We can use LD_PRELOAD environment variable to set a location to fetch shared libraries. By setting this variable, we can overwrite the existing functions and make the standard command work differently, the way we want it to.

The linker links the libraries in the path provided by LD_PRELOAD for compiling the main file. Once a function is linked, when other instance of same function shows up, original location is ignored and the newer location is used.

For example, let us try to overwrite the puts function present in stdio.h file.

Consider a file main.c, with following content:

#include <stdio.h>

int theFunction(const char *s)
{
    return puts(s);
}

int main (int argc, char** argv) {
    theFunction("Hello, this is traditional work flow.");
    printf("%s: puts location: %p\n", __FILE__, puts);
}

Compiling and running the main.c file gives following output with location of puts as 0x7f877af52ef0.

Create another file, unmain.c as follows:

#include <stdio.h>

int puts(const char *__s)
{
    return printf("New puts, hackerman alert!\n");
}

Now, create a shared library of this unmain.c file with command:

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

Update the LD_PRELOAD with the location of the shared library, unmain.so:

export LD_PRELOAD="$PWD/unmain.so"

And then again run the first compiled main.c file’s executable main.o to see that the location of the puts has been updated. For my case, new location has been set to 0x7f6003f5d119.

./main.o

To remove the shared library, use unset to unset value of LD_PRELOAD.

unset LD_PRELOAD