What are the uses of self modifying code? [closed]

Question

Is there any real use for self modifying code?

I know that they can be used to build worms/viruses, but I was wondering whether there is some good reason that a programmer may have to use self modifying code.

Any ideas? Hypothetical situations are welcome too.

Answer 1

Turns out that the Wikipedia entry on "self-modifying code" has a great list:

1. Semi-automatic optimization of a state dependent loop.
2. Runtime code generation, or specialization of an algorithm in runtime or loadtime (which is popular, for example, in the domain of real-time graphics) such as a general sort utility preparing code to perform the key comparison described in a specific invocation.
3. Altering of inlined state of an object, or simulating the high-level construction of closures.
4. Patching of subroutine address calling, as done usually at load time of dynamic libraries, or, on each invocation patching the subroutine's internal references to its parameters so as to use their actual addresses. Whether this is regarded as 'self-modifying code' or not is a case of terminology.
5. Evolutionary computing systems such as genetic programming.
6. Hiding of code to prevent reverse engineering, as through use of a disassembler or debugger.
7. Hiding of code to evade detection by virus/spyware scanning software and the like.
8. Filling 100% of memory (in some architectures) with a rolling pattern of repeating opcodes, to erase all programs and data, or to burn-in hardware.
9. Compression of code to be decompressed and executed at runtime, e.g., when memory or disk space is limited.
10. Some very limited instruction sets leave no option but to use self-modifying code to achieve certain functionality. For example, a "One Instruction Set Computer" machine that uses only the subtract-and-branch-if-negative "instruction" cannot do an indirect copy (something like the equivalent of "*a = **b" in the C programming language) without using self-modifying code.
11. Altering instructions for fault-tolerance

On the point about thwarting hackers using self-modifying code:

Over the course of several firmware updates, DirectTV slowly assembled a program on their smart card to destroy cards that have been hacked to illegally receive unpaid channels. See Jeff's Coding Horror article on the Black Sunday Hack for more information.

Answer 2

I've seen self-modifying code used for:

1. speed optimisation, by having the program write more code for itself on the fly

2. obsfucation, to make reverse engineering much harder