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178 lines
10 KiB
Markdown
178 lines
10 KiB
Markdown
# macOS GCD - Grand Central Dispatch
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<details>
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<summary><a href="https://cloud.hacktricks.xyz/pentesting-cloud/pentesting-cloud-methodology"><strong>☁️ HackTricks Cloud ☁️</strong></a> -<a href="https://twitter.com/hacktricks_live"><strong>🐦 Twitter 🐦</strong></a> - <a href="https://www.twitch.tv/hacktricks_live/schedule"><strong>🎙️ Twitch 🎙️</strong></a> - <a href="https://www.youtube.com/@hacktricks_LIVE"><strong>🎥 Youtube 🎥</strong></a></summary>
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* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
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* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
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* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
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</details>
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## Basic Information
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**Grand Central Dispatch (GCD),** also known as **libdispatch**, is available in both macOS and iOS. It's a technology developed by Apple to optimize application support for concurrent (multithreaded) execution on multicore hardware.
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**GCD** provides and manages **FIFO queues** to which your application can **submit tasks** in the form of **block objects**. Blocks submitted to dispatch queues are **executed on a pool of threads** fully managed by the system. GCD automatically creates threads for executing the tasks in the dispatch queues and schedules those tasks to run on the available cores.
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{% hint style="success" %}
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In summary, to execute code in **parallel**, processes can send **blocks of code to GCD**, which will take care of their execution. Therefore, processes don't create new threads; **GCD executes the given code with its own pool of threads**.
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{% endhint %}
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This is very helpful to manage parallel execution successfully, greatly reducing the number of threads processes create and optimising the parallel execution. This is idea for tasks that require **great parallelism** (brute-forcing?) or for tasks that shouldn't block the main thread: For example, the main thread on iOS handles UI interactions, so any other functionality that could make the app hang (searching, accessing a web, reading a file...) is managed this way.
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## Objective-C
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In Objetive-C there are different functions to send a block to be executed in parallel:
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* [**dispatch\_async**](https://developer.apple.com/documentation/dispatch/1453057-dispatch\_async): Submits a block for asynchronous execution on a dispatch queue and returns immediately.
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* [**dispatch\_sync**](https://developer.apple.com/documentation/dispatch/1452870-dispatch\_sync): Submits a block object for execution and returns after that block finishes executing.
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* [**dispatch\_once**](https://developer.apple.com/documentation/dispatch/1447169-dispatch\_once): Executes a block object only once for the lifetime of an application.
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* [**dispatch\_async\_and\_wait**](https://developer.apple.com/documentation/dispatch/3191901-dispatch\_async\_and\_wait): Submits a work item for execution and returns only after it finishes executing. Unlike [**`dispatch_sync`**](https://developer.apple.com/documentation/dispatch/1452870-dispatch\_sync), this function respects all attributes of the queue when it executes the block.
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These functions expect these parameters: [**`dispatch_queue_t`**](https://developer.apple.com/documentation/dispatch/dispatch\_queue\_t) **`queue,`** [**`dispatch_block_t`**](https://developer.apple.com/documentation/dispatch/dispatch\_block\_t) **`block`**
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This is the **struct of a Block**:
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```c
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struct Block {
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void *isa; // NSConcreteStackBlock,...
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int flags;
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int reserved;
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void *invoke;
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struct BlockDescriptor *descriptor;
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// captured variables go here
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};
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```
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And this is an example to use **parallelism** with **`dispatch_async`**:
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```objectivec
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#import <Foundation/Foundation.h>
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// Define a block
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void (^backgroundTask)(void) = ^{
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// Code to be executed in the background
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for (int i = 0; i < 10; i++) {
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NSLog(@"Background task %d", i);
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sleep(1); // Simulate a long-running task
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}
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};
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int main(int argc, const char * argv[]) {
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@autoreleasepool {
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// Create a dispatch queue
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dispatch_queue_t backgroundQueue = dispatch_queue_create("com.example.backgroundQueue", NULL);
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// Submit the block to the queue for asynchronous execution
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dispatch_async(backgroundQueue, backgroundTask);
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// Continue with other work on the main queue or thread
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for (int i = 0; i < 10; i++) {
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NSLog(@"Main task %d", i);
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sleep(1); // Simulate a long-running task
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}
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}
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return 0;
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}
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```
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## Swift
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**`libswiftDispatch`** is a library that provides **Swift bindings** to the Grand Central Dispatch (GCD) framework which is originally written in C.\
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The **`libswiftDispatch`** library wraps the C GCD APIs in a more Swift-friendly interface, making it easier and more intuitive for Swift developers to work with GCD.
