing
@@ -2735,12 +2690,6 @@ function realpathSync(p, options) {
   validatePath(p);
   p = pathModule.resolve(p);
 
-  // On Windows, strip the extended-length path prefix (\\?\) so that the
-  // path walking logic below works with standard drive-letter or UNC roots.
-  if (isWindows) {
-    p = stripExtendedPathPrefix(p);
-  }
-
   const cache = options[realpathCacheKey];
   const maybeCachedResult = cache?.get(p);
   if (maybeCachedResult) {
@@ -2844,11 +2793,6 @@ function realpathSync(p, options) {
     // Resolve the link, then start over
     p = pathModule.resolve(resolvedLink, StringPrototypeSlice(p, pos));
 
-    // Strip extended path prefix again in case pathModule.resolve re-added it
-    if (isWindows) {
-      p = stripExtendedPathPrefix(p);
-    }
-
     // Skip over roots
     current = base = splitRoot(p);
     pos = current.length;
@@ -2907,12 +2851,6 @@ function realpath(p, options, callback) {
   validatePath(p);
   p = pathModule.resolve(p);
 
-  // On Windows, strip the extended-length path prefix (\\?\) so that the
-  // path walking logic below works with standard drive-letter or UNC roots.
-  if (isWindows) {
-    p = stripExtendedPathPrefix(p);
-  }
-
   const seenLinks = new SafeMap();
   const knownHard = new SafeSet();
 
@@ -3013,12 +2951,6 @@ function realpath(p, options, callback) {
   function gotResolvedLink(resolvedLink) {
     // Resolve the link, then start over
     p = pathModule.resolve(resolvedLink, StringPrototypeSlice(p, pos));
-
-    // Strip extended path prefix again in case pathModule.resolve re-added it
-    if (isWindows) {
-      p = stripExtendedPathPrefix(p);
-    }
-
     current = base = splitRoot(p);
     pos = current.length;
 

lib/path.js

@@ -230,10 +230,36 @@ const win32 = {
         }
       }
 
-      const len = path.length;
       let rootEnd = 0;
       let device = '';
       let isAbsolute = false;
+
+      // Strip extended-length path prefix (\\?\C:\... -> C:\...,
+      // \\?\UNC\... -> \\...) before processing.
+      if (path.length >= 6 &&
+          StringPrototypeCharCodeAt(path, 0) === CHAR_BACKWARD_SLASH &&
+          StringPrototypeCharCodeAt(path, 1) === CHAR_BACKWARD_SLASH &&
+          StringPrototypeCharCodeAt(path, 2) === CHAR_QUESTION_MARK &&
+          StringPrototypeCharCodeAt(path, 3) === CHAR_BACKWARD_SLASH) {
+        const drive = StringPrototypeCharCodeAt(path, 4);
+        if (isWindowsDeviceRoot(drive) &&
+            StringPrototypeCharCodeAt(path, 5) === CHAR_COLON) {
+          // \\?\C:\ -> C:\
+          path = StringPrototypeSlice(path, 4);
+        } else if (path.length >= 8 &&
+                   (drive === 85 || drive === 117) &&  // U/u
+                   (StringPrototypeCharCodeAt(path, 5) === 78 ||
+                    StringPrototypeCharCodeAt(path, 5) === 110) &&  // N/n
+                   (StringPrototypeCharCodeAt(path, 6) === 67 ||
+                    StringPrototypeCharCodeAt(path, 6) === 99) &&  // C/c
+                   StringPrototypeCharCodeAt(path, 7) ===
+                     CHAR_BACKWARD_SLASH) {
+          // \\?\UNC\server\share -> \\server\share
+          path = `\\\\${StringPrototypeSlice(path, 8)}`;
+        }
+      }
+
+      const len = path.length;
       const code = StringPrototypeCharCodeAt(path, 0);
 
