Linux libraries part 0

In this series of articles, I will explain what I understand about Linux libraries.

In this series of articles, I will explain what I understand about Linux libraries.

At the beginning when I started Buffer Overflow I felt stuck because I was missing some basic notions like how Linux libraries work, PLT & GOT, dynamic linker, shared objects, etc …

So I tried to understand and today I thought I would write an article about it and come back later if necessary and also if it could help someone.

What is libraries ?

When you code in C, you use functions like printf to print or scanf to type, etc. These functions, you didn’t code them yourself but you use them.

Where are these functions?

These functions are in a library on your system. This library is a set of pre-coded functions that you don’t need to recode each time but use them directly. You will save a lot of time.

Here I took the example of function but a library does not contain only function, there are also variables, etc…

In short, libraries are an easy way to reuse code.

They allow similar functionality to be grouped together in a single unit. These units allow developers to write their code from modules: this is called modular programming.

Under Linux, there are two types of libraries, the static libraries and the dynamic libraries (shared libraries).

Static libraries

At this level the editing of links is done in a static way.

That is, at compile time all the libraries that the executable needs to run are integrated inside.

advantage:

All symbols are resolved at runtime.

The function calls are fast.

Example:

Disadvantages

The libraries are duplicated in each executable.

So we use a lot of space on the machine.

If you update the printf function in the library, for example, you have to recompile all the executables.

Example:

test.c

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#include <stdio.h>
#include <stdlib.h>

int main(){

  printf("I'm _R4v3n");

  return 0;
}

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gcc -o test test.c
gcc -static -o test_static test.c

-static to compile in static mode.

When it is static the size is high.

Here we have used only the printf function.

As we use more functions in our code, the size of the executable will increase.

To check if the executable is statically linked.

Dynamic libraries

In the next part we will see the dynamic libraries.

Prerequisites: Basic assembly, GDB

References

IBM : Anatomy of Linux dynamic libraries