## `filepath-securejoin` ##

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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`.

> **NOTE**: This code is *only* safe if you are not at risk of other processes
> modifying path components after you've used `SecureJoin`. If it is possible
> for a malicious process to modify path components of the resolved path, then
> you will be vulnerable to some fairly trivial TOCTOU race conditions. [There
> are some Linux kernel patches I'm working on which might allow for a better
> solution.][lwn-obeneath]
>
> In addition, with a slightly modified API it might be possible to use
> `O_PATH` and verify that the opened path is actually the resolved one -- but
> I have not done that yet. I might add it in the future as a helper function
> to help users verify the path (we can't just return `/proc/self/fd/<foo>`
> because that doesn't always work transparently for all users).

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
  `root` 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-existent 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
}
```

[lwn-obeneath]: https://lwn.net/Articles/767547/
[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.