Virtual Memory and Relocatable Code

Question

In a 32 bit system, each process virtually has 2^32 bytes of CONTIGUOUS address space. So why the final executable code generated by a linker needs to be relocatable. What is the requirement since all addresses generated would be virtual addresses in the process's own address space and other process CANNOT use the same. Hence the process can be placed in anywhere it wants to be. Why relocatable?

Answer 1

Some operating systems make the executable code relocatable (this is definitely not universal to all operating systems) to allow for address space layout randomization. This helps mitigate certain attacks.

In the past when stacks were executable a buffer overflow could be exploited by writing executable code directly on the overflowed stack or heap. As operating systems became smarter and started preventing execution of the stack and the heap, attacks became more sophisticated and started using known code sequences in memory by doing return oriented programming. The mitigation to that class of attacks was first done by randomizing the memory layout for shared libraries (since those were easier to exploit) and then when attackers switched to attacking the main executable, by randomizing the memory position of the executable. To make it possible the main executable needs to be relocatable.