Keith Smiley keith

Hunting Down Private Dirty Memory in Linux Shared Libraries

Shared libraries (.so files) on Linux are designed to save memory by sharing physical memory pages across processes via the kernel's Page Cache. But in practice, shared libraries often consume substantially more physical RAM than expected due to Copy-On-Write (COW) dirtying.

When a process writes to any page in a shared object—or when the dynamic linker (ld.so) writes relocated pointers at startup—that 4 KB page is duplicated in physical RAM as Private_Dirty memory. If a library has 5 MB of dirty pages and is loaded by 100 processes, that library silently costs 500 MB of unshareable RAM.

Here is a practical, command-line-driven playbook for identifying private dirty hotspots in shared objects and resolving them in source code.

Modern Arch linux installation guide

macOS Internals

Understand your Mac and iPhone more deeply by tracing the evolution of Mac OS X from prelease to Swift. John Siracusa delivers the details.

Starting Points

How to use this gist

You've got two main options:

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ELF Format Cheatsheet

Introduction

Executable and Linkable Format (ELF), is the default binary format on Linux-based systems.

Compilation

Updating expired GPG keys and their backup 🔑🔐💻

I use a GPG key to sign my git commits.

An error like this one might be a sign of an expired GPG key.

error: gpg failed to sign the data fatal: failed to write commit object

Debugging the Swift Toolchain

Use these steps to debug components of the Swift toolchain. This allows you to see Swift's source code from the debugger – instead of disassembly. The debugger can also provide some variable names and values. This has been initially tested with libswiftCore.dylib.

These instructions were updated as of Swift 5.2.1.

	from os import urandom
	from socket import create_connection

	from cryptography.hazmat.primitives.asymmetric import padding
	from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
	from cryptography.hazmat.primitives.serialization import load_der_public_key


	def read(sock, length):
	result = b''

	// Example usage:
	// Swizzler<NSString , NSDateFormatter , NSDate *> NSDateFormatter_stringFromDate_ {
	// NSDateFormatter.class, @selector(stringFromDate:), [&](auto self, auto date) {
	// if ([NSCalendar.currentCalendar components:NSCalendarUnitWeekday fromDate:date].weekday == 4) {
	// return @"It Is Wednesday My Dudes";
	// } else {
	// return NSDateFormatter_stringFromDate_(self, date);
	// }
	// }
	// };