Use SubstituteName instead of PrintName for reparse point decoding

reparse.go

@@ -68,17 +68,29 @@ func DecodeReparsePointData(tag uint32, b []byte) (*ReparsePoint, error) {
 }
 
 func decodeWindowsReparsePointData(b []byte, isMountPoint bool) (*ReparsePoint, error) {
-	nameOffset := 8 + binary.LittleEndian.Uint16(b[4:6])
+	nameOffset := 8 + binary.LittleEndian.Uint16(b[0:2])
 	if !isMountPoint {
 		nameOffset += 4
 	}
-	nameLength := binary.LittleEndian.Uint16(b[6:8])
+	nameLength := binary.LittleEndian.Uint16(b[2:4])
 	name := make([]uint16, nameLength/2)
 	err := binary.Read(bytes.NewReader(b[nameOffset:nameOffset+nameLength]), binary.LittleEndian, &name)
 	if err != nil {
 		return nil, err
 	}
-	return &ReparsePoint{Target: string(utf16.Decode(name)), IsMountPoint: isMountPoint, IsLxSymlink: false}, nil
+	target := string(utf16.Decode(name))
+	// SubstituteName uses NT path prefixes; convert back to Win32 form.
+	if strings.HasPrefix(target, `\??\UNC\`) {
+		target = `\\` + target[8:]
+	} else if strings.HasPrefix(target, `\??\`) {
+		rest := target[4:]
+		if len(rest) >= 2 && isDriveLetter(rest[0]) && rest[1] == ':' {
+			target = rest
+		} else {
+			target = `\\?\` + rest
+		}
+	}
+	return &ReparsePoint{Target: target, IsMountPoint: isMountPoint, IsLxSymlink: false}, nil
 }
 
