Linear Address Spaces: Unsafe at any speed (2022) • Hacker News Board

Re: Linear Address Spaces: Unsafe at any speed (2022)

#181

Post by aforwardslash » Wed, Jan 07, 2026, 4:23 PM UTC

convolvatron wrote: post #156

aforwardslash wrote: post #98

Earlier quoted context omitted.

If I understood correctly, you'te talking about using descriptors to map segments; the issue with this approach is two-fold: it is slow (as each descriptor needs to be created for each segment - and sometimes more than one, if you need write-execute permissions), and there is a practical limit on the number of descriptors you can have - 8192 total, including call gates and whatnot. To extend this, you need to use LDT…

no, not at all. we weren't using the underlying segmentation support. we just added kernel facilities to support segment ids and ranges and augment the kernel region structure appropriately. A call gate is just a syscall that changes the processes VM tables to include or drop regions (segments) based on the policy of the call.

Humm very interesting approach, are there any publicly available documentation links?

Re: Linear Address Spaces: Unsafe at any speed (2022)

#182

Post by rep_lodsb » Thu, Jan 08, 2026, 1:04 PM UTC

mpweiher wrote: post #180

StillBored wrote: post #178

Earlier quoted context omitted.

I think my point revolves more around what the HW designers were enabling. If they thought that the flat model was the right one, they would have just kept doing what the 286 did, and fixed the segment sizes at 4G.

Yes. The point is that the hardware designers were wrong in thinking that the segmented model was the right one. The hardware designers kept enabling complex segmented models using complex segment machinery. Operating system designers fixed the segments as soon as the hardware made that possible in order to enable a flat (paged) memory model and never looked back.

But were the software people actually right, or did they just follow the well-trodden path of VMS / UNIX, instead of making full use of the x86 hardware?

Having separate segments for every object is problematic because of pointer size and limited number of selectors, but even 3 segments for code/data/stack would have eliminated many security bugs, especially at the time when there was no page-level NX bit. For single-threaded programs, the data and stack segment could have shared the same address space but with a different limit (and the "expand-down" bit set), so that 32-bit pointers could reach both using DS, while preventing [SS:EBP+x] from accessing anything outside the stack.

Re: Linear Address Spaces: Unsafe at any speed (2022)

#183

Post by mpweiher » Tue, Jan 20, 2026, 8:19 AM UTC

rep_lodsb wrote: post #182

mpweiher wrote: post #180

Earlier quoted context omitted.

Yes. The point is that the hardware designers were wrong in thinking that the segmented model was the right one. The hardware designers kept enabling complex segmented models using complex segment machinery. Operating system designers fixed the segments as soon as the hardware made that possible in order to enable a flat (paged) memory model and never looked back.

But were the software people actually right , or did they just follow the well-trodden path of VMS / UNIX, instead of making full use of the x86 hardware? Having separate segments for every object is problematic because of pointer size and limited number of selectors, but even 3 segments for code/data/stack would have eliminated many security bugs, especially at the time when there was no page-level NX bit. For singl…

Inasmuch as hardware exists to run software, so software is the customer, the hardware people were wrong by definition, as they created a product that their customers weren't asking for, didn't want and had no use for.

Might segmentation have been better if the software had wanted it? Well, it's a counterfactual, so in some sense we can't know. And we can argue why we believe one or the other is better, but the evidence seems to be pretty overwhelming. It's not that there weren't (and aren't) operating systems that use segmentation, but somehow their "better" memory model didn't take the world by storm.