1 stable 2 super stable

[honey the codewitch]

That all falls apart on embedded when you're dealing with primitive memory protection schemes and an RTOS at best. :( I was just dealing with a buffer overrun this morning.

Check out my IoT graphics library here: https://honeythecodewitch.com/gfx And my IoT UI/User Experience library here: https://honeythecodewitch.com/uix

[honey the codewitch]

Hard to protect the OS against ill behaved drivers without introducing a nasty performance hit. Drivers basically need to operate in kernel space. Windows does have "user mode" drivers now, but not everything can be run that way. It would destroy your GPU performance, for example.

Check out my IoT graphics library here: https://honeythecodewitch.com/gfx And my IoT UI/User Experience library here: https://honeythecodewitch.com/uix

[trønderen]

Don't forget the ring protection. Drivers are not supposed to run in ring 0.

[trønderen]

Remember the ring protection. Drivers are not supposed to run in ring 0. The (hardware) ring protection is effective at all times; it is not turned on or activated. Making use of it does not affect performance. Then comes the questions of placing the various OS data structures in the appropriate ring. I wouldn't be surprised if there are essential data structures in ring 1 (/2) that really should be located in ring 0. But then the OS code referencing it must be located in ring 0 as well. If the OS wasn't originally architected with a ring protection in mind, cleaning it up later may be a rather nasty job. I've never been inside Windows source code, but my experiences, not the least with (but certainly not limited to) Windows Docker, seems to indicate that it "Everything is deeply intertwingled" (to quote Ted Nelson). Picking apart a crow's nest to put some of the stick in the "ring 0" pile, without breaking any of the other sticks, requires extreme care. Then learning Zephyr, seeing how it is possible to divide OS functionality into clear cut, small pieces so that you load exactly what you need and nothing more, was a revelation to me. It is the diametrical opposite to Windows. Of course: Zephyr and Windows targets (almost) completely different problem domains. Yet I am quite sure that a general OS like Windows could be built with much more of the Zephyr modular philosophy.

[honey the codewitch]

Maybe I misunderstand how things are laid out but I do know a driver can do a kernel wait without switching between "user mode" and "kernel mode" under windows. I assumed kernel mode was equiv to ring 0 but it sounds like the truth is more complicated.

Check out my IoT graphics library here: https://honeythecodewitch.com/gfx And my IoT UI/User Experience library here: https://honeythecodewitch.com/uix

[Calin Negru]

k5054, dandy72, tronderen that’s an interesting read guys

[Daniel Pfeffer]

Well, in OS/2 you had the following levels: Level 0: Kernel and (some) drivers Level 1: Most drivers Level 2: IOPL (In, Out instructions) for use programs Level 3: User programs I don't know much about the Windows kernel, but I assumed that it had a similar structure.

Freedom is the freedom to say that two plus two make four. If that is granted, all else follows. -- 6079 Smith W.

[trønderen]

You've got several different protection mechanisms: First, that of addressability. A process presents a logical address the memory management system, which will translate it to a physical address. The contents of the MMS tables switches when the CPU switches to another process, so each process will see a different selection of physical pages, even if the logical address is the same. No user process has page table entries pointing to OS data structures, so it cannot reference / modify them. The translation from logical to physical pages goes through another translation before getting to the page tables: The logical address space is split into segments. Each segment has a minimum privilege level (i.e. ring). On Intel CPUs, 0 is the highest privilege, 3 is minimum. Even if a driver runs in the same logical address space as OS code, some segments of that space could be marked as requiring level 0 (or 1 or 2) for access. Drivers usually run in ring 1 or 2, and if the OS data lies in a ring 0 segment, the driver cannot access it. A process in a given ring have access to all segments of lower (higher numbered) rings, so a ring 0 kernel process can access whatever it wants, as long as it has a page table entry to it. The segment descriptor tells the length of the segment: An attempt to go to a less restricted segment and address out of bounds, into another segment, will fail. The segment descriptor also indicates the type of segment, one of 16 values (4 bits): A "Read-Only" segment may not be modified, even it it can be read. Typically, the OS will make configuration and state information available to drivers this way, but the drivers cannot modify/corrupt this information. Also, the contents of the segment can not be executed as instructions. An "Execute-Only" segment cannot be read or written, but may be executed. Code segments may allow reading. (The OS may need write access to the data structures, so it constructs a different segment descriptor for its own use.) On the x86/x64 you can also restrict write access on the page level. Even if the segment generally is accessible, sensitive OS structures may be stored in pages that denies writing. (Both the segment and the page must allow writing.) There is another bit restricting code execution, if this bit is set in the page descriptor. The ring (also called Privilege Level) of the current process also can restrict access to I/O devices. E.g. the OS may allow users to write drivers to run in ring 2, to gain access to (at least some) OS structures, but do I/O

[honey the codewitch]

trønderen wrote:

hey may still have access to a lot of the OS data structures, a lot of it read only, that ordinary user processes can't access.

