Pranav Ram Joshi

Man is born free, and everywhere he is in chains. - Jean-Jacques Rousseau; The Social Contract

Preamble​

This post explores how XNU — the kernel behind macOS and iOS — schedules and executes threads across processors. We'll look at preemptive scheduling, the Mach continuation model that allows kernel stacks to be reclaimed, and the hardware topology that underlies multiprocessing: cores, dies, SMT, and the distinction between symmetric and asymmetric multiprocessing.

This post builds on the Mach abstractions covered in Mach Kernel Abstractions and XNU Internals. Familiarity with Mach tasks and threads will be helpful but is not strictly required.

Preamble​

A programming language--in a hosted environment--has two crucial components: language features and runtime capabilities. For example, features such as traits are language features and it describes the functions (methods) a type may implement, borrow checker which statically analyzes program source to find common errors, and checks that are enforced during compilation are language features. Garbage collection, memory access checks, and other features that are available during program execution are runtime capabilities. We'll look into what happens when a hosted environment closes the standard streams of a Rust program along with masking the /dev directory. This directory is available in most UNIX-like systems, which is what I primarily use.

Virtual Memory Management provides the dream of infinite space. Address Sanitizer is the precipice hidden in the fog - the silent pitfall that claims the soul the moment the dream wanders into the void.

Preamble​

I have recently started learning Rust. But while learning about the various types and programming model of Rust, I couldn't help but try to see the differences between Rust and C. I also revisited flexible array member in C structures. While doing that, I poked around the memory and was using various Sanitizers. Until now, I've naively used the address sanitizer. I decided to understand the output of the sanitizer and how the algorithm works. Initially, I looked into compiler specific documentation. Both GCC and clang's documentation ultimately refers to [GOOGLE_ASAN]. I plan to discuss common programming errors which causes memory related issues, how address sanitizer can be used, why it's not always reliable, and understand how to read the vmmap(1) utility on Darwin.

Search for the truth is the noblest occupation of man; its publication is a duty. - Anne Louise Germaine de Staël; De l'Allemagne

Preamble: XNU and the Mach Kernel​

An operating system is a set of system software that acts as a bridge between the application software and the underlying hardware. Among the various software packaged with an operating system is the kernel — a program responsible for communicating with the hardware and managing the system in its entirety. The purpose of this post is to explore the XNU kernel source and understand the core abstractions of the Mach kernel that underpin the macOS kernel architecture. We'll walk through Mach's taxonomy — tasks, threads, IPC ports, processor sets — and examine the XNU-specific implementation details, including pointer authentication and key Mach routines like task_info and mach_task_self.

There are various ways one could use C preprocessor macros. Learning how to properly use them--or failing to--can end up producing some bizarre codebases. If you have ever encountered the <os/availability.h> header file on macOS, you'll find some macros that are confusing to interpret.

The API_AVAILABLE Macro Definition​

The macro is defined as follows:

Preamble​

Writing a portable C program turns out to be more of a nightmare than imagined. C is notorious for being platform dependent--also known as portable assembly language, yet most of the software that runs across different systems is written in this language. The secret lies in understanding what varies between platforms--data type widths, byte ordering, memory alignment, available system calls and library functions--and using the right tools to handle those differences at build time. In this post, we'll walk through the C compilation pipeline, explore how preprocessor directives enable conditional compilation, examine data type portability pitfalls like endianness and struct padding, and introduce GNU Autotools as a build system for portable projects. This is the third in a three-part series. The first post covers UNIX terminal devices, and the second explains how the shell executes programs.

Be conservative in what you do, be liberal in what you accept from others. - RFC 793

Preamble: From Telecommunication to Packet Filtering​

Throughout the course of humanity, we have witnessed various forms of communication. We use language to communicate on a daily basis. Writing letters is also a form of communication, albeit a form which has slow transmission and reception time. Then came the era of telecommunication. Through the use of electronic and electrical means, we soon came to the realization that communication can be instantaneous. This, I believe, is one of the greatest achievement for mankind. This really made the world feel like a small village. The Internet is the closest to the anywhere door we know from Doraemon.

Preamble​

A shell process is a command-line interpreter — the program that reads your input, locates the right program or builtin command, and orchestrates its execution. Whether you use Bourne Shell, Bash, Korn Shell, or another flavor, the underlying mechanics are remarkably similar: the shell forks a child process, the child replaces its image with the target program via exec, and the parent waits for completion. In this post, we'll trace that fork-exec-wait cycle step by step, explore how environment variables are passed to programs, and look at the shell's capabilities as an interpreter — including loops, conditionals, and history expansion. This is the second in a three-part series. The first post covers UNIX terminal devices and line discipline, and the third explores writing portable C code.

Preamble​

When you open a terminal, you are greeted with a prompt and the system expects your input. But have you ever wondered what sort of device the terminal really is? A terminal is more than just a window for typing commands — it is a character special device managed by the kernel, with a layered architecture involving device drivers, line discipline modules, and pseudo-terminal pairs. In this post, we'll explore the internals of the UNIX terminal device: how tty and pty devices work, what terminal line discipline does, and how the kernel mediates between the user and the shell process. This is the first in a three-part series. The second post covers how the shell process executes programs, and the third explores writing portable C code.

Introduction: Why Use-After-Free Doesn't Always Crash​

Memory is a crucial topic when it comes to building software. Programs running on historical devices referenced physical memory locations directly, a practice superseded by a dedicated hardware component: the Memory Management Unit (MMU). We now work with virtual memory addresses that is managed by MMU for us^[0]. For a hands-on exploration of how process memory is laid out on macOS ARM64, see the memory post.

malloc and free: Why Accessing Freed Memory Can Silently Succeed​

I recently stumbled across a video regarding the Rust^TM language. The intent of this post is not to look down on the Rust^TM language but to observe a behaviour of a trivial C program. A program fragment was shown in the video, similar to the one shown in Listing 1. After building the executable, we can notice that the program exits normally. But before that, we tried to access a region of memory that was "freed". In contrast, Rust^TM informs this issue during compilation.