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 liberr = 0;
	int root = -EBADF,
		handle = -EBADF,
		fd = -EBADF;

	root = pathrs_root_open(root_path);
	if (root < 0) {
		liberr = root;
		goto err;
	}

	handle = pathrs_resolve(root, unsafe_path);
	if (handle < 0) {
		liberr = handle;
		goto err;
	}

	fd = pathrs_reopen(handle, O_RDONLY);
	if (fd < 0) {
		liberr = fd;
		goto err;
	}

err:
	if (liberr < 0) {
		pathrs_error_t *error = pathrs_errorinfo(liberr);
		fprintf(stderr, "Uh-oh: %s (errno=%d)\n", error->description, error->saved_errno);
		pathrs_errorinfo_free(error);
	}
	close(root);
	close(handle);
	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 <[email protected]>
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/>.