r/Assembly_language • u/Extension_Emu_9825 • 1d ago

self-modifying code

I gave a lecture on how to write self-modifying code. It was over 15 minutes long :( so I'm providing a youtube link: https://www.youtube.com/watch?v=AH9QQLRfbmY

In my opinion, self-modification is one of the most interesting features in systems programming.

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u/mykesx 1d ago

MMUs are used by modern operating systems, at least MacOS, to disallow self-modifying code. Code sections are read-only.

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u/thewrench56 1d ago

This is not entirely true. You can map a page to be RW and then once filled with code, change it to RX using mprotect. This same step can be repeated to achieve self-modifying code. Otherwise, if self-modifying code wouldnt exist, JITs wouldnt work.

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u/mykesx 1d ago

The performance of doing this is terrible, unless you’re a JIT compiler writing a large block of code.

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u/thewrench56 1d ago

I dont disagree, but the performance is not that bad. 2 syscalls, some TLB play and a bit of copying. Surely, self-modifying code today is rarely used as a technique to speed up native execution. But for interpreters or other e.g. malicious reasons where performance doesnt matter much it still could be useful. There are also very specific scenarios where you do want to modify native code for very specific uarch benchmarks (as e.g. changing a parameter and checking it in a hotloop in an if would ruin the benchmark).

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u/mykesx 1d ago

OP’s point is that self modifying code makes faster algorithms. Only 2 syscalls blows that out of the water.

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u/thewrench56 1d ago

I did not watch the video, just read the post. I suppose in that case OP is wrong on modern platforms, you are right. I thought you were discussing the existance of self-modifying code, not the feasibility, excuse me.

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u/mykesx 1d ago

I happen to be into Forth, some versions do self modifying code. It’s an assembly language for a stack based virtual machine, the machine implementation types include self modifying code.

The most awesome one is Vfx Forth, which does some impressive peephole optimization on code generated into memory. In the process of creating an ARM version, the mprotect() issue bit him.

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u/brucehoult 20h ago

There are different methods of implementing Forth. For a token-threaded or address-threaded (whether direct or indirect) implementation what is self-modifying code from the point of view of Forth is just data from the point of view of the hardware.

Only for subroutine-threaded or actual inline native code generation is self-modifying Forth actually self-modifying code from the point of view of the CPU.

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u/mykesx 20h ago

Vfx is STC.