Update tar path resolution

Fixes bug for resolving symlinks which allowed fully resolving
an existing symlink to a path, causing some symlinks to get
overridden as symlinks to self.
Updates logic to split name into parent path, resolve the parent
path, then safely join back with the base name.
Uses the split code to ensure parent directories are created in
all cases.
Replaces `rootJoin` with filepath.Join to the root, which already
correctly cleans relative symlinks to the root.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

archive/path.go

@@ -28,7 +28,7 @@ func rootPath(root, path string) (string, error) {
 		}
 		path = newpath
 		if i == linksWalked {
-			newpath = rootJoin(newpath)
+			newpath = filepath.Join("/", newpath)
 			if path == newpath {
 				return filepath.Join(root, newpath), nil
 			}
@@ -37,28 +37,12 @@ func rootPath(root, path string) (string, error) {
 	}
 }
 
-// rootJoin joins a path with root, cleaning up any links that
-// reference above root.
-func rootJoin(path string) string {
-	if filepath.IsAbs(path) {
-		path = filepath.Clean(path)
-	}
-
-	// Resolve any ".." or "/.." before joining to root
-	for !filepath.IsAbs(path) {
-		path = "/" + path
-		path = filepath.Clean(path)
-	}
-
-	return path
-}
-
 func walkLink(root, path string, linksWalked *int) (newpath string, islink bool, err error) {
 	if *linksWalked > 255 {
 		return "", false, errTooManyLinks
 	}
 
-	path = rootJoin(path)
+	path = filepath.Join("/", path)
 	if path == "/" {
 		return path, false, nil
 	}