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bind_filter.go
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bind_filter.go
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//go:build windows
// +build windows
package bindfilter
import (
"bytes"
"encoding/binary"
"errors"
"fmt"
"os"
"path/filepath"
"strings"
"unsafe"
"golang.org/x/sys/windows"
)
//go:generate go run github.com/Microsoft/go-winio/tools/mkwinsyscall -output zsyscall_windows.go ./bind_filter.go
//sys bfSetupFilter(jobHandle windows.Handle, flags uint32, virtRootPath string, virtTargetPath string, virtExceptions **uint16, virtExceptionPathCount uint32) (hr error) = bindfltapi.BfSetupFilter?
//sys bfRemoveMapping(jobHandle windows.Handle, virtRootPath string) (hr error) = bindfltapi.BfRemoveMapping?
//sys bfGetMappings(flags uint32, jobHandle windows.Handle, virtRootPath *uint16, sid *windows.SID, bufferSize *uint32, outBuffer *byte) (hr error) = bindfltapi.BfGetMappings?
// BfSetupFilter flags. See:
// https://github.com/microsoft/BuildXL/blob/a6dce509f0d4f774255e5fbfb75fa6d5290ed163/Public/Src/Utilities/Native/Processes/Windows/NativeContainerUtilities.cs#L193-L240
//
//nolint:revive // var-naming: ALL_CAPS
const (
BINDFLT_FLAG_READ_ONLY_MAPPING uint32 = 0x00000001
// Tells bindflt to fail mapping with STATUS_INVALID_PARAMETER if a mapping produces
// multiple targets.
BINDFLT_FLAG_NO_MULTIPLE_TARGETS uint32 = 0x00000040
)
//nolint:revive // var-naming: ALL_CAPS
const (
BINDFLT_GET_MAPPINGS_FLAG_VOLUME uint32 = 0x00000001
BINDFLT_GET_MAPPINGS_FLAG_SILO uint32 = 0x00000002
BINDFLT_GET_MAPPINGS_FLAG_USER uint32 = 0x00000004
)
// ApplyFileBinding creates a global mount of the source in root, with an optional
// read only flag.
// The bind filter allows us to create mounts of directories and volumes. By default it allows
// us to mount multiple sources inside a single root, acting as an overlay. Files from the
// second source will superscede the first source that was mounted.
// This function disables this behavior and sets the BINDFLT_FLAG_NO_MULTIPLE_TARGETS flag
// on the mount.
func ApplyFileBinding(root, source string, readOnly bool) error {
// The parent directory needs to exist for the bind to work. MkdirAll stats and
// returns nil if the directory exists internally so we should be fine to mkdirall
// every time.
if err := os.MkdirAll(filepath.Dir(root), 0); err != nil {
return err
}
if strings.Contains(source, "Volume{") && !strings.HasSuffix(source, "\\") {
// Add trailing slash to volumes, otherwise we get an error when binding it to
// a folder.
source = source + "\\"
}
flags := BINDFLT_FLAG_NO_MULTIPLE_TARGETS
if readOnly {
flags |= BINDFLT_FLAG_READ_ONLY_MAPPING
}
// Set the job handle to 0 to create a global mount.
if err := bfSetupFilter(
0,
flags,
root,
source,
nil,
0,
); err != nil {
return fmt.Errorf("failed to bind target %q to root %q: %w", source, root, err)
}
return nil
}
// RemoveFileBinding removes a mount from the root path.
func RemoveFileBinding(root string) error {
if err := bfRemoveMapping(0, root); err != nil {
return fmt.Errorf("removing file binding: %w", err)
}
return nil
}
// GetBindMappings returns a list of bind mappings that have their root on a
// particular volume. The volumePath parameter can be any path that exists on
// a volume. For example, if a number of mappings are created in C:\ProgramData\test,
// to get a list of those mappings, the volumePath parameter would have to be set to
// C:\ or the VOLUME_NAME_GUID notation of C:\ (\\?\Volume{GUID}\), or any child
// path that exists.
func GetBindMappings(volumePath string) ([]BindMapping, error) {
rootPtr, err := windows.UTF16PtrFromString(volumePath)
if err != nil {
return nil, err
}
flags := BINDFLT_GET_MAPPINGS_FLAG_VOLUME
// allocate a large buffer for results
var outBuffSize uint32 = 256 * 1024
buf := make([]byte, outBuffSize)
if err := bfGetMappings(flags, 0, rootPtr, nil, &outBuffSize, &buf[0]); err != nil {
return nil, err
}
if outBuffSize < 12 {
return nil, fmt.Errorf("invalid buffer returned")
}
result := buf[:outBuffSize]
// The first 12 bytes are the three uint32 fields in getMappingsResponseHeader{}
headerBuffer := result[:12]
// The alternative to using unsafe and casting it to the above defined structures, is to manually
// parse the fields. Not too terrible, but not sure it'd worth the trouble.
header := *(*getMappingsResponseHeader)(unsafe.Pointer(&headerBuffer[0]))
if header.MappingCount == 0 {
// no mappings
return []BindMapping{}, nil
}
mappingsBuffer := result[12 : int(unsafe.Sizeof(mappingEntry{}))*int(header.MappingCount)]
// Get a pointer to the first mapping in the slice
mappingsPointer := (*mappingEntry)(unsafe.Pointer(&mappingsBuffer[0]))
// Get slice of mappings
mappings := unsafe.Slice(mappingsPointer, header.MappingCount)
mappingEntries := make([]BindMapping, header.MappingCount)
for i := 0; i < int(header.MappingCount); i++ {
bindMapping, err := getBindMappingFromBuffer(result, mappings[i])
if err != nil {
return nil, fmt.Errorf("fetching bind mappings: %w", err)
}
mappingEntries[i] = bindMapping
}
return mappingEntries, nil
}
// mappingEntry holds information about where in the response buffer we can
// find information about the virtual root (the mount point) and the targets (sources)
// that get mounted, as well as the flags used to bind the targets to the virtual root.
type mappingEntry struct {
VirtRootLength uint32
VirtRootOffset uint32
Flags uint32
NumberOfTargets uint32
TargetEntriesOffset uint32
}
type mappingTargetEntry struct {
TargetRootLength uint32
TargetRootOffset uint32
}
// getMappingsResponseHeader represents the first 12 bytes of the BfGetMappings() response.
