godoc: update package documentation

The old documentation still referenced the filepath.SecureJoin proposal,
which is long-dead. We should reference the new os.OpenRoot proposal as
well as libpathrs.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

doc.go

@@ -0,0 +1,39 @@
+// Copyright (C) 2014-2015 Docker Inc & Go Authors. All rights reserved.
+// Copyright (C) 2017-2024 SUSE LLC. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package securejoin implements a set of helpers to make it easier to write Go
+// code that is safe against symlink-related escape attacks. The primary idea
+// is to let you resolve a path within a rootfs directory as if the rootfs was
+// a chroot.
+//
+// securejoin has two APIs, a "legacy" API and a "modern" API.
+//
+// The legacy API is [SecureJoin] and [SecureJoinVFS]. These methods are
+// **not** safe against race conditions where an attacker changes the
+// filesystem after (or during) the [SecureJoin] operation.
+//
+// The new API is made up of [OpenInRoot] and [MkdirAll] (and derived
+// functions). These are safe against racing attackers and have several other
+// protections that are not provided by the legacy API. There are many more
+// operations that most programs expect to be able to do safely, but we do not
+// provide explicit support for them because we want to encourage users to
+// switch to [libpathrs](https://github.com/openSUSE/libpathrs) which is a
+// cross-language next-generation library that is entirely designed around
+// operating on paths safely.
+//
+// securejoin has been used by several container runtimes (Docker, runc,
+// Kubernetes, etc) for quite a few years as a de-facto standard for operating
+// on container filesystem paths "safely". However, most users still use the
+// legacy API which is unsafe against various attacks (there is a fairly long
+// history of CVEs in dependent as a result). Users should switch to the modern
+// API as soon as possible (or even better, switch to libpathrs).
+//
+// This project was initially intended to be included in the Go standard
+// library, but [it was rejected](https://go.dev/issue/20126). There is now a
+// [new Go proposal](https://go.dev/issue/67002) for a safe path resolution API
+// that shares some of the goals of filepath-securejoin. However, that design
+// is intended to work like `openat2(RESOLVE_BENEATH)` which does not fit the
+// usecase of container runtimes and most system tools.
+package securejoin

join.go

@@ -3,11 +3,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package securejoin is an implementation of the hopefully-soon-to-be-included
-// SecureJoin helper that is meant to be part of the "path/filepath" package.
-// The purpose of this project is to provide a PoC implementation to make the
-// SecureJoin proposal (https://github.com/golang/go/issues/20126) more
-// tangible.
 package securejoin
 
 import (