Wait for a callback and then using the result

Question

I'm using Frida to run a script on a process, and I want to wait for it to send the result back to me, as a callback.

My current code looks like this:

def check(target): global msg script_to_run = ''' (omitted for brevity) ''' # callback to handle the output of the script def on_message(message, data): global msg if message['type'] == 'send': msg = message['payload'] session = device.attach(target.pid) script = session.create_script(script_to_run) script.on('message', on_message) script.load() # Wait for the script to send us a message while msg == None: pass return msg == 'true'

The while loop in particular looks a bit unoptimized, and I'm also worried about how I used that global variable... it will pollute the global scope.

EDIT: As it turns out, I was approaching this in the wrong direction. See my answer below for details.

At the very least, I found I should change the pass for a sleep(0): stackoverflow.com/a/790246/12735366 — JJTech
– JJTech, Commented Sep 4, 2022 at 19:41
This feels too vague to really answer. How is check(target) used? You're transforming an asynchronous function into a synchronous one, which defeats the entire point. — Teepeemm
– Teepeemm, Commented Sep 4, 2022 at 21:46
Yes, sorry, I realize now that messages aren’t the proper way to go about it, I should use the RPC api. I’ll elaborate both question and answer when I get home — JJTech
– JJTech, Commented Sep 4, 2022 at 21:47

JJTech · Accepted Answer · 2022-09-04 23:08:38Z

So, as it turns out, I was approaching this problem in the wrong direction.

Instead of trying to wait on an asynchronous message, I discovered that Frida has an RPC API.

Here's how I re-wrote my code:

import frida # --snip-- def check(target): session = frida.attach(target.pid) script = session.create_script(''' rpc.exports = { check: function () { // --snip-- return true; } }; ''') script.load() return script.exports.check()

Much more succinct, doesn't use globals, and doesn't use a while loop to block.

Peer Sommerlund · Accepted Answer · 2022-09-04 18:02:32Z

I am not familiar with Frida, but looking at the examples found at https://frida.re/docs/messages/ I think you should do your processing in the callback "on_message", instead of passing it to the main thread.

The Frida examples end with sys.stdin.read() which acts like a wait-forever loop. The callback is probably executing in a different thread.

So change your last lines to

# Wait for the script to send us a message sys.stdin.read()

And change the callback to

def on_message(message, data): if message['type'] == 'send': print(message['payload'] == 'true')

Don't worry about poluting the global namespace. You are the only one using it :-)

Well, see the problem is that this function is going to be part of a larger program. So, I need to get the result back in the main thread, and block until then. — JJTech
– JJTech, Commented Sep 4, 2022 at 19:22

Stack Exchange Network

Wait for a callback and then using the result

2 Answers 2

You must log in to answer this question.

Hot Network Questions

Wait for a callback and then using the result

2 Answers 2

You must log in to answer this question.

Related

Hot Network Questions