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memfs.go
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memfs.go
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// Copyright 2023 Chainguard, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package fs
import (
"archive/tar"
"bytes"
"encoding/binary"
"errors"
"fmt"
"io"
"io/fs"
"os"
"path/filepath"
"strings"
"github.com/blang/vfs"
origMemfs "github.com/blang/vfs/memfs"
"golang.org/x/sys/unix"
)
const (
linkFlagSymlink = 's'
linkFlagHardlink = 'h'
)
func NewMemFS() FullFS {
return &memFS{origMemfs.Create(), map[string]*ownership{}}
}
type ownership struct {
uid, gid int
perms fs.FileMode
}
type memFS struct {
*origMemfs.MemFS
perms map[string]*ownership // used only for overrides
}
func (m *memFS) MkdirAll(path string, perm fs.FileMode) error {
// Fast path: if we can tell whether path is a directory or file, stop with success or error.
dir, err := m.Stat(path)
if err == nil {
if dir.IsDir() {
return nil
}
return &os.PathError{Op: "mkdir", Path: path, Err: errors.New("not a directory")}
}
// Slow path: make sure parent exists and then call Mkdir for path.
i := len(path)
for i > 0 && os.IsPathSeparator(path[i-1]) { // Skip trailing path separator.
i--
}
j := i
for j > 0 && !os.IsPathSeparator(path[j-1]) { // Scan backward over element.
j--
}
if j > 1 {
// Create parent.
err = m.MkdirAll(path[:j-1], fs.ModeDir|perm)
if err != nil {
return err
}
}
// Parent now exists; invoke Mkdir and use its result.
err = m.Mkdir(path, fs.ModeDir|perm)
if err != nil {
// Handle arguments like "foo/." by
// double-checking that directory doesn't exist.
dir, err1 := m.Lstat(path)
if err1 == nil && dir.IsDir() {
return nil
}
return err
}
return nil
}
func (m *memFS) Open(name string) (fs.File, error) {
return m.OpenFile(name, os.O_RDONLY, 0o644)
}
func (m *memFS) OpenFile(name string, flag int, perm fs.FileMode) (File, error) {
// m.MemFS does not support symlinks, so we need to give it a resolved path.
// That means walking through all of the parts until we get to the end, and
// giving it the actual path to the file.
truename, err := m.walkSymlinks(name)
if err != nil {
return nil, err
}
return m.openFile(truename, flag, perm)
}
// openFile opens a file, but assumes all symlinks in the interim path have been resolved.
func (m *memFS) openFile(name string, flag int, perm fs.FileMode) (*memFile, error) {
f, err := m.MemFS.OpenFile(name, flag, perm)
if err != nil {
return nil, err
}
return &memFile{f, m}, nil
}
func (m *memFS) ReadFile(name string) ([]byte, error) {
f, err := m.OpenFile(name, os.O_RDONLY, 0o644)
if err != nil {
return nil, err
}
defer f.Close()
b := bytes.NewBuffer(nil)
if _, err := io.Copy(b, f); err != nil {
return nil, err
}
return b.Bytes(), nil
}
func (m *memFS) WriteFile(name string, b []byte, mode fs.FileMode) error {
f, err := m.OpenFile(name, os.O_RDWR|os.O_CREATE, mode)
if err != nil {
return err
}
defer f.Close()
if _, err := io.Copy(f, bytes.NewBuffer(b)); err != nil {
return err
}
return nil
}
func (m *memFS) ReadDir(name string) ([]fs.DirEntry, error) {
fi, err := m.MemFS.ReadDir(name)
if err != nil {
return nil, err
}
var de = make([]fs.DirEntry, 0, len(fi))
for _, f := range fi {
// what if we had overrides for ownership or permissions?
if o, ok := m.perms[standardizePath(filepath.Join(name, f.Name()))]; ok {
f = &memFileInfo{f, o.perms, o.uid, o.gid}
}
de = append(de, fs.FileInfoToDirEntry(f))
}
return de, nil
}
func (m *memFS) OpenReaderAt(name string) (File, error) {
return m.open(name)
}
func (m *memFS) open(name string) (*memFile, error) {
f, err := m.MemFS.OpenFile(name, os.O_RDONLY, 0o644)
if err != nil {
return nil, err
}
return &memFile{File: f, fs: m}, nil
}
func (m *memFS) Mknod(path string, mode uint32, dev int) error {
file, err := m.OpenFile(
path,
os.O_CREATE|os.O_WRONLY|os.O_TRUNC,
fs.FileMode(mode)|os.ModeCharDevice|os.ModeDevice,
)
if err != nil {
return err
}
defer file.Close()
// save the major and minor numbers in the file itself
devNumbers := []uint32{unix.Major(uint64(dev)), unix.Minor(uint64(dev))}
return binary.Write(file, binary.LittleEndian, devNumbers)
}
func (m *memFS) Readnod(name string) (dev int, err error) {
file, err := m.Open(name)
if err != nil {
return 0, err
}
defer file.Close()
fi, err := file.Stat()
if err != nil {
return 0, err
}
if fi.Mode()&os.ModeCharDevice != os.ModeCharDevice {
return 0, fmt.Errorf("%s not a character device", name)
}
// read the major and minor numbers from the file itself
devNumbers := make([]uint32, 2)
if err := binary.Read(file, binary.LittleEndian, devNumbers); err != nil {
return 0, err
}
