[etune]

ATOM could inject code before or after any basic block in a program: https://dl.acm.org/doi/abs/10.1145/178243.178260. The general technique, IIRC, was to replace the first instruction of the basic block with a jump to code that contained your new code and then the overwritten instructions, and then jump back into the original code.

[dillstead]

Yes, I've implemented this technique before at my job. Relocating assembly instructions especially those that contain branching logic can be tricky as the offsets have to be recomputed or a new instruction needs to be used instead. More often that not, you may not have enough space for that new instruction.

[nicholasmoser]

This is also similar to what is done with video game modification (think Gecko, Game Shark, Game Genie).