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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?

user1863877
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  • check this:http://stackoverflow.com/questions/7193888/load-time-relocation-and-virtual-memory – mohit Jun 07 '13 at 07:41

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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.

Art
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Executable code does not always contain relative addresses. On Windows, for example, addressing is often absolute (e.g. for global data).

Consider two different dynamic libraries. Both were compiled for a fixed base address of 0x00100000. Your program tries to load both of them. Where is the loader to place the 2nd DLL? Its preferred base address is already used by the other DLL.

In this case relocatable code helps placing the 2nd DLL at a different address and patching its internal pointers to the new location. With fixed base addresses, loading the 2nd DLL would just fail.

Krycztij
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It needs to be relocatable because in order to execute your process needs to be put into the actual main memory in a ready queue. Now where in the main memory it shall be placed is not fixed (it is placed wherever sufficient space is available) so the actual addresses of the instructions varies from its virtual address .

Hence statements making calls to functions ,returns etc need to be updated accordingly pointing to the actual address of those functions

user2458228
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