// It gives us the size of the buffer, the status of the call and the number of mappings.
// A response
type getMappingsResponseHeader struct {
Size uint32
Status uint32
MappingCount uint32
}
type BindMapping struct {
MountPoint string
Flags uint32
Targets []string
}
func decodeEntry(buffer []byte) (string, error) {
name := make([]uint16, len(buffer)/2)
err := binary.Read(bytes.NewReader(buffer), binary.LittleEndian, &name)
if err != nil {
return "", fmt.Errorf("decoding name: %w", err)
}
return windows.UTF16ToString(name), nil
}
func getTargetsFromBuffer(buffer []byte, offset, count int) ([]string, error) {
if len(buffer) < offset+count*6 {
return nil, fmt.Errorf("invalid buffer")
}
targets := make([]string, count)
for i := 0; i < count; i++ {
entryBuf := buffer[offset+i*8 : offset+i*8+8]
tgt := *(*mappingTargetEntry)(unsafe.Pointer(&entryBuf[0]))
if len(buffer) < int(tgt.TargetRootOffset)+int(tgt.TargetRootLength) {
return nil, fmt.Errorf("invalid buffer")
}
decoded, err := decodeEntry(buffer[tgt.TargetRootOffset : tgt.TargetRootOffset+tgt.TargetRootLength])
if err != nil {
return nil, fmt.Errorf("decoding name: %w", err)
}
decoded, err = getFinalPath(decoded)
if err != nil {
return nil, fmt.Errorf("fetching final path: %w", err)
}
targets[i] = decoded
}
return targets, nil
}
func getFinalPath(pth string) (string, error) {
// BfGetMappings returns VOLUME_NAME_NT paths like \Device\HarddiskVolume2\ProgramData.
// These can be accessed by prepending \\.\GLOBALROOT to the path. We use this to get the
// DOS paths for these files.
if strings.HasPrefix(pth, `\Device`) {
pth = `\\.\GLOBALROOT` + pth
}
han, err := openPath(pth)
if err != nil {
return "", fmt.Errorf("fetching file handle: %w", err)
}
defer func() {
_ = windows.CloseHandle(han)
}()
buf := make([]uint16, 100)
var flags uint32 = 0x0
for {
n, err := windows.GetFinalPathNameByHandle(han, &buf[0], uint32(len(buf)), flags)
if err != nil {
// if we mounted a volume that does not also have a drive letter assigned, attempting to
// fetch the VOLUME_NAME_DOS will fail with os.ErrNotExist. Attempt to get the VOLUME_NAME_GUID.
if errors.Is(err, os.ErrNotExist) && flags != 0x1 {
flags = 0x1
continue
}
return "", fmt.Errorf("getting final path name: %w", err)
}
if n < uint32(len(buf)) {
break
}
buf = make([]uint16, n)
}
finalPath := windows.UTF16ToString(buf)
// We got VOLUME_NAME_DOS, we need to strip away some leading slashes.
// Leave unchanged if we ended up requesting VOLUME_NAME_GUID
if len(finalPath) > 4 && finalPath[:4] == `\\?\` && flags == 0x0 {
finalPath = finalPath[4:]
if len(finalPath) > 3 && finalPath[:3] == `UNC` {
// return path like \\server\share\...
finalPath = `\` + finalPath[3:]
}
}
return finalPath, nil
}
func getBindMappingFromBuffer(buffer []byte, entry mappingEntry) (BindMapping, error) {
if len(buffer) < int(entry.VirtRootOffset)+int(entry.VirtRootLength) {
return BindMapping{}, fmt.Errorf("invalid buffer")
}
src, err := decodeEntry(buffer[entry.VirtRootOffset : entry.VirtRootOffset+entry.VirtRootLength])
if err != nil {
return BindMapping{}, fmt.Errorf("decoding entry: %w", err)
}
targets, err := getTargetsFromBuffer(buffer, int(entry.TargetEntriesOffset), int(entry.NumberOfTargets))
if err != nil {
return BindMapping{}, fmt.Errorf("fetching targets: %w", err)
}
src, err = getFinalPath(src)
if err != nil {
return BindMapping{}, fmt.Errorf("fetching final path: %w", err)
}
return BindMapping{
Flags: entry.Flags,
Targets: targets,
MountPoint: src,
}, nil
}
func openPath(path string) (windows.Handle, error) {
u16, err := windows.UTF16PtrFromString(path)
if err != nil {
return 0, err
}
h, err := windows.CreateFile(
u16,
0,
windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
nil,
windows.OPEN_EXISTING,
windows.FILE_FLAG_BACKUP_SEMANTICS, // Needed to open a directory handle.
0)
if err != nil {
return 0, &os.PathError{
Op: "CreateFile",
Path: path,
Err: err,
}
}
return h, nil
}