use LD_PRELOAD to patch your program

Env ?

• Linux/Unix platform.
• C/C++

What is it ?

• You can set env variable LD_PRELOAD to "replace" the binary program's functions/subroutines with the functions/subroutines which are defined in the .so shared libraries specified in LD_PRELOAD.

How to use it ?

• For example, if we want to replace "malloc" function of "ls" program with our own defined function.
• Step1: write our self-defined malloc and compile it as .so shared library.
• vim mylib.c:

#include <stdio.h>
void* malloc(size_t size) {
    printf ("hello\n");
    return NULL;
}

• compile mylib.c as shared library

gcc -Wall -fpic -shared -o libmylib.so mylib.c

• Step2: Run it !
• Replace ls "malloc" with our own defined "malloc" in shared library

LD_PRELOAD=./libmylib.so /bin/ls

• Result

• hello
  hello
  hello
  hello
  hello
  hello
  hello
  /bin/ls: memory exhausted

Usage ?

1. I guess for hacker/cracker it may be an important skill.
2. We can fast replace the function with our defined function although we do not have source code.(patching/wrap function)