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251 lines
13 KiB
Markdown
251 lines
13 KiB
Markdown
# macOS GCD - Grand Central Dispatch
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<details>
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<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
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Other ways to support HackTricks:
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* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
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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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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
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* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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## Basic Information
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**Grand Central Dispatch (GCD),** also known as **libdispatch** (`libdispatch.dyld`), 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** (which might increase or decrease as necessary).
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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 ideal 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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### Blocks
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A block is a **self contained section of code** (like a function with arguments returning a value) and can also specify bound variables.\
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However, at compiler level blocks doesn't exist, they are `os_object`s. Each of these objects is formed by two structures:
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* **block literal**: 
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* It starts by the **`isa`** field, pointing to the block's class:
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* `NSConcreteGlobalBlock` (blocks from `__DATA.__const`)
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* `NSConcreteMallocBlock` (blocks in the heap)
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* `NSConcreateStackBlock` (blocks in stack)
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* It has **`flags`** (indicating fields present in the block descriptor) and some reserved bytes
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* The function pointer to call
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* A pointer to the block descriptor
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* Block imported variables (if any)
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* **block descriptor**: It's size depends on the data that is present (as indicated in the previous flags)
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* It has some reserved bytes
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* The size of it
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* It'll usually have a pointer to an Objective-C style signature to know how much space is needed for the params (flag `BLOCK_HAS_SIGNATURE`)
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* If variables are referenced, this block will also have pointers to a copy helper (copying the value at the begining) and dispose helper (freeing it).
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### Queues
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A dispatch queue is a named object providing FIFO ordering of blocks for executions.
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Blocks a set in queues to be executed, and these support 2 modes: `DISPATCH_QUEUE_SERIAL` and `DISPATCH_QUEUE_CONCURRENT`. Of course the **serial** one **won't have race condition** problems as a block won't be executed until the previous one has finished. But **the other type of queue might have it**.
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Default queues:
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* `.main-thread`: From `dispatch_get_main_queue()`
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* `.libdispatch-manager`: GCD's queue manager
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* `.root.libdispatch-manager`: GCD's queue manager
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* `.root.maintenance-qos`: Lowest priority tasks
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* `.root.maintenance-qos.overcommit`
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* `.root.background-qos`: Available as `DISPATCH_QUEUE_PRIORITY_BACKGROUND`
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* `.root.background-qos.overcommit`
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* `.root.utility-qos`: Available as `DISPATCH_QUEUE_PRIORITY_NON_INTERACTIVE`
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* `.root.utility-qos.overcommit`
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* `.root.default-qos`: Available as `DISPATCH_QUEUE_PRIORITY_DEFAULT`
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* `.root.background-qos.overcommit`
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* `.root.user-initiated-qos`: Available as `DISPATCH_QUEUE_PRIORITY_HIGH`
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* `.root.background-qos.overcommit`
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* `.root.user-interactive-qos`: Highest priority
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* `.root.background-qos.overcommit`
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Notice that it will be the system who decides **which threads handle which queues at each time** (multiple threads might work in the same queue or the same thread might work in different queues at some point)
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#### Attributtes
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When creating a queue with **`dispatch_queue_create`** the third argument is a `dispatch_queue_attr_t`, which usually is either `DISPATCH_QUEUE_SERIAL` (which is actually NULL) or `DISPATCH_QUEUE_CONCURRENT` which is a pointer to a `dispatch_queue_attr_t` struct which allow to control some parameters of the queue.
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### Dispatch objects
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There are several objects that libdispatch uses and queues and blocks are just 2 of them. It's possible to create these objects with `dispatch_object_create`:
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* `block`
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* `data`: Data blocks
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* `group`: Group of blocks
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* `io`: Async I/O requests
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* `mach`: Mach ports
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* `mach_msg`: Mach messages
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* `pthread_root_queue`:A queue with a pthread thread pool and not workqueues
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* `queue`
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* `semaphore`
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* `source`: Event source
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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 (1160).png" alt="" width="563"><figcaption></figcaption></figure>
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<figure><img src="../../.gitbook/assets/image (1162).png" alt="" width="563"><figcaption></figcaption></figure>
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<figure><img src="../../.gitbook/assets/image (1163).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 (1164).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 (1165).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 (1166).png" alt="" width="563"><figcaption></figcaption></figure>
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## References
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* [**\*OS Internals, Volume I: User Mode. By Jonathan Levin**](https://www.amazon.com/MacOS-iOS-Internals-User-Mode/dp/099105556X)
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<details>
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<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
|
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|
|
Other ways to support HackTricks:
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|
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* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
|
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* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
|
|
* 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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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
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* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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