[TUHS] Harvard and Von Neumann Architectures and Unix

Warner Losh imp at bsdimp.com
Tue Nov 28 04:14:08 AEST 2017

On Mon, Nov 27, 2017 at 9:50 AM, Larry McVoy <lm at mcvoy.com> wrote:

> On Mon, Nov 27, 2017 at 11:11:41AM -0500, Noel Chiappa wrote:
> >     > From: Doug McIlroy
> >
> >     > But if that had been in D space, it couldn't have been executed.
> >
> > Along those lines, I was wondering about modern OS's, which I gather for
> > security reasons prevent execution of data, and prevent writing to code.
> >
> > Programs which emit these little 'custom code fragments' (I prefer that
> term,
> > since they aren't really 'self-modifying code' - which I define as 'a
> program
> > which _changes_ _existing_ instructions) must have some way of having a
> chunk
> > of memory into which they can write, but which can also be executed.
> Isn't that how dtrace works?

That's not self modifying code. I've written some self-modifying code (I
was young, it was sexy, I thought), and it's all about rewriting your own
instruction stream. And doing so based on data you get so far. Not every
modification of a text page is self-modifying code.

dtrace, and every debugger that doesn't have hardware assist debug
registers (or overflows them) modifies the text pages to allow breakpoints,
watchpoints, tracepoints and the like to work. Even the OS creates code to
run at runtime. The signal trampoline that's used to return from a signal
handler in BSD is a small bit of code the OS fills in bits of out of a
template. It's one of the things that slowed adoption of turning off the X
bit for stack pages in response to many of the buffer overflow attacks.

On modern architectures, a fair amount of PTE and/or cache dancing needs to
happen to make this stuff work sometimes. Some processors make this easy
with dual mappings, others are... what's the right word... tedious.

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