Source file src/internal/syscall/windows/at_windows.go

     1  // Copyright 2024 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package windows
     6  
     7  import (
     8  	"internal/oserror"
     9  	"runtime"
    10  	"structs"
    11  	"syscall"
    12  	"unsafe"
    13  )
    14  
    15  // Openat flags supported by syscall.Open.
    16  const (
    17  	O_DIRECTORY = 0x04000 // target must be a directory
    18  )
    19  
    20  // Openat flags not supported by syscall.Open.
    21  //
    22  // These are invented values, use values in the 33-63 bit range
    23  // to avoid overlap with flags and attributes supported by [syscall.Open].
    24  //
    25  // When adding a new flag here, add an unexported version to
    26  // the set of invented O_ values in syscall/types_windows.go
    27  // to avoid overlap.
    28  const (
    29  	O_NOFOLLOW_ANY = 0x200000000 // disallow symlinks anywhere in the path
    30  	O_WRITE_ATTRS  = 0x800000000 // FILE_WRITE_ATTRIBUTES, used by Chmod
    31  )
    32  
    33  func Openat(dirfd syscall.Handle, name string, flag uint64, perm uint32) (_ syscall.Handle, e1 error) {
    34  	if len(name) == 0 {
    35  		return syscall.InvalidHandle, syscall.ERROR_FILE_NOT_FOUND
    36  	}
    37  
    38  	var access, options uint32
    39  	// Map Win32 file flags to NT create options.
    40  	fileFlags := uint32(flag) & FileFlagsMask
    41  	if fileFlags&^ValidFileFlagsMask != 0 {
    42  		return syscall.InvalidHandle, oserror.ErrInvalid
    43  	}
    44  	if fileFlags&O_FILE_FLAG_OVERLAPPED == 0 {
    45  		options |= FILE_SYNCHRONOUS_IO_NONALERT
    46  	}
    47  	if fileFlags&O_FILE_FLAG_DELETE_ON_CLOSE != 0 {
    48  		access |= DELETE
    49  	}
    50  	setOptionFlag := func(ntFlag, win32Flag uint32) {
    51  		if fileFlags&win32Flag != 0 {
    52  			options |= ntFlag
    53  		}
    54  	}
    55  	setOptionFlag(FILE_NO_INTERMEDIATE_BUFFERING, O_FILE_FLAG_NO_BUFFERING)
    56  	setOptionFlag(FILE_WRITE_THROUGH, O_FILE_FLAG_WRITE_THROUGH)
    57  	setOptionFlag(FILE_SEQUENTIAL_ONLY, O_FILE_FLAG_SEQUENTIAL_SCAN)
    58  	setOptionFlag(FILE_RANDOM_ACCESS, O_FILE_FLAG_RANDOM_ACCESS)
    59  	setOptionFlag(FILE_OPEN_FOR_BACKUP_INTENT, O_FILE_FLAG_BACKUP_SEMANTICS)
    60  	setOptionFlag(FILE_SESSION_AWARE, O_FILE_FLAG_SESSION_AWARE)
    61  	setOptionFlag(FILE_DELETE_ON_CLOSE, O_FILE_FLAG_DELETE_ON_CLOSE)
    62  	setOptionFlag(FILE_OPEN_NO_RECALL, O_FILE_FLAG_OPEN_NO_RECALL)
    63  	setOptionFlag(FILE_OPEN_REPARSE_POINT, O_FILE_FLAG_OPEN_REPARSE_POINT)
    64  
    65  	switch flag & (syscall.O_RDONLY | syscall.O_WRONLY | syscall.O_RDWR) {
    66  	case syscall.O_RDONLY:
    67  		// FILE_GENERIC_READ includes FILE_LIST_DIRECTORY.
    68  		access |= FILE_GENERIC_READ
    69  	case syscall.O_WRONLY:
    70  		access |= FILE_GENERIC_WRITE
    71  		options |= FILE_NON_DIRECTORY_FILE
    72  	case syscall.O_RDWR:
    73  		access |= FILE_GENERIC_READ | FILE_GENERIC_WRITE
    74  		options |= FILE_NON_DIRECTORY_FILE
    75  	default:
    76  		// Stat opens files without requesting read or write permissions,
    77  		// but we still need to request SYNCHRONIZE.
