What are the uses of self modifying code?
I've seen self-modifying code used for:
speed optimisation, by having the program write more code for itself on the fly
obsfucation, to make reverse engineering much harder
In former times where RAM was limited, self modifying code was used to save memory. Nowadays for example application compression utilities like UPX are used to decompress/modify the own code after loading a compressed image of the application.
Turns out that the Wikipedia entry on "self-modifying code" has a great list:
- Semi-automatic optimization of a state dependent loop.
- 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.
- Altering of inlined state of an object, or simulating the high-level construction of closures.
- 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.
- Evolutionary computing systems such as genetic programming.
- Hiding of code to prevent reverse engineering, as through use of a disassembler or debugger.
- Hiding of code to evade detection by virus/spyware scanning software and the like.
- 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.
- Compression of code to be decompressed and executed at runtime, e.g., when memory or disk space is limited.
- 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.
- 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.
Because the Commodore 64 doesn't have many registers and has a 1Mhz processor. When you need to read a memory address offset by a value it is easier to modify the source.
@Reader:
LDA $C000
STA $D020
INC Reader+1
JMP Reader
That's the last time I wrote self-modifying code anyway :-)