What is the LD_PRELOAD Trick on Linux?


Introduction

LD_PRELOAD is a powerful and advanced feature in the Linux dynamic linker that allows users to preload shared object files into the address space of a process (before it starts executing). This can be used to override certain functions in the process with custom implementations or to inject additional code into the process at runtime. LD_PRELOAD is often used for debugging and testing purposes, but it can also be used for malicious purposes, such as injecting malware into processes.

How LD_PRELOAD actually works?

The LD_PRELOAD environment variable specifies a list of shared object files that the dynamic linker should load before any other shared object files. These shared object files are known as "preload libraries". When a process is executed, the dynamic linker searches for the shared object files specified in LD_PRELOAD and loads them into the process's address space. Any function calls made by the process will be directed to the implementations in the preload libraries, rather than the implementations in the system libraries or other shared object files.

Example

For example, consider this simple “C” program that calls the printf function from the stdio.h library −

#include <stdio.h>
int main() {
   printf("Hello, Earth!
"); return 0; }

Output

Hello, Earth

If we compile this program and execute it normally, the printf function will be called from the libc.so library, which is the standard “C” library on most Linux systems. However, if we set the LD_PRELOAD environment variable to point to a shared object file that contains a custom implementation of printf, the dynamic linker will load that shared object file instead of libc.so, and the custom implementation of printf will be called when the program is executed.

To set the LD_PRELOAD environment variable, we can use the export command in the terminal 

$ export LD_PRELOAD=/path/to/custom_printf.so

Then, when we execute the program, the custom implementation of “printf” will be called instead of the one in libc.so 

$ ./a.out
Hello, Earth!

Use Cases for LD_PRELOAD

There are several common use cases for LD_PRELOAD 

Debugging and Testing

One of the most common uses for LD_PRELOAD is to override functions in a program for debugging and testing purposes. For example, we might want to write a custom implementation of printf that logs all calls to the function, or a custom implementation of malloc that checks for memory leaks. By using LD_PRELOAD, we can easily insert these custom implementations into a program without modifying the source code.

Dynamic Linking

LD_PRELOAD can also be used to dynamically link a program to a shared object file that was not linked at compile time. This can be useful if we want to use a library that is not installed on the system, or if we want to use a newer version of a library that is installed on the system.

Malware Injection

Unfortunately, LD_PRELOAD can also be used for malicious purposes, such as injecting malware into processes. For this reason, it is important to be cautious when using LD_PRELOAD and to only use trusted shared object files.

Limitations of LD_PRELOAD

There are a few limitations to consider when using LD_PRELOAD 

Function Overrides Only

LD_PRELOAD can only override functions that are called through the dynamic linker. This means that it cannot override functions that are called directly through the program's code, or functions that are implemented in statically linked libraries.

Shared Object Dependencies

Preload libraries must be self-contained and cannot depend on other shared object files. If a preload library depends on another shared object file, the dynamic linker will not be able to load it.

Order of Preload Libraries

The order in which preload libraries are specified in LD_PRELOAD is important. If two preload libraries both provide implementations for the same function, the implementation in the library that is specified first in LD_PRELOAD will be used.

Security Implications

As mentioned earlier, LD_PRELOAD can be used for malicious purposes, such as injecting malware into processes. It is important to be cautious when using LD_PRELOAD and to only use trusted shared object files.

Example: Overriding printf

To demonstrate how LD_PRELOAD works, let's create a simple shared object file that provides a custom implementation of the printf function. We'll start by creating a file called custom_printf.c with the following contents 

#include <studio.h>
int printf(const char *format, ...) {
   va_list args;
   va_start(args, format);
   vprintf("Custom printf: ", args);
   va_end(args);
   return 0;
}

This implementation of printf simply prepends the string "Custom printf: " to the beginning of the output. Next, we'll compile custom_printf.c into a shared object file using the gcc compiler 

$ gcc -fPIC -shared -o custom_printf.so custom_printf.c

Now, we can use LD_PRELOAD to override the printf function in a program. First, we'll set the LD_PRELOAD environment variable to point to custom_printf.so 

$ export LD_PRELOAD=/path/to/custom_printf.so

Example

Then, we'll run a simple “C” program that calls printf 

#include <stdio.h>
int main() {
   printf("Hello, Earth!
"); return 0; }

Output

Hello, Earth!

When we run the program, the output will be "Custom printf: Hello, world!" 

$ ./a.out
Custom printf: Hello, Earth!

As we can see, the custom implementation of printf provided in custom_printf.so was called instead of the one in libc.so.

Conclusion

Overall, LD_PRELOAD is a powerful feature in the Linux dynamic linker that allows users to preload shared object files into the address space of a process before it starts executing. This can be used for a variety of purposes, such as debugging and testing, dynamic linking, and malware injection. However, it is important to be cautious when using LD_PRELOAD and to only use trusted shared object files. LD_PRELOAD has a few limitations, such as the inability to override statically linked functions and the requirement for preload libraries to be self-contained. Understanding how LD_PRELOAD works and its limitations can help users effectively utilize this feature in their work.

Updated on: 04-Jan-2023

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