       // Try to match a root

src/path.cc

@@ -98,6 +98,28 @@ constexpr bool IsWindowsDeviceRoot(const char c) noexcept {
   return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
 }
 
+// Strip Windows extended-length path prefix (\\?\) only when it wraps a
+// drive letter path (\\?\C:\...) or a UNC path (\\?\UNC\...).
+// Device paths like \\?\PHYSICALDRIVE0 are left unchanged.
+// See: https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file
+static void StripExtendedPathPrefix(std::string& path) {
+  if (path.size() >= 4 && path[0] == '\\' && path[1] == '\\' &&
+      path[2] == '?' && path[3] == '\\') {
+    // \\?\C:\ -> C:\ (extended drive path)
+    if (path.size() >= 6 && IsWindowsDeviceRoot(path[4]) && path[5] == ':') {
+      path = path.substr(4);
+      return;
+    }
+    // \\?\UNC\server\share -> \\server\share (extended UNC path)
+    if (path.size() >= 8 && ToLower(path[4]) == 'u' &&
+        ToLower(path[5]) == 'n' && ToLower(path[6]) == 'c' &&
+        path[7] == '\\') {
+      path = "\\\\" + path.substr(8);
+      return;
+    }
+  }
+}
+
 std::string PathResolve(Environment* env,
                         const std::vector<std::string_view>& paths) {
   std::string resolvedDevice = "";
@@ -132,6 +154,10 @@ std::string PathResolve(Environment* env,
       }
     }
 
+    // Strip extended-length path prefix (\\?\C:\... -> C:\...,
+    // \\?\UNC\... -> \\...) before processing.
+    StripExtendedPathPrefix(path);
+
     const size_t len = path.length();
     int rootEnd = 0;
     std::string device = "";

test/cctest/test_path.cc

@@ -43,6 +43,14 @@ TEST_F(PathTest, PathResolve) {
             "\\\\.\\PHYSICALDRIVE0");
   EXPECT_EQ(PathResolve(*env, {"\\\\?\\PHYSICALDRIVE0"}),
             "\\\\?\\PHYSICALDRIVE0");
+  // Extended-length path prefix (\\?\) should be stripped for drive paths
+  EXPECT_EQ(PathResolve(*env, {"\\\\?\\C:\\foo"}), "C:\\foo");
+  EXPECT_EQ(PathResolve(*env, {"\\\\?\\C:\\"}), "C:\\");
+  // Extended-length UNC path prefix (\\?\UNC\) should be stripped
+  EXPECT_EQ(PathResolve(*env, {"\\\\?\\UNC\\server\\share"}),
+            "\\\\server\\share\\");
+  EXPECT_EQ(PathResolve(*env, {"\\\\?\\UNC\\server\\share\\dir"}),
+            "\\\\server\\share\\dir");
 #else
   EXPECT_EQ(PathResolve(*env, {"/var/lib", "../", "file/"}), "/var/file");
   EXPECT_EQ(PathResolve(*env, {"/var/lib", "/../", "file/"}), "/file");

test/parallel/test-fs-realpath-extended-windows-path.js

@@ -37,6 +37,12 @@ const resolveTests = [
       'C:\\foo\\tmp.3\\cycles\\root.js'],
      [['\\\\.\\PHYSICALDRIVE0'], '\\\\.\\PHYSICALDRIVE0'],
      [['\\\\?\\PHYSICALDRIVE0'], '\\\\?\\PHYSICALDRIVE0'],
+     // Extended-length path prefix (\\?\) should be stripped for drive paths
+     [['\\\\?\\C:\\foo'], 'C:\\foo'],
+     [['\\\\?\\C:\\'], 'C:\\'],
+     // Extended-length UNC path prefix (\\?\UNC\) should be stripped
+     [['\\\\?\\UNC\\server\\share'], '\\\\server\\share\\'],
+     [['\\\\?\\UNC\\server\\share\\dir'], '\\\\server\\share\\dir'],
     ],
   ],
   [ path.posix.resolve,