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moby--moby/pkg/symlink/fs.go

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// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE.BSD file.
// This code is a modified version of path/filepath/symlink.go from the Go standard library.
package symlink
import (
"bytes"
"errors"
"os"
"path/filepath"
"strings"
"github.com/docker/docker/pkg/system"
)
// FollowSymlinkInScope is a wrapper around evalSymlinksInScope that returns an
// absolute path. This function handles paths in a platform-agnostic manner.
func FollowSymlinkInScope(path, root string) (string, error) {
path, err := filepath.Abs(filepath.FromSlash(path))
if err != nil {
return "", err
}
root, err = filepath.Abs(filepath.FromSlash(root))
if err != nil {
return "", err
}
return evalSymlinksInScope(path, root)
}
// evalSymlinksInScope will evaluate symlinks in `path` within a scope `root` and return
// a result guaranteed to be contained within the scope `root`, at the time of the call.
// Symlinks in `root` are not evaluated and left as-is.
// Errors encountered while attempting to evaluate symlinks in path will be returned.
// Non-existing paths are valid and do not constitute an error.
// `path` has to contain `root` as a prefix, or else an error will be returned.
// Trying to break out from `root` does not constitute an error.
//
// Example:
// If /foo/bar -> /outside,
// FollowSymlinkInScope("/foo/bar", "/foo") == "/foo/outside" instead of "/outside"
//
// IMPORTANT: it is the caller's responsibility to call evalSymlinksInScope *after* relevant symlinks
// are created and not to create subsequently, additional symlinks that could potentially make a
// previously-safe path, unsafe. Example: if /foo/bar does not exist, evalSymlinksInScope("/foo/bar", "/foo")
// would return "/foo/bar". If one makes /foo/bar a symlink to /baz subsequently, then "/foo/bar" should
// no longer be considered safely contained in "/foo".
func evalSymlinksInScope(path, root string) (string, error) {
root = filepath.Clean(root)
if path == root {
return path, nil
}
if !strings.HasPrefix(path, root) {
return "", errors.New("evalSymlinksInScope: " + path + " is not in " + root)
}
const maxIter = 255
originalPath := path
// given root of "/a" and path of "/a/b/../../c" we want path to be "/b/../../c"
path = path[len(root):]
if root == string(filepath.Separator) {
path = string(filepath.Separator) + path
}
if !strings.HasPrefix(path, string(filepath.Separator)) {
return "", errors.New("evalSymlinksInScope: " + path + " is not in " + root)
}
path = filepath.Clean(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
// b here will always be considered to be the "current absolute path inside
// root" when we append paths to it, we also append a slash and use
// filepath.Clean after the loop to trim the trailing slash
for n := 0; path != ""; n++ {
if n > maxIter {
return "", errors.New("evalSymlinksInScope: too many links in " + originalPath)
}
// find next path component, p
i := strings.IndexRune(path, filepath.Separator)
var p string
if i == -1 {
p, path = path, ""
} else {
p, path = path[:i], path[i+1:]
}
if p == "" {
continue
}
// this takes a b.String() like "b/../" and a p like "c" and turns it
// into "/b/../c" which then gets filepath.Cleaned into "/c" and then
// root gets prepended and we Clean again (to remove any trailing slash
// if the first Clean gave us just "/")
cleanP := filepath.Clean(string(filepath.Separator) + b.String() + p)
if isDriveOrRoot(cleanP) {
// never Lstat "/" itself, or drive letters on Windows
b.Reset()
continue
}
fullP := filepath.Clean(root + cleanP)
fi, err := os.Lstat(fullP)
if os.IsNotExist(err) {
// if p does not exist, accept it
b.WriteString(p)
b.WriteRune(filepath.Separator)
continue
}
if err != nil {
return "", err
}
if fi.Mode()&os.ModeSymlink == 0 {
b.WriteString(p)
b.WriteRune(filepath.Separator)
continue
}
// it's a symlink, put it at the front of path
dest, err := os.Readlink(fullP)
if err != nil {
return "", err
}
if system.IsAbs(dest) {
b.Reset()
}
path = dest + string(filepath.Separator) + path
}
// see note above on "fullP := ..." for why this is double-cleaned and
// what's happening here
return filepath.Clean(root + filepath.Clean(string(filepath.Separator)+b.String())), nil
}
// EvalSymlinks returns the path name after the evaluation of any symbolic
// links.
// If path is relative the result will be relative to the current directory,
// unless one of the components is an absolute symbolic link.
// This version has been updated to support long paths prepended with `\\?\`.
func EvalSymlinks(path string) (string, error) {
return evalSymlinks(path)
}