kubernetes/vendor/github.com/cyphar/filepath-securejoin/open_linux.go

// OpenatInRoot is equivalent to [OpenInRoot], except that the root is provided
// using an *[os.File] handle, to ensure that the correct root directory is used.
func OpenatInRoot(root *os.File, unsafePath string) (*os.File, error) { … }

// OpenInRoot safely opens the provided unsafePath within the root.
// Effectively, OpenInRoot(root, unsafePath) is equivalent to
//
//	path, _ := securejoin.SecureJoin(root, unsafePath)
//	handle, err := os.OpenFile(path, unix.O_PATH|unix.O_CLOEXEC)
//
// But is much safer. The above implementation is unsafe because if an attacker
// can modify the filesystem tree between [SecureJoin] and [os.OpenFile], it is
// possible for the returned file to be outside of the root.
//
// Note that the returned handle is an O_PATH handle, meaning that only a very
// limited set of operations will work on the handle. This is done to avoid
// accidentally opening an untrusted file that could cause issues (such as a
// disconnected TTY that could cause a DoS, or some other issue). In order to
// use the returned handle, you can "upgrade" it to a proper handle using
// [Reopen].
func OpenInRoot(root, unsafePath string) (*os.File, error) { … }

// Reopen takes an *[os.File] handle and re-opens it through /proc/self/fd.
// Reopen(file, flags) is effectively equivalent to
//
//	fdPath := fmt.Sprintf("/proc/self/fd/%d", file.Fd())
//	os.OpenFile(fdPath, flags|unix.O_CLOEXEC)
//
// But with some extra hardenings to ensure that we are not tricked by a
// maliciously-configured /proc mount. While this attack scenario is not
// common, in container runtimes it is possible for higher-level runtimes to be
// tricked into configuring an unsafe /proc that can be used to attack file
// operations. See [CVE-2019-19921] for more details.
//
// [CVE-2019-19921]: https://github.com/advisories/GHSA-fh74-hm69-rqjw
func Reopen(handle *os.File, flags int) (*os.File, error) { … }