Swift Privileged Helper (XPC Listener) Crashing with Illegal Instruction Error

Question

I’ve created a Swift macOS app which uses SMJobBless to create a helper with escalated privileges. This works fine—the helper gets installed to /Library/Privileged Helper Tools and an accompanying LaunchDaemon gets created in /Library/LaunchDaemons. However, the helper is unable to start successfully. Instead, it crashes with an “Illegal instruction: 4” message.

I’ve prepared the helper to respond to XML connections by implementing the NSXPCListenerDelegate protocol. Here‘s my Helper main.swift code:

import Foundation

class HelperDelegate: NSObject, NSXPCListenerDelegate {    
    func listener(_ listener: NSXPCListener, shouldAcceptNewConnection newConnection: NSXPCConnection) -> Bool {        
    newConnection.exportedInterface = NSXPCInterface(with: HelperToolProtocol.self)
        newConnection.exportedObject = HelperTool()
        newConnection.resume()
        return true
    }    
}

let delegate = HelperDelegate()
let listener = NSXPCListener.service()
listener.delegate = delegate
listener.resume()

The crash occurs on the last line, listener.resume().

I tried to launch the helper app manually from the command line (which is identical to what the LaunchDaemon does) and, again, it crashes with the above error message printed to stdout. I don’t have any more ideas on how to test this for the root cause. My implementation is more than rudimentary, following Apple’s guidlines for implementing XM services. Also, the various posts on SO regarding XML services haven’t helped me in resolving this issue. Has anyone of you tried to create a privileged helper in Swift successfully? BTW, the app is not sandboxed.

For the sake of completeness, here’s the code for the HelperTool class referenced in my HelperDelegate class above:

import Foundation

class HelperTool: NSObject, HelperToolProtocol {
    func getVersion(withReply reply: (NSData?) -> ()) {
        let version = Bundle.main.object(forInfoDictionaryKey: "CFBundleShortVersionString" as String) as? String ?? "<unknown version>"
        let build = Bundle.main.object(forInfoDictionaryKey: kCFBundleVersionKey as String) as? String ?? "<unknown build>"
        if let d = "v\(version) (\(build))".data(using: .utf8, allowLossyConversion: false) {
            reply(d as NSData)
        }
    }
}

And finally the HelperToolProtocol:

import Foundation

@objc(HelperToolProtocol) protocol HelperToolProtocol {
    func getVersion(withReply: (NSData?) -> ())
}

Thanks for any help!

Answer 1

After days of testing I finally found a solution which makes my XPC helper launch correctly and respond to any messages. The problem lies in the last three lines of the main.swift module which currently read

let listener = NSXPCListener.service()
listener.delegate = delegate
listener.resume()

which, as put in the question, make the helper crash immediately upon the very last line.

I took these lines directly from Apple’s Creating XPC Services documentation. Here’s the documentation for the NSXPCListener resume() function:

If called on the service() object, this method never returns. Therefore, you should call it as the last step inside the XPC service's main function after setting up any desired initial state and configuring the listener itself.

The solution is to not call the NSXPCListener.service() singleton object but rather instantiate a new NSXPCListener object using the init(machServiceName:)initializer passing the same Mach service name that is being used on the main app’s XPC connection. As resume() in this case would resume immediately—thus terminating the helper—you have to put it on the current run loop to have it run indeterminately. Here’s the new, working code:

let listener = NSXPCListener(machServiceName: "Privilege-Escalation-Sample.Helper")
listener.delegate = delegate
listener.resume()
RunLoop.current.run()