Mastering Frida: A Guide to Fixing Early Instrumentation in Gadget Mode

Introduction

One of the challenges faced in Frida’s Gadget mode is an issue where the process you’re injecting into resumes immediately after injecting the JavaScript. This behavior prevents early instrumentation, which can be crucial for certain analysis tasks. In this blog post, I will guide you through a workaround to fix this issue.

When it comes to dynamic analysis and instrumentation of applications, Frida stands out as an indispensable tool in the kit of developers, reverse engineers, and security researchers alike. Its Gadget mode, in particular, offers unparalleled flexibility, allowing users to inject custom scripts into their target applications seamlessly. However, there’s a snag in the system that’s been tripping up professionals—early instrumentation.

Imagine setting up a meticulous script, ready to unravel the inner workings from the very inception of a process only to find that Frida has already allowed the application to resume before your instrumentation could catch the critical initial functions. This challenge has been particularly pronounced with the latest release, version 16.1.11, posing a significant obstacle to those needing to hook functions at the outset of program execution.

This blog post is forged from the need for a solution—crafted for persistence, it’s a beacon of hope for those grappling with this issue. Herein, we’ll walk through the intricacies of Frida’s Gadget mode and unveil a strategic approach to modify the resume_on_attach flag, a crucial step in gaining the control you require for early instrumentation. By the end of this guide, you’ll have mastered the method to hold the execution reins, ensuring your scripts are primed to hook at the starting line.

Join me as we delve into this technical deep-dive, transforming a point of frustration into a triumph of technical prowess.

Choosing Gadget Mode over Server Mode

In gernal, Frida provides 2 modes for injection, Sever mode and Dadget mode. I am debug an ARM32 program on an embedded patlform. And I tested, the server mode is not very reliable , may because the limit RAM on the dev board, and the dev board may reboot after I start frida-server, It’s very annonying, So I select Gadget mode,and reboot issue goes away. Frida offers two distinct modes for script injection: Server Mode and Gadget Mode. When working with ARM32 programs on embedded platforms, one must consider system constraints. My experience revealed that Server Mode could be unstable, potentially due to the limited RAM available on the development board. This instability often resulted in the board rebooting after initiating frida-server, which was disruptive to the debugging process.

Given these challenges, I opted for Gadget Mode. This alternative proved to be more stable under the same conditions, eliminating the frustrating reboot issues. Gadget Mode’s lightweight footprint makes it better suited for environments with resource limitations, thereby providing a more seamless and reliable debugging experience for ARM32 embedded systems.

The Solution: Modifying resume_on_attach

The root of this issue lies within the default behavior of Frida’s Gadget mode, which is controlled by the resume_on_attach field of the ControlChannel class. This field dictates whether the process should resume immediately after Frida attaches to it.

Step by Step Fix

Here’s a step-by-step guide to addressing this issue:

1. Modifying the ControlChannel Class

Locate the ControlChannel class definition within the frida-core/lib/gadget/gadget.vala file. Search for the resume_on_attach field and change its default value to false.

class ControlChannel {
  private bool resume_on_attach = false; // Change from true to false
  // ...
}

2. Compile Frida with Modified Code

After making this change, recompile Frida to incorporate the modification. Ensure you have all required dependencies and follow the standard build instructions provided in the Frida documentation. This github repository provides docker images for compiling Frida.

3. Using the Modified libGadget.so

Replace the original libGadget.so with the newly compiled version. With this change, the process will no longer resume immediately, allowing your JavaScript to hook functions at the very start of the execution.

4. Utilizing Python Script for Process Resumption in Frida Gadget Mode

While working in Frida Gadget mode, a common approach to inject a JavaScript payload into the target process would involve using the frida command from the Frida-tools package, like so:

frida -H <Board IP address> -n Gadget -l <Javascript file>

Executing this command initiates the injection, but it doesn’t automatically resume the process once the JavaScript file is loaded. Attempts to resume the process manually with %resume in the Frida REPL might not yield success.

To overcome this hurdle, I developed a Python script—a more reliable solution. The script not only injects the JavaScript code but also effectively resumes the process afterwards. Below is the functional part of the script where the device.resume(pid); command is used to resume the process:

# Python script to inject JavaScript and resume process in Frida Gadget mode
import frida

# Establish a connection to the target device
device = frida.get_device_manager().add_remote_device("<Board IP address>")

# Specify the target process identifier (replace 'pid' with the actual process ID)
pid = 'pid'

# Path to the JavaScript file you want to inject
js_file_path = "<Javascript file>"

# Read the JavaScript code from the file
with open(js_file_path, 'r') as file:
    js_code = file.read()

# Inject the JavaScript code into the process
session = device.attach(pid)
script = session.create_script(js_code)

# Load and execute the JavaScript code
script.load()

# Resume the process to continue its execution with the injected code
device.resume(pid)

print("JavaScript injected and process resumed.")

By employing this script, we ensure that the injection and process flow are controlled as expected, thus maintaining the stability of the debugging session.

Conclusion

This blog post addressed early instrumentation challenges in Frida’s Gadget mode and provided a solution by modifying the resume_on_attach flag. Gadget mode was recommended over Server mode for ARM32 programs on embedded platforms due to its stability. A Python script was introduced for injecting JavaScript and resuming the process, ensuring a controlled debugging session. Overall, this post offers a solution for fixing early instrumentation in Frida, empowering developers and researchers to gain insights into application behavior from the beginning.