8350869: os::stat doesn't follow symlinks on Windows

Reviewed-by: dholmes, mgronlun

src/hotspot/os/windows/os_windows.cpp

@@ -4626,6 +4626,63 @@ static void set_path_prefix(char* buf, LPCWSTR* prefix, int* prefix_off, bool* n
   }
 }
 
+// This method checks if a wide path is actually a symbolic link
+static bool is_symbolic_link(const wchar_t* wide_path) {
+  WIN32_FIND_DATAW fd;
+  HANDLE f = ::FindFirstFileW(wide_path, &fd);
+  if (f != INVALID_HANDLE_VALUE) {
+    const bool result = fd.dwFileAttributes & FILE_ATTRIBUTE_REPARSE_POINT && fd.dwReserved0 == IO_REPARSE_TAG_SYMLINK;
+    if (::FindClose(f) == 0) {
+      errno = ::GetLastError();
+      log_debug(os)("is_symbolic_link() failed to FindClose: GetLastError->%ld.", errno);
+    }
+    return result;
+  } else {
+    errno = ::GetLastError();
+    log_debug(os)("is_symbolic_link() failed to FindFirstFileW: GetLastError->%ld.", errno);
+    return false;
+  }
+}
+
+// This method dereferences a symbolic link
+static WCHAR* get_path_to_target(const wchar_t* wide_path) {
+  HANDLE hFile = CreateFileW(wide_path, GENERIC_READ, FILE_SHARE_READ, nullptr,
+                             OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
+  if (hFile == INVALID_HANDLE_VALUE) {
+    errno = ::GetLastError();
+    log_debug(os)("get_path_to_target() failed to CreateFileW: GetLastError->%ld.", errno);
+    return nullptr;
+  }
+
+  // Returned value includes the terminating null character.
+  const size_t target_path_size = ::GetFinalPathNameByHandleW(hFile, nullptr, 0,
+                                                              FILE_NAME_NORMALIZED);
+  if (target_path_size == 0) {
+    errno = ::GetLastError();
+    log_debug(os)("get_path_to_target() failed to GetFinalPathNameByHandleW: GetLastError->%ld.", errno);
+    return nullptr;
+  }
+
+  WCHAR* path_to_target = NEW_C_HEAP_ARRAY(WCHAR, target_path_size, mtInternal);
+
+  // The returned size is the length excluding the terminating null character.
+  const size_t res = ::GetFinalPathNameByHandleW(hFile, path_to_target, static_cast<DWORD>(target_path_size),
+                                                 FILE_NAME_NORMALIZED);
+  if (res != target_path_size - 1) {
+    errno = ::GetLastError();
+    log_debug(os)("get_path_to_target() failed to GetFinalPathNameByHandleW: GetLastError->%ld.", errno);
+    return nullptr;
+  }
+
+  if (::CloseHandle(hFile) == 0) {
+    errno = ::GetLastError();
+    log_debug(os)("get_path_to_target() failed to CloseHandle: GetLastError->%ld.", errno);
+    return nullptr;
+  }
+
+  return path_to_target;
+}
+
 // Returns the given path as an absolute wide path in unc format. The returned path is null
 // on error (with err being set accordingly) and should be freed via os::free() otherwise.
 // additional_space is the size of space, in wchar_t, the function will additionally add to
@@ -4694,15 +4751,35 @@ int os::stat(const char *path, struct stat *sbuf) {
     return -1;
   }
 
-  WIN32_FILE_ATTRIBUTE_DATA file_data;;
-  BOOL bret = ::GetFileAttributesExW(wide_path, GetFileExInfoStandard, &file_data);
-  os::free(wide_path);
+  const bool is_symlink = is_symbolic_link(wide_path);
+  WCHAR* path_to_target = nullptr;
 
+  if (is_symlink) {
+    path_to_target = get_path_to_target(wide_path);
+    if (path_to_target == nullptr) {
+      // it is a symbolic link, but we failed to resolve it,
+      // errno has been set in the call to get_path_to_target(),
+      // no need to overwrite it
+      os::free(wide_path);
+      return -1;
+    }
+  }
+
+  WIN32_FILE_ATTRIBUTE_DATA file_data;;
+  BOOL bret = ::GetFileAttributesExW(is_symlink ? path_to_target : wide_path, GetFileExInfoStandard, &file_data);
+
+  // if getting attributes failed, GetLastError should be called immediately after that
   if (!bret) {
     errno = ::GetLastError();
+    log_debug(os)("os::stat() failed to GetFileAttributesExW: GetLastError->%ld.", errno);
+    os::free(wide_path);
+    os::free(path_to_target);
     return -1;
   }
 