This must be why they can do things like enter a wait state without doing a switch.

trønderen wrote:

Simpler machines often have just two privilege levels, comparable to ring 0 and ring 3 on the x86/x64. Then, drivers usually have all the privileges of the OS, running in "kernel mode".

When I learned things, this is how it was done. I knew things had changed, but nevertheless that probably contributed to my misunderstanding.

Check out my IoT graphics library here: https://honeythecodewitch.com/gfx And my IoT UI/User Experience library here: https://honeythecodewitch.com/uix

[honey the codewitch]

tronden did a pretty good rundown in a reply to me. My information was woefully out of date.

Check out my IoT graphics library here: https://honeythecodewitch.com/gfx And my IoT UI/User Experience library here: https://honeythecodewitch.com/uix

[dandy72]

Granted, but this thread was started discussing Windows only, which tries a lot harder protecting itself than embedded systems.

[dandy72]

That *is* kernel mode (ring 0) vs usermode (ring 3), no?

[trønderen]

(Most) drivers are supposed to run in ring 1 or 2 - call that "driver mode" if you like. I guess that the OS kernel has some central drivers of its own running in ring 0. The borderline is fuzzy between drivers and code for manipulating CPU resources (such as the interrupt system or MMS). The OS may trust itself. It should not trust "foreign" drivers, e.g. those developed by manufacturers/vendors of "foreign" peripherals, to run in ring 0.

[pmauriks]

3 Super duper stable. :) Obligatory mention of Tannenbaum and Minix 3 ([Minix 3 - Wikipedia](https://en.wikipedia.org/wiki/Minix\_3))

[dandy72]

What (admittedly little) exposure I've had to this had led me to conclude that, for all practical purposes, there was no such thing as Ring 1 and Ring 2, at least when it comes to Windows. I think Mark Russinovich even said so himself, but don't quote me on that. Although it does make sense that this is where drivers ought to live.

[dandy72]

trønderen wrote:

The OS may trust itself.

That's a mistake if that's the case. I thought Windows these days only established trust by signing *every* file in the Windows folder (where practical).

[trønderen]

"Trust" in the sense "May allow its own components write access to data structures that noone else trusted to modify". If it didn't trust itself to set up segment/paging tables, interrupt handlers etc., nothing could ever work. Signing is a different sort of trust.

[Calin Negru]

> super duper stable I don’t think that is out yet. Who knows, maybe Windows 12 will fit the requirements. Although some might say there’s not much space for improvement left.

[trønderen]

The hardware offering certainly is there. A specific OS is not obliged to make use of it. The 386 segment system looks like it was hand crafted to Windows of the day. But MS made attempts to make Window the universal OS for all sorts of processors (such as MIPS, Alpha, PowerPC, Itanium, ...). Since they do not all provide the same hardware support, MS chose to limit themselves to the bare minimum available on all processors and do their own software simulation, identical for all of it. The same goes for the protection system: They ignored all advanced mechanisms. All users shall have the same offering; we cannot give x86/x64 users a protection mechanism unavailable to users on other platforms. Of course they could have designed a hardware abstraction layer that modelled both segment and ring mechanisms that could very easily be mapped to Intel hardware, but required far more software emulation on other processors (giving them a competitive disadvantage). Maybe they would have done that, if they had known at that time how other processors would dwindle away anyway. They didn't. A major factor may be that they picked up OS experts from other architectures that didn't provide such things, major design architects simply unfamiliar with how such mechanisms are used, so they ignored them. I am quite sure that at least you were right, that Windows ignored the intermediate privilege levels (along with the segment mechanisms). I thought that in order to make a more robust kernel, they started pushing external drivers down to ring 1 a while ago. I may very well be wrong. Maybe what I have read is unfounded speculations, and I found it such a natural thing to do that I took for granted that the change was carried through. If the OS code is structured like a crow's nest it might be very difficult to realize, though. I have strong suspicions that the current state of the Windows implementation is not quite what you would use as a model system if you were teaching a university level course in well-designed OS implementation.

[Matt Bond]

"Intend your puns, weaklings" - Old Fire Emblem game.

Bond Keep all things as simple as possible, but no simpler. -said someone, somewhere