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

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package symlink
import (
"fmt"
"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)
if err != nil {
return "", err
}
link, err = filepath.Abs(link)
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
}