Is there any way to deal with race condition?

[qingpengchen2011]

The question is: I hook two functions that are from different threads. But they access the same global variable which is a javascript array I define global. Is there any way to cope with this? I don't know how to use Il2CppObject.enter() and Il2CppObject.exit(). Any example will be good.

[vfsfitvnm]

I never used those api, however the idea is to declare a lock object (or use whatever Il2Cpp.Object you have):

const lock = Il2Cpp.Image.corlib.class("System.Object").new();

And then call the monitor methods you need (e.g. https://stackoverflow.com/a/6029829/16885569)

[vfsfitvnm]

Uhm, however the solution above may have a problem. If you create the lock object within the frida thread, it may be garbage collected after the call to Il2Cpp.perform ends (because the frida thread gets detached from il2cpp), so, in theory, you could either:

pin the lock:

const lock = Il2Cpp.Image.corlib.class("System.Object").new();
lock.ref(true);

create the lock within the main thread:

Il2Cpp.perform(async () => { // <-- notice the async
    const lock = await Il2Cpp.attachedThreads[0].schedule(() => Il2Cpp.Image.corlib.class("System.Object").new());

    // ...
});

[qingpengchen2011]

Thanks for the solution. I'll try it out.

[qingpengchen2011]

Sorry. I haven't succeeded to create an appropriate environment to determine whether the solution is working. It's hard to simulate.

[vfsfitvnm]

This is sufficient:

Il2Cpp.perform(() => {
    const lock = Il2Cpp.Image.corlib.class("System.Object").new();
    lock.ref(true);

    const SystemBooelan = Il2Cpp.Image.corlib.class("System.Boolean");

    const str = Il2Cpp.String.from("true");

    let c = 0;

    const promise = Il2Cpp.scheduleOnInitializerThread(() => {
        for (let i = 0; i < 500; i++) {
            // lock.enter();
            c += +SystemBooelan.method<boolean>("Parse").invoke(str);
            console.log(`il2cpp[${i}] = ${c}`); 
            // lock.exit();
        }
    });

    for (let i = 0; i < 500; i++) {
        // lock.enter();
        c += +SystemBooelan.method<boolean>("Parse").invoke(str);
        console.log(`frida[${i}] = ${c}`); 
        // lock.exit();
    };

    promise.then(() => {
        console.log(`c = ${c}`);
    });
});

Uncomment the commented lines to make it print the correct value