    78  		access |= SYNCHRONIZE
    79  	}
    80  	if flag&syscall.O_CREAT != 0 {
    81  		access |= FILE_GENERIC_WRITE
    82  	}
    83  	if fileFlags&O_FILE_FLAG_NO_BUFFERING != 0 {
    84  		// Disable buffering implies no implicit append access.
    85  		access &^= FILE_APPEND_DATA
    86  	}
    87  	if flag&syscall.O_APPEND != 0 {
    88  		access |= FILE_APPEND_DATA
    89  		// Remove FILE_WRITE_DATA access unless O_TRUNC is set,
    90  		// in which case we need it to truncate the file.
    91  		if flag&syscall.O_TRUNC == 0 {
    92  			access &^= FILE_WRITE_DATA
    93  		}
    94  	}
    95  	if flag&O_DIRECTORY != 0 {
    96  		options |= FILE_DIRECTORY_FILE
    97  		access |= FILE_LIST_DIRECTORY
    98  	}
    99  	if flag&syscall.O_SYNC != 0 {
   100  		options |= FILE_WRITE_THROUGH
   101  	}
   102  	if flag&O_WRITE_ATTRS != 0 {
   103  		access |= FILE_WRITE_ATTRIBUTES
   104  	}
   105  	// Allow File.Stat.
   106  	access |= STANDARD_RIGHTS_READ | FILE_READ_ATTRIBUTES | FILE_READ_EA
   107  
   108  	objAttrs := &OBJECT_ATTRIBUTES{}
   109  	if flag&O_NOFOLLOW_ANY != 0 {
   110  		objAttrs.Attributes |= OBJ_DONT_REPARSE
   111  	}
   112  	if flag&syscall.O_CLOEXEC == 0 {
   113  		objAttrs.Attributes |= OBJ_INHERIT
   114  	}
   115  	if fileFlags&O_FILE_FLAG_POSIX_SEMANTICS == 0 {
   116  		objAttrs.Attributes |= OBJ_CASE_INSENSITIVE
   117  	}
   118  	if err := objAttrs.init(dirfd, name); err != nil {
   119  		return syscall.InvalidHandle, err
   120  	}
   121  
   122  	// We don't use FILE_OVERWRITE/FILE_OVERWRITE_IF, because when opening
   123  	// a file with FILE_ATTRIBUTE_READONLY these will replace an existing
   124  	// file with a new, read-only one.
   125  	//
   126  	// Instead, we ftruncate the file after opening when O_TRUNC is set.
   127  	var disposition uint32
   128  	switch {
   129  	case flag&(syscall.O_CREAT|syscall.O_EXCL) == (syscall.O_CREAT | syscall.O_EXCL):
   130  		disposition = FILE_CREATE
   131  		options |= FILE_OPEN_REPARSE_POINT // don't follow symlinks
   132  	case flag&syscall.O_CREAT == syscall.O_CREAT:
   133  		disposition = FILE_OPEN_IF
   134  	default:
   135  		disposition = FILE_OPEN
   136  	}
   137  
   138  	fileAttrs := uint32(FILE_ATTRIBUTE_NORMAL)
   139  	if perm&syscall.S_IWRITE == 0 {
   140  		fileAttrs = FILE_ATTRIBUTE_READONLY
   141  	}
   142  
   143  	var h syscall.Handle
   144  	err := NtCreateFile(
   145  		&h,
   146  		SYNCHRONIZE|access,
   147  		objAttrs,
   148  		&IO_STATUS_BLOCK{},
   149  		nil,
   150  		fileAttrs,
   151  		FILE_SHARE_READ|FILE_SHARE_WRITE|FILE_SHARE_DELETE,
   152  		disposition,
   153  		FILE_OPEN_FOR_BACKUP_INTENT|options,
   154  		nil,
   155  		0,
   156  	)
   157  	if err != nil {
   158  		return h, ntCreateFileError(err, flag)
   159  	}
   160  
   161  	if flag&syscall.O_TRUNC != 0 {
   162  		err = syscall.Ftruncate(h, 0)
   163  		if err == ERROR_INVALID_PARAMETER {
   164  			// ERROR_INVALID_PARAMETER means truncation is not supported on this file handle.
   165  			// Unix's O_TRUNC specification says to ignore O_TRUNC on named pipes and terminal devices.
