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Simplify FollowSymlinkInScope based on Go 1.3.3's EvalSymlinks

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Signed-off-by: Andrew Page <admwiggin@gmail.com>
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Tianon Gravi 2014-12-05 14:58:46 -07:00 committed by Tibor Vass
parent 74bbb93571
commit fa3ec89515
6 changed files with 344 additions and 88 deletions
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Copyright (c) 2014 The Docker & Go Authors. All rights reserved.
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* Redistributions of source code must retain the above copyright
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pkg/symlink/MAINTAINERS
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Michael Crosby <michael@crosbymichael.com> (@crosbymichael)
Victor Vieux <vieux@docker.com> (@vieux)
Tibor Vass <teabee89@gmail.com> (@tiborvass)
Cristian Staretu <cristian.staretu@gmail.com> (@unclejack)
Tianon Gravi <admwiggin@gmail.com> (@tianon)

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Package symlink implements EvalSymlinksInScope which is an extension of filepath.EvalSymlinks
from the [Go standard library](https://golang.org/pkg/path/filepath).
The code from filepath.EvalSymlinks has been adapted in fs.go.
Please read the LICENSE.BSD file that governs fs.go and LICENSE.APACHE for fs_test.go.

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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 (
"fmt"
"bytes"
"errors"
"os"
"path"
"path/filepath"
"strings"
)
const maxLoopCounter = 100
// FollowSymlink will follow an existing link and scope it to the root
// path provided.
// The role of this function is to return an absolute path in the root
// or normalize to the root if the symlink leads to a path which is
// outside of the root.
// Errors encountered while attempting to follow the symlink in path
// will be reported.
// Normalizations to the root don't constitute errors.
func FollowSymlinkInScope(link, root string) (string, error) {
root, err := filepath.Abs(root)
// FollowSymlinkInScope is a wrapper around evalSymlinksInScope that returns an absolute path
func FollowSymlinkInScope(path, root string) (string, error) {
path, err := filepath.Abs(path)
if err != nil {
return "", err
}
link, err = filepath.Abs(link)
root, err = filepath.Abs(root)
if err != nil {
return "", err
}
if link == root {
return root, nil
}
if !strings.HasPrefix(filepath.Dir(link), root) {
return "", fmt.Errorf("%s is not within %s", link, root)
}
prev := "/"
for _, p := range strings.Split(link, "/") {
prev = filepath.Join(prev, p)
loopCounter := 0
for {
loopCounter++
if loopCounter >= maxLoopCounter {
return "", fmt.Errorf("loopCounter reached MAX: %v", loopCounter)
}
if !strings.HasPrefix(prev, root) {
// Don't resolve symlinks outside of root. For example,
// we don't have to check /home in the below.
//
// /home -> usr/home
// FollowSymlinkInScope("/home/bob/foo/bar", "/home/bob/foo")
break
}
stat, err := os.Lstat(prev)
if err != nil {
if os.IsNotExist(err) {
break
}
return "", err
}
// let's break if we're not dealing with a symlink
if stat.Mode()&os.ModeSymlink != os.ModeSymlink {
break
}
// process the symlink
dest, err := os.Readlink(prev)
if err != nil {
return "", err
}
if path.IsAbs(dest) {
prev = filepath.Join(root, dest)
} else {
prev, _ = filepath.Abs(prev)
dir := filepath.Dir(prev)
prev = filepath.Join(dir, dest)
if dir == root && !strings.HasPrefix(prev, root) {
prev = root
}
if len(prev) < len(root) || (len(prev) == len(root) && prev != root) {
prev = filepath.Join(root, filepath.Base(dest))
}
}
}
}
if prev == "/" {
prev = root
}
return prev, nil
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 "/oustide"
//
// 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 cleanP == string(filepath.Separator) {
// never Lstat "/" itself
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 + string(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 filepath.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
}

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// Licensed under the Apache License, Version 2.0; See LICENSE.APACHE
package symlink
import (