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* **`DispatchQueue.global().sync{ ... }`**
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* **`DispatchQueue.global().async{ ... }`**
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* **`let onceToken = DispatchOnce(); onceToken.perform { ... }`**
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* **`async await`**
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* **`var (data, response) = await URLSession.shared.data(from: URL(string: "https://api.example.com/getData"))`**
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**Code example**:
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```swift
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import Foundation
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// Define a closure (the Swift equivalent of a block)
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let backgroundTask: () -> Void = {
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for i in 0..<10 {
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print("Background task \(i)")
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sleep(1) // Simulate a long-running task
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}
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}
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// Entry point
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autoreleasepool {
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// Create a dispatch queue
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let backgroundQueue = DispatchQueue(label: "com.example.backgroundQueue")
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// Submit the closure to the queue for asynchronous execution
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backgroundQueue.async(execute: backgroundTask)
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// Continue with other work on the main queue
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for i in 0..<10 {
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print("Main task \(i)")
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sleep(1) // Simulate a long-running task
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}
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}
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```
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## Frida
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The following Frida script can be used to **hook into several `dispatch`** functions and extract the queue name, the backtrace and the block: [**https://github.com/seemoo-lab/frida-scripts/blob/main/scripts/libdispatch.js**](https://github.com/seemoo-lab/frida-scripts/blob/main/scripts/libdispatch.js)
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```bash
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frida -U <prog_name> -l libdispatch.js
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dispatch_sync
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Calling queue: com.apple.UIKit._UIReusePool.reuseSetAccess
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Callback function: 0x19e3a6488 UIKitCore!__26-[_UIReusePool addObject:]_block_invoke
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Backtrace:
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0x19e3a6460 UIKitCore!-[_UIReusePool addObject:]
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0x19e3a5db8 UIKitCore!-[UIGraphicsRenderer _enqueueContextForReuse:]
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0x19e3a57fc UIKitCore!+[UIGraphicsRenderer _destroyCGContext:withRenderer:]
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[...]
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```
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## Ghidra
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Currently Ghidra doesn't understand neither the ObjectiveC **`dispatch_block_t`** structure, neither the **`swift_dispatch_block`** one.
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So if you want it to understand them, you could just **declare them**:
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<figure><img src="../../.gitbook/assets/image (688).png" alt="" width="563"><figcaption></figcaption></figure>
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<figure><img src="../../.gitbook/assets/image (690).png" alt="" width="563"><figcaption></figcaption></figure>
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<figure><img src="../../.gitbook/assets/image (691).png" alt="" width="563"><figcaption></figcaption></figure>
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Then, find a place in the code where they are **used**:
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{% hint style="success" %}
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Note all of references made to "block" to understand how you could figure out that the struct is being used.
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{% endhint %}
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<figure><img src="../../.gitbook/assets/image (692).png" alt="" width="563"><figcaption></figcaption></figure>
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Right click on the variable -> Retype Variable and select in this case **`swift_dispatch_block`**:
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<figure><img src="../../.gitbook/assets/image (693).png" alt="" width="563"><figcaption></figcaption></figure>
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Ghidra will automatically rewrite everything:
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<figure><img src="../../.gitbook/assets/image (694).png" alt="" width="563"><figcaption></figcaption></figure>
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<details>
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<summary><a href="https://cloud.hacktricks.xyz/pentesting-cloud/pentesting-cloud-methodology"><strong>☁️ HackTricks Cloud ☁️</strong></a> -<a href="https://twitter.com/hacktricks_live"><strong>🐦 Twitter 🐦</strong></a> - <a href="https://www.twitch.tv/hacktricks_live/schedule"><strong>🎙️ Twitch 🎙️</strong></a> - <a href="https://www.youtube.com/@hacktricks_LIVE"><strong>🎥 Youtube 🎥</strong></a></summary>
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* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
|
|
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
|
|
* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
|
|
* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
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</details>
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