   166  			// We do the same here.
   167  			if t, err1 := syscall.GetFileType(h); err1 == nil && (t == syscall.FILE_TYPE_PIPE || t == syscall.FILE_TYPE_CHAR) {
   168  				err = nil
   169  			}
   170  		}
   171  		if err != nil {
   172  			syscall.CloseHandle(h)
   173  			return syscall.InvalidHandle, err
   174  		}
   175  	}
   176  
   177  	return h, nil
   178  }
   179  
   180  // ntCreateFileError maps error returns from NTCreateFile to user-visible errors.
   181  func ntCreateFileError(err error, flag uint64) error {
   182  	s, ok := err.(NTStatus)
   183  	if !ok {
   184  		// Shouldn't really be possible, NtCreateFile always returns NTStatus.
   185  		return err
   186  	}
   187  	switch s {
   188  	case STATUS_REPARSE_POINT_ENCOUNTERED:
   189  		return syscall.ELOOP
   190  	case STATUS_NOT_A_DIRECTORY:
   191  		// ENOTDIR is the errno returned by open when O_DIRECTORY is specified
   192  		// and the target is not a directory.
   193  		//
   194  		// NtCreateFile can return STATUS_NOT_A_DIRECTORY under other circumstances,
   195  		// such as when opening "file/" where "file" is not a directory.
   196  		// (This might be Windows version dependent.)
   197  		//
   198  		// Only map STATUS_NOT_A_DIRECTORY to ENOTDIR when O_DIRECTORY is specified.
   199  		if flag&O_DIRECTORY != 0 {
   200  			return syscall.ENOTDIR
   201  		}
   202  	case STATUS_FILE_IS_A_DIRECTORY:
   203  		return syscall.EISDIR
   204  	case STATUS_OBJECT_NAME_COLLISION:
   205  		return syscall.EEXIST
   206  	}
   207  	return s.Errno()
   208  }
   209  
   210  func Mkdirat(dirfd syscall.Handle, name string, mode uint32) error {
   211  	objAttrs := &OBJECT_ATTRIBUTES{}
   212  	if err := objAttrs.init(dirfd, name); err != nil {
   213  		return err
   214  	}
   215  	var h syscall.Handle
   216  	err := NtCreateFile(
   217  		&h,
   218  		FILE_GENERIC_READ,
   219  		objAttrs,
   220  		&IO_STATUS_BLOCK{},
   221  		nil,
   222  		syscall.FILE_ATTRIBUTE_NORMAL,
   223  		syscall.FILE_SHARE_READ|syscall.FILE_SHARE_WRITE|syscall.FILE_SHARE_DELETE,
   224  		FILE_CREATE,
   225  		FILE_DIRECTORY_FILE,
   226  		nil,
   227  		0,
   228  	)
   229  	if err != nil {
   230  		return ntCreateFileError(err, 0)
   231  	}
   232  	syscall.CloseHandle(h)
   233  	return nil
   234  }
   235  
   236  func Deleteat(dirfd syscall.Handle, name string, options uint32) error {
   237  	if name == "." {
   238  		// NtOpenFile's documentation isn't explicit about what happens when deleting ".".
   239  		// Make this an error consistent with that of POSIX.
   240  		return syscall.EINVAL
   241  	}
   242  	objAttrs := &OBJECT_ATTRIBUTES{}
   243  	if err := objAttrs.init(dirfd, name); err != nil {
   244  		return err
   245  	}
   246  	var h syscall.Handle
   247  	err := NtOpenFile(
   248  		&h,
   249  		SYNCHRONIZE|FILE_READ_ATTRIBUTES|DELETE,
   250  		objAttrs,
   251  		&IO_STATUS_BLOCK{},
   252  		FILE_SHARE_DELETE|FILE_SHARE_READ|FILE_SHARE_WRITE,
   253  		FILE_OPEN_REPARSE_POINT|FILE_OPEN_FOR_BACKUP_INTENT|FILE_SYNCHRONOUS_IO_NONALERT|options,
   254  	)
   255  	if err != nil {
   256  		return ntCreateFileError(err, 0)
   257  	}
   258  	defer syscall.CloseHandle(h)
   259  
   260  	if TestDeleteatFallback {
   261  		return deleteatFallback(h)
   262  	}
   263  
   264  	const FileDispositionInformationEx = 64
   265  
   266  	// First, attempt to delete the file using POSIX semantics
   267  	// (which permit a file to be deleted while it is still open).
