Self-modifying code: Difference between revisions

Revision as of 23:46, 27 January 2004

Revision as of 23:48, 27 January 2004

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Because of the security implications of self-modifying code, some [[operating system]]s go to lengths to rule it out. Recent versions of [[OpenBSD]], for instance, have a feature known as W^X (for "write [[xor]] execute", meaning a program can only write, or execute, ''but not both'') which inhibits alteration of memory pages which harbor executable code. Programs which depend upon rewriting their own machine code cannot execute in such an environment.

Most modern processors loadsload the mashinecodemachine code before they executesexecute it, wichwhich means that if an instruction that is too near the [[instruction pointer]] is modified, the processor will not notice, but instead execute the code as it was <i>before</i> it was modified. See [[Prefetch Input Queue]] (PIQ)

==Example [[NASM]]-[[syntax]] self-modifying [[x86]]-assembly algorithm that determines the size of [[Prefetch Input Queue|PIQ]]==