containerd/pkg/symlink/fs_windows.go

/*
   Copyright The containerd Authors.

   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.
*/

// This file is a copy of moby/moby/pkg/symlink/fs_windows.go

package symlink

import (
	"bytes"
	"errors"
	"os"
	"path/filepath"
	"strings"

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

func toShort(path string) (string, error) {
	p, err := windows.UTF16FromString(path)
	if err != nil {
		return "", err
	}
	b := p // GetShortPathName says we can reuse buffer
	n, err := windows.GetShortPathName(&p[0], &b[0], uint32(len(b)))
	if err != nil {
		return "", err
	}
	if n > uint32(len(b)) {
		b = make([]uint16, n)
		if _, err = windows.GetShortPathName(&p[0], &b[0], uint32(len(b))); err != nil {
			return "", err
		}
	}
	return windows.UTF16ToString(b), nil
}

func toLong(path string) (string, error) {
	p, err := windows.UTF16FromString(path)
	if err != nil {
		return "", err
	}
	b := p // GetLongPathName says we can reuse buffer
	n, err := windows.GetLongPathName(&p[0], &b[0], uint32(len(b)))
	if err != nil {
		return "", err
	}
	if n > uint32(len(b)) {
		b = make([]uint16, n)
		n, err = windows.GetLongPathName(&p[0], &b[0], uint32(len(b)))
		if err != nil {
			return "", err
		}
	}
	b = b[:n]
	return windows.UTF16ToString(b), nil
}

func evalSymlinks(path string) (string, error) {
	path, err := walkSymlinks(path)
	if err != nil {
		return "", err
	}

	p, err := toShort(path)
	if err != nil {
		return "", err
	}
	p, err = toLong(p)
	if err != nil {
		return "", err
	}
	// windows.GetLongPathName does not change the case of the drive letter,
	// but the result of EvalSymlinks must be unique, so we have
	// EvalSymlinks(`c:\a`) == EvalSymlinks(`C:\a`).
	// Make drive letter upper case.
	if len(p) >= 2 && p[1] == ':' && 'a' <= p[0] && p[0] <= 'z' {
		p = string(p[0]+'A'-'a') + p[1:]
	} else if len(p) >= 6 && p[5] == ':' && 'a' <= p[4] && p[4] <= 'z' {
		p = p[:3] + string(p[4]+'A'-'a') + p[5:]
	}
	return filepath.Clean(p), nil
}

const (
	utf8RuneSelf   = 0x80
	longPathPrefix = `\\?\`
)

func walkSymlinks(path string) (string, error) {
	const maxIter = 255
	originalPath := path
	// consume path by taking each frontmost path element,
	// expanding it if it's a symlink, and appending it to b
	var b bytes.Buffer
	for n := 0; path != ""; n++ {
		if n > maxIter {
			return "", errors.New("EvalSymlinks: too many links in " + originalPath)
		}

		// A path beginning with `\\?\` represents the root, so automatically
		// skip that part and begin processing the next segment.
		if strings.HasPrefix(path, longPathPrefix) {
			b.WriteString(longPathPrefix)
			path = path[4:]
			continue
		}

		// find next path component, p
		var i = -1
		for j, c := range path {
			if c < utf8RuneSelf && os.IsPathSeparator(uint8(c)) {
				i = j
				break
			}
		}
		var p string
		if i == -1 {
			p, path = path, ""
		} else {
			p, path = path[:i], path[i+1:]
		}

		if p == "" {
			if b.Len() == 0 {
				// must be absolute path
				b.WriteRune(filepath.Separator)
			}
			continue
		}

		// If this is the first segment after the long path prefix, accept the
		// current segment as a volume root or UNC share and move on to the next.
		if b.String() == longPathPrefix {
			b.WriteString(p)
			b.WriteRune(filepath.Separator)
			continue
		}

		fi, err := os.Lstat(b.String() + p)
		if err != nil {
			return "", err
		}
		if fi.Mode()&os.ModeSymlink == 0 {
			b.WriteString(p)
			if path != "" || (b.Len() == 2 && len(p) == 2 && p[1] == ':') {
				b.WriteRune(filepath.Separator)
			}
			continue
		}

		// it's a symlink, put it at the front of path
		dest, err := os.Readlink(b.String() + p)
		if err != nil {
			return "", err
		}
		if isAbs(dest) {
			b.Reset()
		}
		path = dest + string(filepath.Separator) + path
	}
	return filepath.Clean(b.String()), nil
}

func isDriveOrRoot(p string) bool {
	if p == string(filepath.Separator) {
		return true
	}

	length := len(p)
	if length >= 2 {
		if p[length-1] == ':' && (('a' <= p[length-2] && p[length-2] <= 'z') || ('A' <= p[length-2] && p[length-2] <= 'Z')) {
			return true
		}
	}
	return false
}

// isAbs is a platform-specific wrapper for filepath.IsAbs. On Windows,
// golang filepath.IsAbs does not consider a path \windows\system32 as absolute
// as it doesn't start with a drive-letter/colon combination. However, in
// docker we need to verify things such as WORKDIR /windows/system32 in
// a Dockerfile (which gets translated to \windows\system32 when being processed
// by the daemon. This SHOULD be treated as absolute from a docker processing
// perspective.
func isAbs(path string) bool {
	if filepath.IsAbs(path) || strings.HasPrefix(path, string(os.PathSeparator)) {
		return true
	}
	return false
}