   268  	// This matches the behavior of DeleteFileW.
   269  	//
   270  	// The following call uses features available on different Windows versions:
   271  	// - FILE_DISPOSITION_INFORMATION_EX: Windows 10, version 1607 (aka RS1)
   272  	// - FILE_DISPOSITION_POSIX_SEMANTICS: Windows 10, version 1607 (aka RS1)
   273  	// - FILE_DISPOSITION_IGNORE_READONLY_ATTRIBUTE: Windows 10, version 1809 (aka RS5)
   274  	//
   275  	// Also, some file systems, like FAT32, don't support POSIX semantics.
   276  	err = NtSetInformationFile(
   277  		h,
   278  		&IO_STATUS_BLOCK{},
   279  		unsafe.Pointer(&FILE_DISPOSITION_INFORMATION_EX{
   280  			Flags: FILE_DISPOSITION_DELETE |
   281  				FILE_DISPOSITION_FORCE_IMAGE_SECTION_CHECK |
   282  				FILE_DISPOSITION_POSIX_SEMANTICS |
   283  				// This differs from DeleteFileW, but matches os.Remove's
   284  				// behavior on Unix platforms of permitting deletion of
   285  				// read-only files.
   286  				FILE_DISPOSITION_IGNORE_READONLY_ATTRIBUTE,
   287  		}),
   288  		uint32(unsafe.Sizeof(FILE_DISPOSITION_INFORMATION_EX{})),
   289  		FileDispositionInformationEx,
   290  	)
   291  	switch err {
   292  	case nil:
   293  		return nil
   294  	case STATUS_INVALID_INFO_CLASS, // the operating system doesn't support FileDispositionInformationEx
   295  		STATUS_INVALID_PARAMETER, // the operating system doesn't support one of the flags
   296  		STATUS_NOT_SUPPORTED:     // the file system doesn't support FILE_DISPOSITION_INFORMATION_EX or one of the flags
   297  		return deleteatFallback(h)
   298  	default:
   299  		return err.(NTStatus).Errno()
   300  	}
   301  }
   302  
   303  // TestDeleteatFallback should only be used for testing purposes.
   304  // When set, [Deleteat] uses the fallback path unconditionally.
   305  var TestDeleteatFallback bool
   306  
   307  // deleteatFallback is a deleteat implementation that strives
   308  // for compatibility with older Windows versions and file systems
   309  // over performance.
   310  func deleteatFallback(h syscall.Handle) error {
   311  	var data syscall.ByHandleFileInformation
   312  	if err := syscall.GetFileInformationByHandle(h, &data); err == nil && data.FileAttributes&syscall.FILE_ATTRIBUTE_READONLY != 0 {
   313  		// Remove read-only attribute. Reopen the file, as it was previously open without FILE_WRITE_ATTRIBUTES access
   314  		// in order to maximize compatibility in the happy path.
   315  		wh, err := ReOpenFile(h,
   316  			FILE_WRITE_ATTRIBUTES,
   317  			FILE_SHARE_READ|FILE_SHARE_WRITE|FILE_SHARE_DELETE,
   318  			syscall.FILE_FLAG_OPEN_REPARSE_POINT|syscall.FILE_FLAG_BACKUP_SEMANTICS,
   319  		)
   320  		if err != nil {
   321  			return err
   322  		}
   323  		err = SetFileInformationByHandle(
   324  			wh,
   325  			FileBasicInfo,
   326  			unsafe.Pointer(&FILE_BASIC_INFO{
   327  				FileAttributes: data.FileAttributes &^ FILE_ATTRIBUTE_READONLY,
   328  			}),
   329  			uint32(unsafe.Sizeof(FILE_BASIC_INFO{})),
   330  		)
   331  		syscall.CloseHandle(wh)
   332  		if err != nil {
   333  			return err
   334  		}
   335  	}
   336  
   337  	return SetFileInformationByHandle(
   338  		h,
   339  		FileDispositionInfo,
   340  		unsafe.Pointer(&FILE_DISPOSITION_INFO{
   341  			DeleteFile: true,
   342  		}),
   343  		uint32(unsafe.Sizeof(FILE_DISPOSITION_INFO{})),
   344  	)
   345  }
   346  
   347  func Renameat(olddirfd syscall.Handle, oldpath string, newdirfd syscall.Handle, newpath string) error {
   348  	objAttrs := &OBJECT_ATTRIBUTES{}
   349  	if err := objAttrs.init(olddirfd, oldpath); err != nil {
   350  		return err
   351  	}
   352  	var h syscall.Handle
