CPython 3.12 Cross-Compilation for iOS

The Motivation

The idea for this project was born out of frustration. While working on mobile security research, I realized that the existing Python packages for jailbroken iOS devices were severely outdated, being stuck on version 3.9. This caused major compatibility issues, as most modern pip packages simply wouldn't work.

My primary goal was to run Frida directly on my iPhone. Usually, you need a computer to run the Frida client, but I wanted a fully standalone, on-device instrumentation setup. The old Python version couldn't handle the modern frida-tools package.

I looked for a newer version, but to my surprise, despite the massive size of the Jailbreak community, no one had ported a modern Python. So, I decided to take matters into my own hands.

Community Impact

After successfully building and testing the package, I open-sourced it and shared the results on the r/jailbreak subreddit. The response was overwhelming. I was surprised by how many developers and researchers were facing the exact same bottleneck. It was incredibly rewarding to see my tool enabling others to run modern scripts and tools right on their devices.

Running Python on iOS has always been a challenge due to the platform's restrictions on dynamic code loading and the lack of a native compiler. This project aimed to solve that by cross-compiling the latest CPython 3.12.5 source code specifically for the arm64 architecture used by modern iPhones and iPads.

The Challenge

Porting CPython to iOS is not just about compiling code; it's about navigating a restrictive sandbox and a missing system layer. The biggest hurdle is the cross-compilation process itself. The build system must run on macOS but generate binaries for arm64-apple-ios, which often confuses CPython's configure script into assuming it can execute what it builds.

Beyond the build environment, iOS lacks many standard POSIX functions like system(), forkpty(), and getentropy(), causing standard builds to fail immediately. Furthermore, dynamic linking is incredibly tricky on this platform. To ensure core modules like pip and ssl work reliably, dependencies cannot simply be assumed to exist on the user's device; they must be carefully managed.

The Implementation

To overcome these hurdles, I engineered a custom build pipeline that handles every step of the process. You can check the full source code on GitHub.

First, I tackled the dependency issue by compiling OpenSSL 1.1.1 as a static library. By forcing CPython to link against these static .a files using -lssl -lcrypto, I guaranteed that the ssl module works out-of-the-box. This is critical for enabling pip install to function correctly over HTTPS without relying on external system libraries.

Instead of patching hundreds of source files to fix the missing system calls, I created a config.site file to pre-seed the configure script. This allowed me to manually define cache variables to disable unsupported features like forkpty and bypass cross-compilation checks efficiently.

Finally, since iOS refuses to run unsigned binaries, I built an automated signing step into the pipeline. The script iterates through every generated .so and .dylib file, signing them with ldid before packaging. The entire process is automated via GitHub Actions, which fetches the source, applies patches, builds for arm64, and packages the result into a .deb file ready for Sileo and Zebra.

Result

A fully functional Python 3.12 interpreter running on iOS, capable of installing packages via pip and running complex scripts.