How to use LD_PRELOAD to mock system time for testing?

Use libraries like libfaketime or timeskew from the Awesome LD_PRELOAD list. Set the LD_PRELOAD environment variable to the library path and configure it to return specific times, allowing you to test time-dependent code without source changes.

What are the best LD_PRELOAD tools for debugging filesystem access?

Tools like fsatrace for tracing filesystem operations and cleancache for managing page cache are listed. These help monitor or modify file access patterns in applications, useful for debugging backup or logging software.

LD_PRELOAD vs Frida for dynamic instrumentation – which is better?

LD_PRELOAD is Linux-specific and lower-level, ideal for simple library function overriding. Frida is cross-platform and more flexible for complex hooking. For basic interception on Linux, LD_PRELOAD is lightweight; for advanced cross-platform needs, Frida might be preferred.

How to create a custom LD_PRELOAD library to override functions like malloc?

Write a shared library in C with functions matching the target signatures, compile with -shared -fPIC flags, and set LD_PRELOAD to its path. Refer to examples in the Awesome LD_PRELOAD list, such as unrandom for srand overrides, for guidance.

Are there security risks with using LD_PRELOAD?

Yes, LD_PRELOAD can be exploited for privilege escalation or malware injection, as it allows arbitrary code execution. Always verify library sources and use in trusted environments; some systems disable it for security hardening.

Can LD_PRELOAD add TLS support to legacy applications?

Yes, tools like ktlswrapper in the list intercept network calls to enable TLS encryption without modifying source code. This is useful for securing older applications that lack built-in TLS support.

Open-Awesome

awesome-ld-preload

A curated list of resources and libraries for LD_PRELOAD, a Linux mechanism for runtime function interception and behavior modification.

GitHub

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What is awesome-ld-preload?

Awesome LD_PRELOAD is a curated list of resources, libraries, and tools related to LD_PRELOAD, a Linux mechanism that allows developers to intercept and override library functions at runtime. It solves problems like modifying application behavior without source changes, testing time-dependent code, and adding features like file locking or TLS support to existing binaries.

Target Audience

System programmers, Linux developers, and security researchers who need to debug, test, or modify application behavior at the library level without recompilation.

Value Proposition

Developers choose Awesome LD_PRELOAD for its comprehensive collection of ready-to-use libraries and tools that demonstrate practical applications of LD_PRELOAD, saving time and providing inspiration for low-level system hacking and instrumentation.

Overview

List of resources related to LD_PRELOAD, a mechanism for changing application behavior at run-time

Use Cases

Best For

Testing time-dependent code by mocking system time functions
Debugging filesystem access patterns in applications
Adding TLS support to legacy applications without source changes
Optimizing memory usage with huge pages for performance-critical apps
Disabling disk writes for faster testing of backup or logging software
Intercepting network calls to redirect ports or log SSL traffic

Not Ideal For

Projects requiring cross-platform compatibility on Windows or mobile operating systems where LD_PRELOAD is not supported
Teams needing official support, versioning, or maintenance guarantees for the tools listed
Applications running in security-restricted environments that disable library injection via LD_PRELOAD

Pros & Cons

Pros

Comprehensive Tool Curation

Aggregates over 20 specialized libraries like libfaketime for time mocking and fsatrace for filesystem access tracing, providing a one-stop reference for various use cases.

Practical Use Case Focus

Emphasizes real-world applications such as testing, debugging, and optimization, with tools specifically for adding TLS support via ktlswrapper or disabling disk writes with libeatmydata.

Clear Implementation Guidance

Offers straightforward instructions on using LD_PRELOAD with environment variables, making it accessible for developers familiar with Linux system programming.

Versatile Functionality Coverage

Includes tools for performance enhancement (libhugetlbfs), security (force-urandom), and behavior modification, demonstrating the broad utility of LD_PRELOAD.

Cons

Variable Tool Maintenance

As a curated list, it doesn't indicate which tools are actively maintained, risking reliance on outdated or abandoned projects like some older libraries in the collection.

Limited Documentation Depth

Many linked libraries have sparse or minimal documentation, forcing users to rely on source code or external resources for detailed implementation and troubleshooting.

Platform-Specific Focus

Primarily targets Linux, with only a note on macOS alternatives via DYLD_INSERT_LIBRARIES; tools may not work seamlessly on other Unix-like systems without porting.

Frequently Asked Questions

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