syncthing/lib/symlinks/symlink_windows.go

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// Copyright (C) 2014 The Syncthing Authors.
//
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// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at http://mozilla.org/MPL/2.0/.
// +build windows
package symlinks
import (
"os"
"path/filepath"
"github.com/syncthing/protocol"
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"github.com/syncthing/syncthing/lib/osutil"
"syscall"
"unicode/utf16"
"unsafe"
)
const (
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Win32FsctlGetReparsePoint = 0x900a8
Win32FileFlagOpenReparsePoint = 0x00200000
Win32FileAttributeReparsePoint = 0x400
Win32IOReparseTagSymlink = 0xA000000C
Win32SymbolicLinkFlagDirectory = 0x1
)
var (
modkernel32 = syscall.NewLazyDLL("kernel32.dll")
procDeviceIoControl = modkernel32.NewProc("DeviceIoControl")
procCreateSymbolicLink = modkernel32.NewProc("CreateSymbolicLinkW")
Supported = false
)
func init() {
defer func() {
if err := recover(); err != nil {
// Ensure that the supported flag is disabled when we hit an
// error, even though it should already be. Also, silently swallow
// the error since it's fine for a system not to support symlinks.
Supported = false
}
}()
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// Needs administrator privileges.
// Let's check that everything works.
// This could be done more officially:
// http://stackoverflow.com/questions/2094663/determine-if-windows-process-has-privilege-to-create-symbolic-link
// But I don't want to define 10 more structs just to look this up.
base := os.TempDir()
path := filepath.Join(base, "symlinktest")
defer os.Remove(path)
err := Create(path, base, protocol.FlagDirectory)
if err != nil {
return
}
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stat, err := osutil.Lstat(path)
if err != nil || stat.Mode()&os.ModeSymlink == 0 {
return
}
target, flags, err := Read(path)
if err != nil || osutil.NativeFilename(target) != base || flags&protocol.FlagDirectory == 0 {
return
}
Supported = true
}
type reparseData struct {
reparseTag uint32
reparseDataLength uint16
reserved uint16
substitueNameOffset uint16
substitueNameLength uint16
printNameOffset uint16
printNameLength uint16
flags uint32
// substituteName - 264 widechars max = 528 bytes
// printName - 260 widechars max = 520 bytes
// = 1048 bytes total
buffer [1048]uint16
}
func (r *reparseData) PrintName() string {
// No clue why the offset and length is doubled...
offset := r.printNameOffset / 2
length := r.printNameLength / 2
return string(utf16.Decode(r.buffer[offset : offset+length]))
}
func (r *reparseData) SubstituteName() string {
// No clue why the offset and length is doubled...
offset := r.substitueNameOffset / 2
length := r.substitueNameLength / 2
return string(utf16.Decode(r.buffer[offset : offset+length]))
}
func Read(path string) (string, TargetType, error) {
ptr, err := syscall.UTF16PtrFromString(path)
if err != nil {
return "", TargetUnknown, err
}
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handle, err := syscall.CreateFile(ptr, 0, syscall.FILE_SHARE_READ|syscall.FILE_SHARE_WRITE|syscall.FILE_SHARE_DELETE, nil, syscall.OPEN_EXISTING, syscall.FILE_FLAG_BACKUP_SEMANTICS|Win32FileFlagOpenReparsePoint, 0)
if err != nil || handle == syscall.InvalidHandle {
return "", TargetUnknown, err
}
defer syscall.Close(handle)
var ret uint16
var data reparseData
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r1, _, err := syscall.Syscall9(procDeviceIoControl.Addr(), 8, uintptr(handle), Win32FsctlGetReparsePoint, 0, 0, uintptr(unsafe.Pointer(&data)), unsafe.Sizeof(data), uintptr(unsafe.Pointer(&ret)), 0, 0)
if r1 == 0 {
return "", TargetUnknown, err
}
tt := TargetUnknown
if attr, err := syscall.GetFileAttributes(ptr); err == nil {
if attr&syscall.FILE_ATTRIBUTE_DIRECTORY != 0 {
tt = TargetDirectory
} else {
tt = TargetFile
}
}
return osutil.NormalizedFilename(data.PrintName()), tt, nil
}
func Create(source, target string, tt TargetType) error {
srcp, err := syscall.UTF16PtrFromString(source)
if err != nil {
return err
}
trgp, err := syscall.UTF16PtrFromString(osutil.NativeFilename(target))
if err != nil {
return err
}
// Sadly for Windows we need to specify the type of the symlink,
// whether it's a directory symlink or a file symlink.
// If the flags doesn't reveal the target type, try to evaluate it
// ourselves, and worst case default to the symlink pointing to a file.
mode := 0
if tt == TargetUnknown {
path := target
if !filepath.IsAbs(target) {
path = filepath.Join(filepath.Dir(source), target)
}
stat, err := os.Stat(path)
if err == nil && stat.IsDir() {
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mode = Win32SymbolicLinkFlagDirectory
}
} else if tt == TargetDirectory {
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mode = Win32SymbolicLinkFlagDirectory
}
r0, _, err := syscall.Syscall(procCreateSymbolicLink.Addr(), 3, uintptr(unsafe.Pointer(srcp)), uintptr(unsafe.Pointer(trgp)), uintptr(mode))
if r0 == 1 {
return nil
}
return err
}
func ChangeType(path string, tt TargetType) error {
target, exTt, err := Read(path)
if err != nil {
return err
}
// If it's the same type, nothing to do.
if tt == exTt {
return nil
}
// If the actual type is unknown, but the new type is file, nothing to do
if exTt == TargetUnknown && tt != TargetDirectory {
return nil
}
return osutil.InWritableDir(func(path string) error {
// It should be a symlink as well hence no need to change permissions on
// the file.
os.Remove(path)
return Create(path, target, tt)
}, path)
}