Can LD_PRELOAD replace a function resolved via dlopen()/dlsym()?

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Summary

The question revolves around whether LD_PRELOAD can replace a function resolved via dlopen() and dlsym(). In a scenario where a program dynamically loads liba.so, resolves and calls func1() from it, and another library libb.so redefines func1(), the goal is to use LD_PRELOAD to force the program to use func1() from libb.so instead of the original from liba.so. The key takeaway is that LD_PRELOAD has limitations when dealing with dynamically loaded libraries.

Root Cause

The root cause of the issue lies in how LD_PRELOAD works and its interaction with dlopen() and dlsym(). The main reasons include:

  • LD_PRELOAD is used to preload libraries before any other library, but it does not affect libraries loaded with dlopen().
  • dlopen() and dlsym() provide a way to dynamically load libraries and resolve symbols, which bypasses the normal library loading mechanism where LD_PRELOAD applies.
  • The program explicitly requests func1() from liba.so, making it difficult for LD_PRELOAD to intervene.

Why This Happens in Real Systems

This scenario occurs in real systems due to the following reasons:

  • Dynamic loading of libraries is a common practice for extending program functionality without recompilation.
  • Function overriding is used for various purposes, including testing, debugging, and providing alternative implementations.
  • The combination of dynamic loading and function overriding can lead to complex interactions with LD_PRELOAD.

Real-World Impact

The real-world impact of this issue includes:

  • Difficulty in testing: It becomes challenging to test programs that dynamically load libraries with overridden functions.
  • Limited debugging capabilities: The inability to easily replace functions can limit debugging options.
  • Inflexibility in deployment: The rigid nature of dynamic loading and LD_PRELOAD interaction can make it hard to deploy programs in different environments.

Example or Code

// liba.so
void func1() {
    // Original implementation
}

// libb.so
void func1() {
    // Redefinition of func1
}

// main program
int main() {
    void* handle = dlopen("liba.so", RTLD_LAZY);
    void (*func1_ptr)() = dlsym(handle, "func1");
    func1_ptr(); // Calls the original func1 from liba.so
    return 0;
}

How Senior Engineers Fix It

Senior engineers address this issue by:

  • Modifying the program to use a different approach for dynamic loading that allows LD_PRELOAD to work, such as using dlsym with RTLD_DEFAULT.
  • Adjusting liba.so or libb.so to accommodate the requirements, possibly by renaming functions or using versioning.
  • Employing alternative debugging and testing strategies that do not rely on LD_PRELOAD for function replacement.

Why Juniors Miss It

Junior engineers might miss this issue due to:

  • Lack of understanding of how LD_PRELOAD, dlopen(), and dlsym() interact.
  • Insufficient experience with dynamic loading and function overriding in real-world scenarios.
  • Overlooking the implications of using LD_PRELOAD with dynamically loaded libraries.