src/java.base/windows/classes/sun/nio/fs/WindowsLinkSupport.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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
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 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * 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).
 *
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 * 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
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package sun.nio.fs;

import java.nio.file.*;
import java.io.IOException;
import java.io.IOError;
import jdk.internal.misc.Unsafe;

import static sun.nio.fs.WindowsNativeDispatcher.*;
import static sun.nio.fs.WindowsConstants.*;

/**
 * Utility methods for symbolic link support on Windows Vista and newer.
 */

class WindowsLinkSupport {
    private static final Unsafe unsafe = Unsafe.getUnsafe();

    private WindowsLinkSupport() {
    }

    /**
     * Creates a symbolic link, retyring if not privileged
     */
    static void createSymbolicLink(String link, String target, int flags)
        throws WindowsException
    {
        try {
            CreateSymbolicLink(link, target, flags);
        } catch (WindowsException x) {
            // Retry if the privilege to create symbolic links is not held
            if (x.lastError() == ERROR_PRIVILEGE_NOT_HELD) {
                flags |= SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE;
                try {
                    CreateSymbolicLink(link, target, flags);
                    return;
                } catch (WindowsException y) {
                    // Throw an exception if and only if it is not due to symbolic link creation
                    // privilege not being held (ERROR_PRIVILEGE_NOT_HELD) nor the
                    // SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE flag not being recognized
                    // (ERROR_INVALID_PARAMETER). The latter will occur for Windows builds
                    // older than 14972.
                    int lastError = y.lastError();
                    if (lastError != ERROR_PRIVILEGE_NOT_HELD && lastError != ERROR_INVALID_PARAMETER)
                        throw y;
                }
            }
            throw x;
        }
    }

    /**
     * Returns the target of a symbolic link
     */
    static String readLink(WindowsPath path) throws IOException {
        long handle = 0L;
        try {
            handle = path.openForReadAttributeAccess(false); // don't follow links
        } catch (WindowsException x) {
            x.rethrowAsIOException(path);
        }
        try {
            return readLinkImpl(path, handle);
        } finally {
            CloseHandle(handle);
        }
    }

    /**
     * Returns the final path (all symbolic links resolved) or null if this
     * operation is not supported.
     */
    static String getFinalPath(WindowsPath input) throws IOException {
        long h = 0;
        try {
            h = input.openForReadAttributeAccess(true);
        } catch (WindowsException x) {
            x.rethrowAsIOException(input);
        }
        try {
            return stripPrefix(GetFinalPathNameByHandle(h));
        } catch (WindowsException x) {
            // ERROR_INVALID_LEVEL is the error returned when not supported
            // (a sym link to file on FAT32 or Samba server for example)
            if (x.lastError() != ERROR_INVALID_LEVEL)
                x.rethrowAsIOException(input);
        } finally {
            CloseHandle(h);
        }
        return null;
    }

    /**
     * Returns the final path of a given path as a String. This should be used
     * prior to calling Win32 system calls that do not follow links.
     */
    static String getFinalPath(WindowsPath input, boolean followLinks)
        throws IOException
    {
        WindowsFileSystem fs = input.getFileSystem();
        try {
            // if not following links then don't need final path
            if (!followLinks)
                return input.getPathForWin32Calls();

            // if file is not a sym link then don't need final path
            if (!WindowsFileAttributes.get(input, false).isSymbolicLink()) {
                return input.getPathForWin32Calls();
            }
        } catch (WindowsException x) {
            x.rethrowAsIOException(input);
        }

        // The file is a symbolic link so attempt to get the final path
        String result = getFinalPath(input);
        if (result != null)
            return result;

        // Fallback: read target of link, resolve against parent, and repeat
        // until file is not a link.
        WindowsPath target = input;
        int linkCount = 0;
        do {
            try {
                WindowsFileAttributes attrs =
                    WindowsFileAttributes.get(target, false);
                // non a link so we are done
                if (!attrs.isSymbolicLink()) {
                    return target.getPathForWin32Calls();
                }
            } catch (WindowsException x) {
                x.rethrowAsIOException(target);
            }
            WindowsPath link = WindowsPath
                .createFromNormalizedPath(fs, readLink(target));
            WindowsPath parent = target.getParent();
            if (parent == null) {
                // no parent so use parent of absolute path
                final WindowsPath t = target;
                target = t.toAbsolutePath();
                parent = target.getParent();
            }
            target = parent.resolve(link);

        } while (++linkCount < 32);

        throw new FileSystemException(input.getPathForExceptionMessage(), null,
            "Too many links");
    }

