simple-sandbox

A simple linux sandbox with Node.js API. Used by SYZOJ.

Prerequisites

Packages

You need to have the build-essentials (g++, make, etc.) and the fmt library installed in your system in order to build the C++ part.

The minimal g++ version required is g++-8. A newer version of clang++ with C++17 file system support is recommended.

Install them by (in Ubuntu 18.04):

apt install build-essential clang++-9 libfmt-dev

Kernel

You need to have the memory swap account (disabled by default in Debian 8) enabled with your kernel. You can verify this by checking the existence of /sys/fs/cgroup/memory/memory.memsw.usage_in_bytes

If that file does not exist, then you may have to turn on that with your grub.

Add swapaccount=1 to GRUB_CMDLINE_LINUX_DEFAULT section in /etc/default/grub.

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash cgroup_enable=memory swapaccount=1"

Some distro enables cgroup v2 by default in their new versions, including Arch Linux (since April 2021), Fedora (since 31) and Debian. If you cannot find the directory /sys/fs/cgroup/memory/, this is the case for you. In this case, you also need to add the parameter systemd.unified_cgroup_hierarchy=0 to enable cgroup v1:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash cgroup_enable=memory swapaccount=1 systemd.unified_cgroup_hierarchy=0"

And then run

update-grub && reboot

To enjoy the modified kernel.

Build

Pull the repository to somewhere on your computer and run

CXX=clang++-9 yarn install # Install required packages and compile C++ code with the clang++-9 compiler
yarn run build # Compile typescript code.

To build the project. If you don't want to use yarn, just change yarn to npm.

You can use yarn run build:watch to watch for the change of typescript file.

Use

The library is with a simple API. To start the sandbox, use the following code:

const sandbox = require('simple-sandbox');
const myProcess = await sandbox.startSandbox(parameters);

where parameters is an instance of the SandboxParameters interface. The detail explanation is available in the comments in that file.

The startSandbox function returns a Promise, from which you can get an instance of the sandboxProcess class, reprensenting your sandboxed Process.

To terminate the sandboxed process, just use the stop() function:

myProcess.stop();

To wait for the sandboxed process to end, use the waitForStop() function, which returns a Promise:

myProcess.waitForStop().then((result) => {
    console.log("OK! " + JSON.stringify(result));
}).catch((err) => {
    console.log("Whooops!" + err.toString());
});

Note that myProcess itself is a EventEmitter, so you can register exit (indicates that the child process exited), and error (indicates that some error happens) event listener on it.

Note

When a sandbox is started, a event listener for the exit event on the process object is registered. When Node.js is about to exit, it will kill the sandboxed process.

However, the exit event won't be called if there are SIGTERM or SIGINT (Ctrl-C) signals sent to the Node.js process. You should let the SIGTERM and SIGINT handler to call process.exit() on the main process:

const terminationHandler = () => {
    process.exit(1);
};

process.on('SIGTERM', terminationHandler);
process.on('SIGINT', terminationHandler);

Example

A demostration is available in the demo directory. In order to get the demostration running for every one, we create the directory /opt/sandbox-test.

sudo mkdir /opt/sandbox-test
sudo chown $(whoami) /opt/sandbox-test
mkdir /opt/sandbox-test/rootfs /opt/sandbox-test/binary /opt/sandbox-test/working
curl -L https://nl.alpinelinux.org/alpine/v3.5/releases/x86_64/alpine-minirootfs-3.5.2-x86_64.tar.gz | tar -xzvf - -C /opt/sandbox-test/rootfs

(to be continued)