Honneywell multics? from panels. the inline phots in this message folks -smecc

Mouse mouse at Rodents-Montreal.ORG
Wed Mar 16 08:55:53 CDT 2016

>> Also, Multics stacks grow upward -- great for protection against
>> buffer overrun attacks, but a pain in a modern architecture.
> Sorry, I don't follow that?  Why does the stack growth direction make
> a difference?  It's just a convention, isn't it, which direction is
> 'push' and which is 'pop'?

Well, some architectures have autodecrement move-to-memory and
autoincrement move-from-memory, but not autoincrement move-to-memory or
autodecrement move-from-memory.  The Super-H is an example; I don't
know whether x86 is another.

As for buffer overruns, the point there is that a buffer overrun
clobbers memory addressed higher than the buffer.  If the stack grows
down, this can overwrite stack frames and/or callers' locals.  If the
stack grows up, all it can overwrite is locals for the current frame
and unused stack space.

Actually, it might be interesting to do a VAX OS that used P1 for
program text and data and P0 for an upwards-growing stack.  (It'd mean
not using the VAX stack instructions, since they have a
downward-growing mindset wired into them, but the VAX _does_ allow
mixing either kind of auto-modify with either direction of data move,
so that would be a minor annoyance at worst.)

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