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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) Tibor Vass <teabee89@gmail.com> (@tiborvass)
Victor Vieux <vieux@docker.com> (@vieux) 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 package symlink
import ( import (
"fmt" "bytes"
"errors"
"os" "os"
"path"
"path/filepath" "path/filepath"
"strings" "strings"
) )
const maxLoopCounter = 100 // FollowSymlinkInScope is a wrapper around evalSymlinksInScope that returns an absolute path
func FollowSymlinkInScope(path, root string) (string, error) {
// FollowSymlink will follow an existing link and scope it to the root path, err := filepath.Abs(path)
// 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)
if err != nil { if err != nil {
return "", err return "", err
} }
root, err = filepath.Abs(root)
link, err = filepath.Abs(link)
if err != nil { if err != nil {
return "", err return "", err
} }
return evalSymlinksInScope(path, root)
if link == root { }
return root, nil
} // 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.
if !strings.HasPrefix(filepath.Dir(link), root) { // Symlinks in `root` are not evaluated and left as-is.
return "", fmt.Errorf("%s is not within %s", link, root) // 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.
prev := "/" // Trying to break out from `root` does not constitute an error.
//
for _, p := range strings.Split(link, "/") { // Example:
prev = filepath.Join(prev, p) // If /foo/bar -> /outside,
// FollowSymlinkInScope("/foo/bar", "/foo") == "/foo/outside" instead of "/oustide"
loopCounter := 0 //
for { // IMPORTANT: it is the caller's responsibility to call evalSymlinksInScope *after* relevant symlinks
loopCounter++ // 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")
if loopCounter >= maxLoopCounter { // would return "/foo/bar". If one makes /foo/bar a symlink to /baz subsequently, then "/foo/bar" should
return "", fmt.Errorf("loopCounter reached MAX: %v", loopCounter) // no longer be considered safely contained in "/foo".
} func evalSymlinksInScope(path, root string) (string, error) {
root = filepath.Clean(root)
if !strings.HasPrefix(prev, root) { if path == root {
// Don't resolve symlinks outside of root. For example, return path, nil
// we don't have to check /home in the below. }
// if !strings.HasPrefix(path, root) {
// /home -> usr/home return "", errors.New("evalSymlinksInScope: " + path + " is not in " + root)
// FollowSymlinkInScope("/home/bob/foo/bar", "/home/bob/foo") }
break const maxIter = 255
} originalPath := path
// given root of "/a" and path of "/a/b/../../c" we want path to be "/b/../../c"
stat, err := os.Lstat(prev) path = path[len(root):]
if err != nil { if root == string(filepath.Separator) {
if os.IsNotExist(err) { path = string(filepath.Separator) + path
break }
} if !strings.HasPrefix(path, string(filepath.Separator)) {
return "", err return "", errors.New("evalSymlinksInScope: " + path + " is not in " + root)
} }
path = filepath.Clean(path)
// let's break if we're not dealing with a symlink // consume path by taking each frontmost path element,
if stat.Mode()&os.ModeSymlink != os.ModeSymlink { // expanding it if it's a symlink, and appending it to b
break 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
// process the symlink // filepath.Clean after the loop to trim the trailing slash
dest, err := os.Readlink(prev) for n := 0; path != ""; n++ {
if err != nil { if n > maxIter {
return "", err return "", errors.New("evalSymlinksInScope: too many links in " + originalPath)
} }
if path.IsAbs(dest) { // find next path component, p
prev = filepath.Join(root, dest) i := strings.IndexRune(path, filepath.Separator)
} else { var p string
prev, _ = filepath.Abs(prev) if i == -1 {
p, path = path, ""
dir := filepath.Dir(prev) } else {
prev = filepath.Join(dir, dest) p, path = path[:i], path[i+1:]
if dir == root && !strings.HasPrefix(prev, root) { }
prev = root
} if p == "" {
if len(prev) < len(root) || (len(prev) == len(root) && prev != root) { continue
prev = filepath.Join(root, filepath.Base(dest)) }
}
} // 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 prev == "/" { // if the first Clean gave us just "/")
prev = root cleanP := filepath.Clean(string(filepath.Separator) + b.String() + p)
} if cleanP == string(filepath.Separator) {
return prev, nil // 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 package symlink
import ( import (