   353  	err := NtOpenFile(
   354  		&h,
   355  		SYNCHRONIZE|DELETE,
   356  		objAttrs,
   357  		&IO_STATUS_BLOCK{},
   358  		FILE_SHARE_DELETE|FILE_SHARE_READ|FILE_SHARE_WRITE,
   359  		FILE_OPEN_REPARSE_POINT|FILE_OPEN_FOR_BACKUP_INTENT|FILE_SYNCHRONOUS_IO_NONALERT,
   360  	)
   361  	if err != nil {
   362  		return ntCreateFileError(err, 0)
   363  	}
   364  	defer syscall.CloseHandle(h)
   365  
   366  	renameInfoEx := FILE_RENAME_INFORMATION_EX{
   367  		Flags: FILE_RENAME_REPLACE_IF_EXISTS |
   368  			FILE_RENAME_POSIX_SEMANTICS,
   369  		RootDirectory: newdirfd,
   370  	}
   371  	p16, err := syscall.UTF16FromString(newpath)
   372  	if err != nil {
   373  		return err
   374  	}
   375  	if len(p16) > len(renameInfoEx.FileName) {
   376  		return syscall.EINVAL
   377  	}
   378  	copy(renameInfoEx.FileName[:], p16)
   379  	renameInfoEx.FileNameLength = uint32((len(p16) - 1) * 2)
   380  
   381  	const (
   382  		FileRenameInformation   = 10
   383  		FileRenameInformationEx = 65
   384  	)
   385  	err = NtSetInformationFile(
   386  		h,
   387  		&IO_STATUS_BLOCK{},
   388  		unsafe.Pointer(&renameInfoEx),
   389  		uint32(unsafe.Sizeof(FILE_RENAME_INFORMATION_EX{})),
   390  		FileRenameInformationEx,
   391  	)
   392  	if err == nil {
   393  		return nil
   394  	}
   395  
   396  	// If the prior rename failed, the filesystem might not support
   397  	// POSIX semantics (for example, FAT), or might not have implemented
   398  	// FILE_RENAME_INFORMATION_EX.
   399  	//
   400  	// Try again.
   401  	renameInfo := FILE_RENAME_INFORMATION{
   402  		ReplaceIfExists: true,
   403  		RootDirectory:   newdirfd,
   404  	}
   405  	copy(renameInfo.FileName[:], p16)
   406  	renameInfo.FileNameLength = renameInfoEx.FileNameLength
   407  
   408  	err = NtSetInformationFile(
   409  		h,
   410  		&IO_STATUS_BLOCK{},
   411  		unsafe.Pointer(&renameInfo),
   412  		uint32(unsafe.Sizeof(FILE_RENAME_INFORMATION{})),
   413  		FileRenameInformation,
   414  	)
   415  	if st, ok := err.(NTStatus); ok {
   416  		return st.Errno()
   417  	}
   418  	return err
   419  }
   420  
   421  func Linkat(olddirfd syscall.Handle, oldpath string, newdirfd syscall.Handle, newpath string) error {
   422  	objAttrs := &OBJECT_ATTRIBUTES{}
   423  	if err := objAttrs.init(olddirfd, oldpath); err != nil {
   424  		return err
   425  	}
   426  	var h syscall.Handle
   427  	err := NtOpenFile(
   428  		&h,
   429  		SYNCHRONIZE|FILE_WRITE_ATTRIBUTES,
   430  		objAttrs,
   431  		&IO_STATUS_BLOCK{},
   432  		FILE_SHARE_DELETE|FILE_SHARE_READ|FILE_SHARE_WRITE,
   433  		FILE_OPEN_REPARSE_POINT|FILE_OPEN_FOR_BACKUP_INTENT|FILE_SYNCHRONOUS_IO_NONALERT,
   434  	)
   435  	if err != nil {
   436  		return ntCreateFileError(err, 0)
   437  	}
   438  	defer syscall.CloseHandle(h)
   439  
   440  	linkInfo := FILE_LINK_INFORMATION{
   441  		RootDirectory: newdirfd,
   442  	}
   443  	p16, err := syscall.UTF16FromString(newpath)
   444  	if err != nil {
   445  		return err
   446  	}
   447  	if len(p16) > len(linkInfo.FileName) {
   448  		return syscall.EINVAL
   449  	}
   450  	copy(linkInfo.FileName[:], p16)
   451  	linkInfo.FileNameLength = uint32((len(p16) - 1) * 2)
   452  
   453  	const (
   454  		FileLinkInformation = 11
   455  	)
   456  	err = NtSetInformationFile(
   457  		h,
   458  		&IO_STATUS_BLOCK{},
   459  		unsafe.Pointer(&linkInfo),
   460  		uint32(unsafe.Sizeof(FILE_LINK_INFORMATION{})),
   461  		FileLinkInformation,
   462  	)
   463  	if st, ok := err.(NTStatus); ok {
   464  		return st.Errno()
   465  	}
   466  	return err
   467  }
   468  
   469  // SymlinkatFlags configure Symlinkat.
