Simplify FollowSymlinkInScope based on Go 1.3.3's EvalSymlinks

Signed-off-by: Andrew Page <admwiggin@gmail.com>

symlink/LICENSE.APACHE

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symlink/LICENSE.BSD

@@ -0,0 +1,27 @@
+Copyright (c) 2014 The Docker & Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

symlink/MAINTAINERS

@@ -1,2 +1,3 @@
-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)

symlink/README.md

@@ -0,0 +1,5 @@
+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.

symlink/fs.go

@@ -1,101 +1,131 @@
+// 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
+// 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 {
 		return "", err
 	}
-
+	root, err = filepath.Abs(root)
-	link, err = filepath.Abs(link)
 	if err != nil {
 		return "", err
 	}
-
+	return evalSymlinksInScope(path, root)
-	if link == root {
-		return root, nil
 }
 
-	if !strings.HasPrefix(filepath.Dir(link), root) {
+// evalSymlinksInScope will evaluate symlinks in `path` within a scope `root` and return
-		return "", fmt.Errorf("%s is not within %s", link, root)
+// 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.
-	prev := "/"
+// 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.
-	for _, p := range strings.Split(link, "/") {
+// Trying to break out from `root` does not constitute an error.
-		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
+// Example:
-				//   FollowSymlinkInScope("/home/bob/foo/bar", "/home/bob/foo")
+//   If /foo/bar -> /outside,
-				break
+//   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)
 		}
 
-			stat, err := os.Lstat(prev)
+		// find next path component, p
-			if err != nil {
+		i := strings.IndexRune(path, filepath.Separator)
-				if os.IsNotExist(err) {
+		var p string
-					break
+		if i == -1 {
-				}
+			p, path = path, ""
-				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)
+			p, path = path[:i], path[i+1:]
+		}
 
-				dir := filepath.Dir(prev)
+		if p == "" {
-				prev = filepath.Join(dir, dest)
+			continue
-				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))
+		// 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
 		}
-	if prev == "/" {
+
-		prev = root
+		// it's a symlink, put it at the front of path
+		dest, err := os.Readlink(fullP)
+		if err != nil {
+			return "", err
 		}
-	return prev, nil
+		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
 }

symlink/fs_test.go

@@ -1,3 +1,5 @@
+// Licensed under the Apache License, Version 2.0; See LICENSE.APACHE
+
 package symlink
 
 import (