Kext ring level

[José Cornado]

Prelude:

*) Literature sates that task switching is performed by software instead of hardware in most modern OS. 

In which ring level does a kext run?  As far as I have researched the cpu will not allow a task switch from outside the GDT. Does GDT is memory protected? If so would a kext have access to it?

I have several avenues to take and I would like to explore all. 

Thanks a lot in advance and happy holidays!

--
José Cornado

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Andre Gide

[Quinn "The Eskimo!"]

On 27 Dec 2011, at 21:49, José Cornado wrote:

> In which ring level does a kext run?

The Darwin kernel does not use the concept of rings. All kernel code runs in supervisor mode, and all user processes run in user mode. On x86 CPUs these map to rings 0 and 3, respectively, but to some extent that's an implementation detail. The kernel has and continues to run on a variety of different CPU architectures, most of which aren't based on rings.

> Does GDT is memory protected? If so would a kext have access to it?

What are you planning to do with the GDT? In most cases a KEXT can achieve its memory management goals without having to access the GDT directly, and accessing the GDT directly is likely to result in long-term binary compatibility problems.

Share and Enjoy
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Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

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[Shantonu Sen]

The kernel is open source, you can build it yourself and add instrumentation to determine these things.

None of these things are considered KPI or stable interfaces. Perhaps the Linux kernel might be more interesting for your research.

Shantonu

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[José Cornado]

Thanks, it would be a little overkill for now. I can stay out of kernel space. I will minimize the noise though :-}

Thanks anyway for the help!

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