   470  //
   471  // Symbolic links have two properties: They may be directory or file links,
   472  // and they may be absolute or relative.
   473  //
   474  // The Windows API defines flags describing these properties
   475  // (SYMBOLIC_LINK_FLAG_DIRECTORY and SYMLINK_FLAG_RELATIVE),
   476  // but the flags are passed to different system calls and
   477  // do not have distinct values, so we define our own enumeration
   478  // that permits expressing both.
   479  type SymlinkatFlags uint
   480  
   481  const (
   482  	SYMLINKAT_DIRECTORY = SymlinkatFlags(1 << iota)
   483  	SYMLINKAT_RELATIVE
   484  )
   485  
   486  func Symlinkat(oldname string, newdirfd syscall.Handle, newname string, flags SymlinkatFlags) error {
   487  	// Temporarily acquire symlink-creating privileges if possible.
   488  	// This is the behavior of CreateSymbolicLinkW.
   489  	//
   490  	// (When passed the SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE flag,
   491  	// CreateSymbolicLinkW ignores errors in acquiring privileges, as we do here.)
   492  	return withPrivilege("SeCreateSymbolicLinkPrivilege", func() error {
   493  		return symlinkat(oldname, newdirfd, newname, flags)
   494  	})
   495  }
   496  
   497  func symlinkat(oldname string, newdirfd syscall.Handle, newname string, flags SymlinkatFlags) error {
   498  	oldnameu16, err := syscall.UTF16FromString(oldname)
   499  	if err != nil {
   500  		return err
   501  	}
   502  	oldnameu16 = oldnameu16[:len(oldnameu16)-1] // trim off terminal NUL
   503  
   504  	var options uint32
   505  	if flags&SYMLINKAT_DIRECTORY != 0 {
   506  		options |= FILE_DIRECTORY_FILE
   507  	} else {
   508  		options |= FILE_NON_DIRECTORY_FILE
   509  	}
   510  
   511  	objAttrs := &OBJECT_ATTRIBUTES{}
   512  	if err := objAttrs.init(newdirfd, newname); err != nil {
   513  		return err
   514  	}
   515  	var h syscall.Handle
   516  	err = NtCreateFile(
   517  		&h,
   518  		SYNCHRONIZE|FILE_WRITE_ATTRIBUTES|DELETE,
   519  		objAttrs,
   520  		&IO_STATUS_BLOCK{},
   521  		nil,
   522  		syscall.FILE_ATTRIBUTE_NORMAL,
   523  		0,
   524  		FILE_CREATE,
   525  		FILE_OPEN_REPARSE_POINT|FILE_OPEN_FOR_BACKUP_INTENT|FILE_SYNCHRONOUS_IO_NONALERT|options,
   526  		nil,
   527  		0,
   528  	)
   529  	if err != nil {
   530  		return ntCreateFileError(err, 0)
   531  	}
   532  	defer syscall.CloseHandle(h)
   533  
   534  	// https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_reparse_data_buffer
   535  	type reparseDataBufferT struct {
   536  		_ structs.HostLayout
   537  
   538  		ReparseTag        uint32
   539  		ReparseDataLength uint16
   540  		Reserved          uint16
   541  
   542  		SubstituteNameOffset uint16
   543  		SubstituteNameLength uint16
   544  		PrintNameOffset      uint16
   545  		PrintNameLength      uint16
   546  		Flags                uint32
   547  	}
   548  
   549  	const (
   550  		headerSize = uint16(unsafe.Offsetof(reparseDataBufferT{}.SubstituteNameOffset))
   551  		bufferSize = uint16(unsafe.Sizeof(reparseDataBufferT{}))
   552  	)
   553  
   554  	// Data buffer containing a SymbolicLinkReparseBuffer followed by the link target.