    /**
     * Returns the actual path of a file, optionally resolving all symbolic
     * links.
     */
    static String getRealPath(WindowsPath input, boolean resolveLinks)
        throws IOException
    {
        WindowsFileSystem fs = input.getFileSystem();

        // Start with absolute path
        String path = null;
        try {
            path = input.toAbsolutePath().toString();
        } catch (IOError x) {
            throw (IOException)(x.getCause());
        }

        // Collapse "." and ".."
        if (path.indexOf('.') >= 0) {
            try {
                path = GetFullPathName(path);
            } catch (WindowsException x) {
                x.rethrowAsIOException(input);
            }
        }

        // string builder to build up components of path
        StringBuilder sb = new StringBuilder(path.length());

        // Copy root component
        int start;
        char c0 = path.charAt(0);
        char c1 = path.charAt(1);
        if ((c0 <= 'z' && c0 >= 'a' || c0 <= 'Z' && c0 >= 'A') &&
            c1 == ':' && path.charAt(2) == '\\') {
            // Driver specifier
            sb.append(Character.toUpperCase(c0));
            sb.append(":\\");
            start = 3;
        } else if (c0 == '\\' && c1 == '\\') {
            // UNC pathname, begins with "\\\\host\\share"
            int last = path.length() - 1;
            int pos = path.indexOf('\\', 2);
            // skip both server and share names
            if (pos == -1 || (pos == last)) {
                // The UNC does not have a share name (collapsed by GetFullPathName)
                throw new FileSystemException(input.getPathForExceptionMessage(),
                    null, "UNC has invalid share");
            }
            pos = path.indexOf('\\', pos+1);
            if (pos < 0) {
                pos = last;
                sb.append(path).append("\\");
            } else {
                sb.append(path, 0, pos+1);
            }
            start = pos + 1;
        } else {
            throw new AssertionError("path type not recognized");
        }

        // if the result is only a root component then we simply check it exists
        if (start >= path.length()) {
            String result = sb.toString();
            try {
                GetFileAttributes(result);
            } catch (WindowsException x) {
                x.rethrowAsIOException(path);
            }
            return result;
        }

        // iterate through each component to get its actual name in the
        // directory
        int curr = start;
        while (curr < path.length()) {
            int next = path.indexOf('\\', curr);
            int end = (next == -1) ? path.length() : next;
            String search = sb.toString() + path.substring(curr, end);
            try {
                FirstFile fileData = FindFirstFile(WindowsPath.addPrefixIfNeeded(search));
                FindClose(fileData.handle());

                // if a reparse point is encountered then we must return the
                // final path.
                if (resolveLinks &&
                    WindowsFileAttributes.isReparsePoint(fileData.attributes()))
                {
                    String result = getFinalPath(input);
                    if (result == null) {
                        // Fallback to slow path, usually because there is a sym
                        // link to a file system that doesn't support sym links.
                        WindowsPath resolved = resolveAllLinks(
                            WindowsPath.createFromNormalizedPath(fs, path));
                        result = getRealPath(resolved, false);
                    }
                    return result;
                }

                // add the name to the result
                sb.append(fileData.name());
                if (next != -1) {
                    sb.append('\\');
                }
            } catch (WindowsException e) {
                e.rethrowAsIOException(path);
            }
            curr = end + 1;
        }

        return sb.toString();
    }

    /**
     * Returns target of a symbolic link given the handle of an open file
     * (that should be a link).
     */
    private static String readLinkImpl(WindowsPath path, long handle)
        throws IOException
    {
        int size = MAXIMUM_REPARSE_DATA_BUFFER_SIZE;
        try (NativeBuffer buffer = NativeBuffers.getNativeBuffer(size)) {
            try {
                DeviceIoControlGetReparsePoint(handle, buffer.address(), size);
            } catch (WindowsException x) {
                String pathname = path.getPathForExceptionMessage();
                if (x.lastError() == ERROR_NOT_A_REPARSE_POINT)
                    throw new NotLinkException(pathname, null, x.errorString());
                x.rethrowAsIOException(pathname + ": " + x.errorString());
            }