return int(unix.Mkdev(devNumbers[0], devNumbers[1])), nil
}
func (m *memFS) Chmod(path string, perm fs.FileMode) error {
p := standardizePath(path)
o, ok := m.perms[p]
if !ok {
// take any existing perms from the memfs; we are overring uid/gid anyways
fi, err := m.MemFS.Stat(path)
if err != nil {
return err
}
o = &ownership{perms: fi.Mode()}
m.perms[p] = o
}
// perms must reflect the file type as well
o.perms = perm | (o.perms & os.ModeType)
return nil
}
func (m *memFS) Chown(path string, uid int, gid int) error {
p := standardizePath(path)
o, ok := m.perms[p]
if !ok {
// take any existing perms from the memfs; we are overring uid/gid anyways
fi, err := m.MemFS.Stat(path)
if err != nil {
return err
}
o = &ownership{perms: fi.Mode()}
m.perms[p] = o
}
o.uid = uid
o.gid = gid
return nil
}
func (m *memFS) Create(name string) (File, error) {
return m.OpenFile(name, os.O_CREATE|os.O_TRUNC|os.O_RDWR, 0o666)
}
func (m *memFS) Symlink(oldname, newname string) error {
return m.multilink(true, oldname, newname)
}
func (m *memFS) Link(oldname, newname string) error {
return m.multilink(false, oldname, newname)
}
func (m *memFS) multilink(symlink bool, oldname, newname string) error {
file, err := m.OpenFile(
newname,
os.O_CREATE|os.O_WRONLY|os.O_TRUNC,
0777|os.ModeSymlink)
if err != nil {
return err
}
defer file.Close()
var linkType = linkFlagHardlink
if symlink {
linkType = linkFlagSymlink
}
// hardlinks in tar always are absolute paths, so we need to include the base /
if !symlink {
oldname = filepath.Clean(fmt.Sprintf("%c%s", filepath.Separator, oldname))
}
// save the target in the file itself
_, err = file.Write([]byte(fmt.Sprintf("%c:%s", linkType, oldname)))
return err
}
func (m *memFS) Readlink(name string) (target string, symlink bool, err error) {
truename, err := m.walkSymlinks(filepath.Dir(name))
if err != nil {
return "", false, err
}
return m.readSymlink(filepath.Join(truename, filepath.Base(name)))
}
// readHardlink reads the hardlink target directly from the memfs.
func (m *memFS) readHardlink(name string) (string, bool, error) {
target, linkType, err := m.readLinkBase(name)
return target, linkType == linkFlagHardlink, err
}
// readSymlink reads the symlink target directly from the memfs.
func (m *memFS) readSymlink(name string) (string, bool, error) {
target, linkType, err := m.readLinkBase(name)
return target, linkType == linkFlagSymlink, err
}
// readLinkBase reads the link base target directly from the memfs.
func (m *memFS) readLinkBase(name string) (string, byte, error) {
file, err := m.openFile(name, os.O_RDONLY, 0o644)
if err != nil {
return "", 0, err
}
defer file.Close()
fi, err := file.Stat()
if err != nil {
return "", 0, err
}
buf := make([]byte, fi.Size())
if _, err = file.Read(buf); err != nil {
return "", 0, err
}
// first 2 bytes are the link type ("h" or "s") and a separator ":"
if len(buf) < 3 {
return "", 0, fmt.Errorf("invalid link %s", name)
}
return string(buf[2:]), buf[0], nil
}
func (m *memFS) walkSymlinks(path string) (string, error) {
var final string
segments := strings.Split(path, "/")
for i, seg := range segments {
if seg == "" {
continue
}
// see if this section exists; if it is not a symlink, just add it
segmentName := filepath.Join(final, seg)
fi, err := m.Stat(segmentName)
// we can ignore the last segment not existing
if err != nil {
if i == len(segments)-1 {
final = filepath.Join(final, seg)
continue
}
return "", err
}
// not a symlink, so just append it
if fi.Mode()&os.ModeSymlink != os.ModeSymlink {
final = filepath.Join(final, seg)
} else {
// it is a symlink, so resolve it
target, _, err := m.readSymlink(segmentName)
// what if it was not found?
switch {
case err != nil && i != len(segments)-1:
// we can handle the last one not existing, but no interim one
return "", err
case strings.HasPrefix(target, "/"):
// was it an absolute path?
final = target
default:
// relative path, so add it to the current path
final = filepath.Join(final, target)
}
}
}
return final, nil
}
type memFile struct {
vfs.File
fs *memFS
}
func (f *memFile) Stat() (fs.FileInfo, error) {
fi, err := f.fs.MemFS.Stat(f.Name())
if err != nil {
return nil, err
}
// what if we had overrides for ownership or permissions?
o, ok := f.fs.perms[f.Name()]
if !ok {
return fi, nil
}
return &memFileInfo{fi, o.perms, o.uid, o.gid}, nil
}
type memFileInfo struct {
fs.FileInfo
mode fs.FileMode
uid, gid int
}
func (f *memFileInfo) Mode() fs.FileMode {
return f.mode
}
func (f *memFileInfo) Sys() any {
return &tar.Header{
Mode: int64(f.mode),
Uid: f.uid,
Gid: f.gid,
}
}
func standardizePath(p string) string {
if p[0] == '/' {
p = p[1:]
}
return p
}