restic/internal/fs/freadlink_windows.go

package fs

import (
	"os"
	"syscall"
	"unsafe"

	"golang.org/x/sys/windows"
)

func Freadlink(fd uintptr, name string) (string, error) {
	link, err := readReparseLink(windows.Handle(fd))
	if err != nil {
		return "", &os.PathError{Op: "readlink", Path: name, Err: err}
	}
	return link, nil
}

// based on src/os/file_windows.go from Go 1.23.2
// internally readReparseLink from the std library uses a filehandle, however,
// the external interface is based on a path. Thus, copy everything and minimally
// tweak it to allow passing in a file handle.

// normaliseLinkPath converts absolute paths returned by
// DeviceIoControl(h, FSCTL_GET_REPARSE_POINT, ...)
// into paths acceptable by all Windows APIs.
// For example, it converts
//
//	\??\C:\foo\bar into C:\foo\bar
//	\??\UNC\foo\bar into \\foo\bar
//	\??\Volume{abc}\ into \\?\Volume{abc}\
func normaliseLinkPath(path string) (string, error) {
	if len(path) < 4 || path[:4] != `\??\` {
		// unexpected path, return it as is
		return path, nil
	}
	// we have path that start with \??\
	s := path[4:]
	switch {
	case len(s) >= 2 && s[1] == ':': // \??\C:\foo\bar
		return s, nil
	case len(s) >= 4 && s[:4] == `UNC\`: // \??\UNC\foo\bar
		return `\\` + s[4:], nil
	}

	// \??\Volume{abc}\
	return `\\?\` + path[4:], nil
	// modified to remove the legacy codepath for winreadlinkvolume == 0
}

func readReparseLink(h windows.Handle) (string, error) {
	rdbbuf := make([]byte, windows.MAXIMUM_REPARSE_DATA_BUFFER_SIZE)
	var bytesReturned uint32
	err := windows.DeviceIoControl(h, windows.FSCTL_GET_REPARSE_POINT, nil, 0, &rdbbuf[0], uint32(len(rdbbuf)), &bytesReturned, nil)
	if err != nil {
		return "", err
	}

	rdb := (*reparseDataBuffer)(unsafe.Pointer(&rdbbuf[0]))
	switch rdb.ReparseTag {
	case syscall.IO_REPARSE_TAG_SYMLINK:
		rb := (*symbolicLinkReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME))
		s := rb.Path()
		if rb.Flags&symlinkFlagRelative != 0 {
			return s, nil
		}
		return normaliseLinkPath(s)
	case windows.IO_REPARSE_TAG_MOUNT_POINT:
		return normaliseLinkPath((*mountPointReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME)).Path())
	default:
		// the path is not a symlink or junction but another type of reparse
		// point
		return "", syscall.ENOENT
	}
}

// copied from src/internal/syscall/windows/reparse_windows.go from Go 1.23.0
// renamed to not export symbols

const symlinkFlagRelative = 1

type reparseDataBuffer struct {
	ReparseTag        uint32
	ReparseDataLength uint16
	Reserved          uint16
	DUMMYUNIONNAME    byte
}

type symbolicLinkReparseBuffer struct {
	// The integer that contains the offset, in bytes,
	// of the substitute name string in the PathBuffer array,
	// computed as an offset from byte 0 of PathBuffer. Note that
	// this offset must be divided by 2 to get the array index.
	SubstituteNameOffset uint16
	// The integer that contains the length, in bytes, of the
	// substitute name string. If this string is null-terminated,
	// SubstituteNameLength does not include the Unicode null character.
	SubstituteNameLength uint16
	// PrintNameOffset is similar to SubstituteNameOffset.
	PrintNameOffset uint16
	// PrintNameLength is similar to SubstituteNameLength.
	PrintNameLength uint16
	// Flags specifies whether the substitute name is a full path name or
	// a path name relative to the directory containing the symbolic link.
	Flags      uint32
	PathBuffer [1]uint16
}

// Path returns path stored in rb.
func (rb *symbolicLinkReparseBuffer) Path() string {
	n1 := rb.SubstituteNameOffset / 2
	n2 := (rb.SubstituteNameOffset + rb.SubstituteNameLength) / 2
	return syscall.UTF16ToString((*[0xffff]uint16)(unsafe.Pointer(&rb.PathBuffer[0]))[n1:n2:n2])
}

type mountPointReparseBuffer struct {
	// The integer that contains the offset, in bytes,
	// of the substitute name string in the PathBuffer array,
	// computed as an offset from byte 0 of PathBuffer. Note that
	// this offset must be divided by 2 to get the array index.
	SubstituteNameOffset uint16
	// The integer that contains the length, in bytes, of the
	// substitute name string. If this string is null-terminated,
	// SubstituteNameLength does not include the Unicode null character.
	SubstituteNameLength uint16
	// PrintNameOffset is similar to SubstituteNameOffset.
	PrintNameOffset uint16
	// PrintNameLength is similar to SubstituteNameLength.
	PrintNameLength uint16
	PathBuffer      [1]uint16
}

// Path returns path stored in rb.
func (rb *mountPointReparseBuffer) Path() string {
	n1 := rb.SubstituteNameOffset / 2
	n2 := (rb.SubstituteNameOffset + rb.SubstituteNameLength) / 2
	return syscall.UTF16ToString((*[0xffff]uint16)(unsafe.Pointer(&rb.PathBuffer[0]))[n1:n2:n2])
}