Launch a process with a custom LD_PRELOAD without modifying the environment

How to Launch a Process with a Custom `LD_PRELOAD` Without Modifying the Environment: A Practical Guide for Linux

Introduction to LD_PRELOAD in Linux

In Linux, LD_PRELOAD is an environment variable used to load specific libraries before any other when a program is run. This can be used to alter the behavior of existing programs without changing their source code by injecting your own custom functions. However, there might be scenarios when you want to set LD_PRELOAD temporarily without altering the environment or affecting other running applications. This Q&A guide covers the essentials of achieving this.

Q1: What does LD_PRELOAD do?

A: LD_PRELOAD specifies one or more shared libraries that a program should load before any other when it runs. This can be used to override functions in other shared libraries, providing a mechanism to alter the behavior of programs. For example, you can intercept system calls or library functions calls to add logging, modify outputs, etc.

Q2: How can I set LD_PRELOAD for a single process execution without affecting the environment globally?

A: You can utilize the command line to set LD_PRELOAD momentarily by prepending the environment variable to the command of the executable. For instance, if you want to launch myapp with a custom library customlib.so, you would run:

LD_PRELOAD=/path/to/customlib.so ./myapp

This command sets LD_PRELOAD only for the duration of myapp and does not affect other processes.

Q3: Are there security concerns I should be aware of when using LD_PRELOAD?

A: Yes, improper use of LD_PRELOAD can pose security risks, particularly if unauthorized users can set it to load malicious code. It should be used carefully, especially on multi-user systems or in environments with strict security requirements.

Background: A Closer Look at LD_PRELOAD with Simple Examples

The power of LD_PRELOAD can be illustrated with an example where we intercept the time() system call to modify its behavior. Suppose you have a function that replaces time() to return a fixed timestamp:

Create a C file, fake_time.c:

#include <stdio.h>
#include <time.h>

time_t time(time_t *t) {
    return 915148800; // This corresponds to a specific date and time
}

Compile this file into a shared library:

gcc -shared -fPIC -o fake_time.so fake_time.c

Running a simple program that uses time() with and without our LD_PRELOAD:

gcc -o demo_time program_using_time.c
LD_PRELOAD=./fake_time.so ./demo_time

Without LD_PRELOAD, demo_time would show the current system time, but with it, it would display the time we defined in fake_time.so.

Executable Script: Demonstrate LD_PRELOAD in Action

Here’s a script that demonstrates the use of LD_PRELOAD to prepend a message to all printed content in a program using stdout:

fake_puts.c:

#include <stdio.h>
ssize_t write(int fd, const void *buf, size_t count) {
    const char *prefix = "LD_PRELOAD: ";
    write(1, prefix, strlen(prefix));
    return write(1, buf, count);
}

Compile and run using:

gcc -shared -fPIC -o fake_puts.so fake_puts.c
LD_PRELOAD=./fake_puts.so echo "Hello, world!"

Conclusion: Understanding the Power of LD_PRELOAD

LD_PRELOAD is a flexible tool that can be extraordinarily powerful in the right hands. It allows users and developers to modify the behavior of compiled binaries in a controlled fashion, which can be invaluable for debugging, modifying third-party software without source code access, or enriching features without invasive changes. As with any powerful tool, however, caution is advised to prevent unintended behavior or security vulnerabilities.

How to Launch a Process with a Custom LD_PRELOAD Without Modifying the Environment: A Practical Guide for Linux

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How to Launch a Process with a Custom `LD_PRELOAD` Without Modifying the Environment: A Practical Guide for Linux