            /*
             * typedef struct _REPARSE_DATA_BUFFER {
             *     ULONG  ReparseTag;
             *     USHORT  ReparseDataLength;
             *     USHORT  Reserved;
             *     union {
             *         struct {
             *             USHORT  SubstituteNameOffset;
             *             USHORT  SubstituteNameLength;
             *             USHORT  PrintNameOffset;
             *             USHORT  PrintNameLength;
             *             WCHAR  PathBuffer[1];
             *         } SymbolicLinkReparseBuffer;
             *         struct {
             *             USHORT  SubstituteNameOffset;
             *             USHORT  SubstituteNameLength;
             *             USHORT  PrintNameOffset;
             *             USHORT  PrintNameLength;
             *             WCHAR  PathBuffer[1];
             *         } MountPointReparseBuffer;
             *         struct {
             *             UCHAR  DataBuffer[1];
             *         } GenericReparseBuffer;
             *     };
             * } REPARSE_DATA_BUFFER
             */
            final short OFFSETOF_REPARSETAG = 0;
            final short OFFSETOF_PATHOFFSET = 8;
            final short OFFSETOF_PATHLENGTH = 10;
            final short OFFSETOF_PATHBUFFER = 16 + 4;   // check this

            int tag = (int)unsafe.getLong(buffer.address() + OFFSETOF_REPARSETAG);
            if (tag != IO_REPARSE_TAG_SYMLINK) {
                String pathname = path.getPathForExceptionMessage();
                throw new NotLinkException(pathname, null, "Reparse point is not a symbolic link");
            }

            // get offset and length of target
            short nameOffset = unsafe.getShort(buffer.address() + OFFSETOF_PATHOFFSET);
            short nameLengthInBytes = unsafe.getShort(buffer.address() + OFFSETOF_PATHLENGTH);
            if ((nameLengthInBytes % 2) != 0)
                throw new FileSystemException(null, null, "Symbolic link corrupted");

            // copy into char array
            char[] name = new char[nameLengthInBytes/2];
            unsafe.copyMemory(null, buffer.address() + OFFSETOF_PATHBUFFER + nameOffset,
                name, Unsafe.ARRAY_CHAR_BASE_OFFSET, nameLengthInBytes);

            // remove special prefix
            String target = stripPrefix(new String(name));
            if (target.isEmpty()) {
                throw new IOException("Symbolic link target is invalid");
            }
            return target;
        }
    }

    /**
     * Resolve all symbolic-links in a given absolute and normalized path
     */
    private static WindowsPath resolveAllLinks(WindowsPath path)
        throws IOException
    {
        assert path.isAbsolute();
        WindowsFileSystem fs = path.getFileSystem();

        // iterate through each name element of the path, resolving links as
        // we go.
        int linkCount = 0;
        int elem = 0;
        while (elem < path.getNameCount()) {
            WindowsPath current = path.getRoot().resolve(path.subpath(0, elem+1));

            WindowsFileAttributes attrs = null;
            try {
                attrs = WindowsFileAttributes.get(current, false);
            } catch (WindowsException x) {
                x.rethrowAsIOException(current);
            }

            /**
             * If a symbolic link then we resolve it against the parent
             * of the current name element. We then resolve any remaining
             * part of the path against the result. The target of the link
             * may have "." and ".." components so re-normalize and restart
             * the process from the first element.
             */
            if (attrs.isSymbolicLink()) {
                linkCount++;
                if (linkCount > 32)
                    throw new IOException("Too many links");
                WindowsPath target = WindowsPath
                    .createFromNormalizedPath(fs, readLink(current));
                WindowsPath remainder = null;
                int count = path.getNameCount();
                if ((elem+1) < count) {
                    remainder = path.subpath(elem+1, count);
                }
                path = current.getParent().resolve(target);
                try {
                    String full = GetFullPathName(path.toString());
                    if (!full.equals(path.toString())) {
                        path = WindowsPath.createFromNormalizedPath(fs, full);
                    }
                } catch (WindowsException x) {
                    x.rethrowAsIOException(path);
                }
                if (remainder != null) {
                    path = path.resolve(remainder);
                }

                // reset
                elem = 0;
            } else {
                // not a link
                elem++;
            }
        }

        return path;
    }

    /**
     * Strip long path or symbolic link prefix from path
     */
    private static String stripPrefix(String path) {
        // prefix for resolved/long path
        if (path.startsWith("\\\\?\\")) {
            if (path.startsWith("\\\\?\\UNC\\")) {
                path = "\\" + path.substring(7);
            } else {
                path = path.substring(4);
            }
            return path;
        }

        // prefix for target of symbolic link
        if (path.startsWith("\\??\\")) {
            if (path.startsWith("\\??\\UNC\\")) {
                path = "\\" + path.substring(7);
            } else {
                path = path.substring(4);
            }
            return path;
        }
        return path;
    }
}