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+## `filepath-securejoin` ##
+
+[![Build Status](https://travis-ci.org/cyphar/filepath-securejoin.svg?branch=master)](https://travis-ci.org/cyphar/filepath-securejoin)
+
+An implementation of `SecureJoin`, a [candidate for inclusion in the Go
+standard library][go#20126]. The purpose of this function is to be a "secure"
+alternative to `filepath.Join`, and in particular it provides certain
+guarantees that are not provided by `filepath.Join`.
+
+This is the function prototype:
+
+```go
+func SecureJoin(root, unsafePath string) (string, error)
+```
+
+This library **guarantees** the following:
+
+* If no error is set, the resulting string **must** be a child path of
+  `SecureJoin` and will not contain any symlink path components (they will all
+  be expanded).
+
+* When expanding symlinks, all symlink path components **must** be resolved
+  relative to the provided root. In particular, this can be considered a
+  userspace implementation of how `chroot(2)` operates on file paths. Note that
+  these symlinks will **not** be expanded lexically (`filepath.Clean` is not
+  called on the input before processing).
+
+* Non-existant path components are unaffected by `SecureJoin` (similar to
+  `filepath.EvalSymlinks`'s semantics).
+
+* The returned path will always be `filepath.Clean`ed and thus not contain any
+  `..` components.
+
+A (trivial) implementation of this function on GNU/Linux systems could be done
+with the following (note that this requires root privileges and is far more
+opaque than the implementation in this library, and also requires that
+`readlink` is inside the `root` path):
+
+```go
+package securejoin
+
+import (
+	"os/exec"
+	"path/filepath"
+)
+
+func SecureJoin(root, unsafePath string) (string, error) {
+	unsafePath = string(filepath.Separator) + unsafePath
+	cmd := exec.Command("chroot", root,
+		"readlink", "--canonicalize-missing", "--no-newline", unsafePath)
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		return "", err
+	}
+	expanded := string(output)
+	return filepath.Join(root, expanded), nil
+}
+```
+
+[go#20126]: https://github.com/golang/go/issues/20126
+
+### License ###
+
+The license of this project is the same as Go, which is a BSD 3-clause license
+available in the `LICENSE` file.