This library implements a set of C-friendly APIs (written in Rust) to make path resolution within a potentially-untrusted directory safe on GNU/Linux. There are countless examples of security vulnerabilities caused by bad handling of paths (symlinks make the issue significantly worse).

I have been working on kernel patches to make this trivial to do safely (which morphed into a new syscall), but in order to safely use the new kernel API you need to restructure how you handle paths quite significantly. Since a restructure is necessary anyway, having a new library is not too much of a downside. In addition, this gives us the ability to implement the core safety features through userspace emulation on older kernels.

Here is a toy example of using this library to open a path (/etc/passwd) inside a root filesystem (/path/to/root) safely. More detailed examples can be found in examples/ and tests/.

#include <pathrs.h>

int get_my_fd(void)
{
	const char *root_path = "/path/to/root";
	const char *unsafe_path = "/etc/passwd";

	int fd = -1;
	pathrs_root_t *root = NULL;
	pathrs_handle_t *handle = NULL;
	pathrs_error_t *error = NULL;

	root = pathrs_open(root_path);
	error = pathrs_error(PATHRS_ROOT, root);
	if (error)
		goto err;

	handle = pathrs_resolve(root, unsafe_path);
	error = pathrs_error(PATHRS_ROOT, root);
	if (error) /* or (!handle) */
		goto err;

	fd = pathrs_reopen(handle, O_RDONLY);
	error = pathrs_error(PATHRS_HANDLE, handle);
	if (error) /* or (fd < 0) */
		goto err;

err:
	if (error)
		fprintf(stderr, "Uh-oh: %s (errno=%d)\n", error->description, error->saved_errno);
	pathrs_free(PATHRS_ROOT, root);
	pathrs_free(PATHRS_HANDLE, handle);
	pathrs_free(PATHRS_ERROR, error);
	return fd;
}

libpathrs is licensed under the GNU LGPLv3 (or any later version).

libpathrs: safe path resolution on Linux
Copyright (C) 2019-2021 Aleksa Sarai <cyphar@cyphar.com>
Copyright (C) 2019-2021 SUSE LLC

This program is free software: you can redistribute it and/or modify it under
the terms of the GNU Lesser General Public License as published by the Free
Software Foundation, either version 3 of the License, or (at your option) any
later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along
with this program. If not, see <https://www.gnu.org/licenses/>.