   555  	rdbbuf := make([]byte, bufferSize+uint16(2*len(oldnameu16)))
   556  
   557  	rdb := (*reparseDataBufferT)(unsafe.Pointer(&rdbbuf[0]))
   558  	rdb.ReparseTag = syscall.IO_REPARSE_TAG_SYMLINK
   559  	rdb.ReparseDataLength = uint16(len(rdbbuf)) - uint16(headerSize)
   560  	rdb.SubstituteNameOffset = 0
   561  	rdb.SubstituteNameLength = uint16(2 * len(oldnameu16))
   562  	rdb.PrintNameOffset = 0
   563  	rdb.PrintNameLength = rdb.SubstituteNameLength
   564  	if flags&SYMLINKAT_RELATIVE != 0 {
   565  		rdb.Flags = SYMLINK_FLAG_RELATIVE
   566  	}
   567  
   568  	namebuf := rdbbuf[bufferSize:]
   569  	copy(namebuf, unsafe.String((*byte)(unsafe.Pointer(&oldnameu16[0])), 2*len(oldnameu16)))
   570  
   571  	err = syscall.DeviceIoControl(
   572  		h,
   573  		FSCTL_SET_REPARSE_POINT,
   574  		&rdbbuf[0],
   575  		uint32(len(rdbbuf)),
   576  		nil,
   577  		0,
   578  		nil,
   579  		nil)
   580  	if err != nil {
   581  		// Creating the symlink has failed, so try to remove the file.
   582  		const FileDispositionInformation = 13
   583  		NtSetInformationFile(
   584  			h,
   585  			&IO_STATUS_BLOCK{},
   586  			unsafe.Pointer(&FILE_DISPOSITION_INFORMATION{
   587  				DeleteFile: true,
   588  			}),
   589  			uint32(unsafe.Sizeof(FILE_DISPOSITION_INFORMATION{})),
   590  			FileDispositionInformation,
   591  		)
   592  		return err
   593  	}
   594  
   595  	return nil
   596  }
   597  
   598  // withPrivilege temporariliy acquires the named privilege and runs f.
   599  // If the privilege cannot be acquired it runs f anyway,
   600  // which should fail with an appropriate error.
   601  func withPrivilege(privilege string, f func() error) error {
   602  	runtime.LockOSThread()
   603  	defer runtime.UnlockOSThread()
   604  
   605  	err := ImpersonateSelf(SecurityImpersonation)
   606  	if err != nil {
   607  		return f()
   608  	}
   609  	defer RevertToSelf()
   610  
   611  	curThread, err := GetCurrentThread()
   612  	if err != nil {
   613  		return f()
   614  	}
   615  	var token syscall.Token
   616  	err = OpenThreadToken(curThread, syscall.TOKEN_QUERY|TOKEN_ADJUST_PRIVILEGES, false, &token)
   617  	if err != nil {
   618  		return f()
   619  	}
   620  	defer syscall.CloseHandle(syscall.Handle(token))
   621  
   622  	privStr, err := syscall.UTF16PtrFromString(privilege)
   623  	if err != nil {
   624  		return f()
   625  	}
   626  	var tokenPriv TOKEN_PRIVILEGES
   627  	err = LookupPrivilegeValue(nil, privStr, &tokenPriv.Privileges[0].Luid)
   628  	if err != nil {
   629  		return f()
   630  	}
   631  
   632  	tokenPriv.PrivilegeCount = 1
   633  	tokenPriv.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED
   634  	err = AdjustTokenPrivileges(token, false, &tokenPriv, 0, nil, nil)
   635  	if err != nil {
   636  		return f()
   637  	}
   638  
   639  	return f()
   640  }
   641  

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