+  os::free(wide_path);
+  os::free(path_to_target);
+
   file_attribute_data_to_stat(sbuf, file_data);
   return 0;
 }
@@ -4887,12 +4964,30 @@ int os::open(const char *path, int oflag, int mode) {
     errno = err;
     return -1;
   }
-  int fd = ::_wopen(wide_path, oflag | O_BINARY | O_NOINHERIT, mode);
-  os::free(wide_path);
 
+  const bool is_symlink = is_symbolic_link(wide_path);
+  WCHAR* path_to_target = nullptr;
+
+  if (is_symlink) {
+    path_to_target = get_path_to_target(wide_path);
+    if (path_to_target == nullptr) {
+      // it is a symbolic link, but we failed to resolve it,
+      // errno has been set in the call to get_path_to_target(),
+      // no need to overwrite it
+      os::free(wide_path);
+      return -1;
+    }
+  }
+
+  int fd = ::_wopen(is_symlink ? path_to_target : wide_path, oflag | O_BINARY | O_NOINHERIT, mode);
+
+  // if opening files failed, GetLastError should be called immediately after that
   if (fd == -1) {
     errno = ::GetLastError();
+    log_debug(os)("os::open() failed to _wopen: GetLastError->%ld.", errno);
   }
+  os::free(wide_path);
+  os::free(path_to_target);
 
   return fd;
 }

test/hotspot/jtreg/runtime/LoadClass/TestSymlinkLoad.java

@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @summary JVM should be able to handle loading class via symlink on windows
+ * @requires vm.flagless
+ * @library /test/lib
+ * @run testng/othervm TestSymlinkLoad
+ */
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import jdk.test.lib.compiler.CompilerUtils;
+import jdk.test.lib.process.OutputAnalyzer;
+import jdk.test.lib.process.ProcessTools;
+import jdk.test.lib.util.FileUtils;
+import org.testng.SkipException;
+import org.testng.annotations.Test;
+
+public class TestSymlinkLoad {
+
+    @Test
+    public void testSymlinkClassLoading() throws Exception {
+        Path sourceDir = Paths.get(System.getProperty("test.src"), "test-classes");
+
+        String subPath = "compiled";
+        Path destDir = Paths.get(System.getProperty("test.classes"), subPath);
+
+        CompilerUtils.compile(sourceDir, destDir);
+
+        String bootCP = "-Xbootclasspath/a:" + destDir.toString();
+
+        String className = "Hello";
+
+        // try to load a class itself directly, i.e. not via a symlink
+        ProcessBuilder pb = ProcessTools.createLimitedTestJavaProcessBuilder(
+                bootCP, className);
+
+        // make sure it runs as expected
+        OutputAnalyzer output = new OutputAnalyzer(pb.start());
+        output.shouldContain("Hello World")
+              .shouldHaveExitValue(0);
+
+        // create a symlink to the classfile in a subdir with a given name
+        Path classFile = Path.of(destDir + File.separator + className + ".class");
+        final String subdir = "remote";
+        final String pathToFolderForSymlink = destDir + File.separator + subdir + File.separator;
+        createLinkInSeparateFolder(pathToFolderForSymlink, classFile, className);
+
+        // try to load class via its symlink, which is in a different directory
+        pb = ProcessTools.createLimitedTestJavaProcessBuilder(
+                bootCP + File.separator + subdir, className);
+        output = new OutputAnalyzer(pb.start());
+        output.shouldContain("Hello World")
+              .shouldHaveExitValue(0);
+
+        // remove the subdir
+        FileUtils.deleteFileTreeWithRetry(Path.of(pathToFolderForSymlink));
+    }
+
+    public static void createLinkInSeparateFolder(final String pathToFolderForSymlink, final Path target, final String className) throws IOException {
+        File theDir = new File(pathToFolderForSymlink);
+        if (!theDir.exists()) {
+            theDir.mkdirs();
+        }
+        Path link = Paths.get(pathToFolderForSymlink, className + ".class");
+        if (Files.exists(link)) {
+            Files.delete(link);
+        }
+        try {
+            Files.createSymbolicLink(link, target);
+        } catch (UnsupportedOperationException uoe) {
+            throw new SkipException("Symbolic link creation not supported.", uoe);
+        } catch (IOException ioe) {
+            throw new SkipException("Probably insufficient privileges to create symbolic links (Windows)", ioe);
+        }
+    }
+}