Re: Arbitrary Sandbox

["Randy Brukardt" <randy@rrsoftware.com>]

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message 
news:wccvcn1nl0x.fsf@shell01.TheWorld.com...
...
>> Maybe so. But this is a different case, because memory protection and
>> segmentation support is already there. Why no high-level language uses 
>> it?
>
> You mean "already there" in 386/pentium?  Languages don't use it because
> it's inefficient, and because whatever segmentation can do can be done
> better by some combination of software and paging.

The only reason it is "inefficient" was because OSes didn't use it. That is, 
it was quite efficient on an 386 (the only big expense occurs when reloading 
segment registers, and that was something that occurred very rarely in the 
two-segment model we used). Nowdays, the segment prefixes themselves have 
become very expensive, and thus it's no longer practical to use the segments 
at all. Boo.

As far as "software" and "paging" being able to do anything, this makes no 
sense at all. Pages on the x86 architecture don't (or at least didn't until 
fairly recently) have any memory permissions associated with them, so it was 
impossible to use that to prevent executing data. Moreover, Unix-derived 
file formats had no practical way to tell the OS what was not executed (and 
the little information that exists was not used in any Unix or Windows 
system that I worked on prior to XPsp3).

As a practical matter, OSes made it completely impossible for a compiler to 
do anything in this area, even if it wanted to (as we strongly did -- we 
continued to use the DOS Extender compiler for development for a decade 
simply because it detected bugs a lot earlier than NT could - executing 
stack by mistake is extremely common). So sad. And nowdays they're 
reinventing the wheel such that it still is impossible for a compiler to do 
much in this area, except a handful of very weak technologies like DEP.

                                   Randy.