 func decodeLxReparsePointData(b []byte) (*ReparsePoint, error) {

reparse_test.go

@@ -0,0 +1,177 @@
+//go:build windows
+
+package winio
+
+import (
+	"bytes"
+	"encoding/binary"
+	"testing"
+	"unicode/utf16"
+)
+
+// buildMountPointBuffer builds a raw REPARSE_DATA_BUFFER for a mount point.
+func buildMountPointBuffer(substituteName, printName string) []byte {
+	subUTF16 := utf16.Encode([]rune(substituteName + "\x00"))
+	printUTF16 := utf16.Encode([]rune(printName + "\x00"))
+
+	subBytes := len(subUTF16) * 2
+	printBytes := len(printUTF16) * 2
+
+	// Length fields exclude the trailing NUL.
+	subNameLen := uint16((len(subUTF16) - 1) * 2)
+	printNameLen := uint16((len(printUTF16) - 1) * 2)
+
+	if printName == "" {
+		printUTF16 = nil
+		printBytes = 0
+		printNameLen = 0
+	}
+
+	pathBufSize := subBytes + printBytes
+	dataLen := 8 + pathBufSize
+
+	var buf bytes.Buffer
+	_ = binary.Write(&buf, binary.LittleEndian, uint32(reparseTagMountPoint))
+	_ = binary.Write(&buf, binary.LittleEndian, uint16(dataLen))
+	_ = binary.Write(&buf, binary.LittleEndian, uint16(0))
+	_ = binary.Write(&buf, binary.LittleEndian, uint16(0))
+	_ = binary.Write(&buf, binary.LittleEndian, subNameLen)
+	_ = binary.Write(&buf, binary.LittleEndian, uint16(subBytes))
+	_ = binary.Write(&buf, binary.LittleEndian, printNameLen)
+	_ = binary.Write(&buf, binary.LittleEndian, subUTF16)
+	if printUTF16 != nil {
+		_ = binary.Write(&buf, binary.LittleEndian, printUTF16)
+	}
+
+	return buf.Bytes()
+}
+
+func TestDecodeJunctionEmptyPrintName(t *testing.T) {
+	raw := buildMountPointBuffer(`\??\C:\test\tgt`, "")
+
+	rp, err := DecodeReparsePoint(raw)
+	if err != nil {
+		t.Fatalf("decode reparse point failed: %v", err)
+	}
+	if rp.Target != `C:\test\tgt` {
+		t.Errorf("target = %q, want %q", rp.Target, `C:\test\tgt`)
+	}
+	if !rp.IsMountPoint {
+		t.Error("ismountpoint should be true")
+	}
+}
+
+func TestDecodeJunctionBothNames(t *testing.T) {
+	raw := buildMountPointBuffer(`\??\C:\test\tgt`, `C:\test\tgt`)
+
+	rp, err := DecodeReparsePoint(raw)
+	if err != nil {
+		t.Fatalf("decode reparse point failed: %v", err)
+	}
+	if rp.Target != `C:\test\tgt` {
+		t.Errorf("target = %q, want %q", rp.Target, `C:\test\tgt`)
+	}
+	if !rp.IsMountPoint {
+		t.Error("ismountpoint should be true")
+	}
+}
+
+func TestDecodeJunctionUNCPath(t *testing.T) {
+	raw := buildMountPointBuffer(`\??\UNC\server\share`, `\\server\share`)
+
+	rp, err := DecodeReparsePoint(raw)
+	if err != nil {
+		t.Fatalf("decode reparse point failed: %v", err)
+	}
+	if rp.Target != `\\server\share` {
+		t.Errorf("target = %q, want %q", rp.Target, `\\server\share`)
+	}
+}
+
+func TestDecodeJunctionVolumeGUIDPath(t *testing.T) {
+	raw := buildMountPointBuffer(`\??\Volume{00000000-0000-0000-0000-000000000000}\test`, `\\?\Volume{00000000-0000-0000-0000-000000000000}\test`)
+
+	rp, err := DecodeReparsePoint(raw)
+	if err != nil {
+		t.Fatalf("decode reparse point failed: %v", err)
+	}
+	if rp.Target != `\\?\Volume{00000000-0000-0000-0000-000000000000}\test` {
+		t.Errorf("target = %q, want %q", rp.Target, `\\?\Volume{00000000-0000-0000-0000-000000000000}\test`)
+	}
+}
+
+func TestRoundTripMountPointUNC(t *testing.T) {
+	original := &ReparsePoint{
+		Target:       `\\server\share`,
+		IsMountPoint: true,
+	}
+
+	encoded := EncodeReparsePoint(original)
+	decoded, err := DecodeReparsePoint(encoded)
+	if err != nil {
+		t.Fatalf("decode reparse point failed: %v", err)
+	}
+	if decoded.Target != original.Target {
+		t.Errorf("target = %q, want %q", decoded.Target, original.Target)
+	}
+	if !decoded.IsMountPoint {
+		t.Error("ismountpoint should be true")
+	}
+}
+
+func TestRoundTripMountPoint(t *testing.T) {
+	original := &ReparsePoint{
+		Target:       `C:\test\tgt`,
+		IsMountPoint: true,
+	}
+
+	encoded := EncodeReparsePoint(original)
+	decoded, err := DecodeReparsePoint(encoded)
+	if err != nil {
+		t.Fatalf("decode reparse point failed: %v", err)
+	}
+	if decoded.Target != original.Target {
+		t.Errorf("target = %q, want %q", decoded.Target, original.Target)
+	}
+	if !decoded.IsMountPoint {
+		t.Error("ismountpoint should be true")
+	}
+}
+
+func TestRoundTripSymlink(t *testing.T) {
+	original := &ReparsePoint{
+		Target:       `C:\test\tgt`,
+		IsMountPoint: false,
+	}
+
+	encoded := EncodeReparsePoint(original)
+	decoded, err := DecodeReparsePoint(encoded)
+	if err != nil {
+		t.Fatalf("decode reparse point failed: %v", err)
+	}
+	if decoded.Target != original.Target {
+		t.Errorf("target = %q, want %q", decoded.Target, original.Target)
+	}
+	if decoded.IsMountPoint {
+		t.Error("ismountpoint should be false")
+	}
+}
+
+func TestRoundTripSymlinkVolumeGUIDPath(t *testing.T) {
+	original := &ReparsePoint{
+		Target:       `\\?\Volume{00000000-0000-0000-0000-000000000000}\test`,
+		IsMountPoint: false,
+	}
+
+	encoded := EncodeReparsePoint(original)
+	decoded, err := DecodeReparsePoint(encoded)
+	if err != nil {
+		t.Fatalf("decode reparse point failed: %v", err)
+	}
+	if decoded.Target != original.Target {
+		t.Errorf("target = %q, want %q", decoded.Target, original.Target)
+	}
+	if decoded.IsMountPoint {
+		t.Error("ismountpoint should be false")
+	}
+}