16-bit code and L4

Espen Skoglund esk at ira.uka.de
Fri Jun 22 13:18:42 CEST 2007

[Adam Lackorzynski]
> On Fri Jun 22, 2007 at 15:37:30 +1300, Valery V. Sedletski wrote:
>> And how to access these GDT descriptors from userland? Is this
>> documented somewhere? Can usermode task change attributes of these
>> descriptors (such as "16-bits" attribute)?  Are they (descriptors)
>> somehow mapped to requestor's address space or how else can
>> usermode task modify these descriptors? Or, can usermode task only
>> load selectors corresponding to these descriptors, but not modify
>> descriptors itself?

> Userland can load descriptors that are sanity checked by the kernel
> and then stored in the GDT. Using 16-bit segments should work too
> although I never tried. Similar features are offered for the
> LDT. Our l4sys package contains segment.h headers which describe the
> kernel interface.

>> So it is with Fiasco, and what about Pistachio? (Is this mailing
>> list the right place to ask such questions, or it is better to ask
>> them at l4ka at ira.uka.de?)

> I'm talking about Fiasco only. You can ask any L4 related question
> on l4-hackers.

>> And what about DS/ES? Can usermode task load these registers with
>> another selector, as with FS/GS?

> Loading should work I guess but DS and ES are currently not restored
> on exit to user.

I was working on allowing applications to use segmentation in
Pistachio as well.  The modifications enabled apps to install segment
descriptors in a per address space LDT (after sanity checking by the
kernel of course).  Modifying the GDT is not supported.

By setting a thread in a special mode the kernel will never modify the
ES, FS, and GS of the thread.  For optimization reasons the kernel
still retains the right to modify DS and SS though, so in effect these
can not be freely used by the app.  Also, the kernel only allows data
segments to be installed --- no code or other segment types.

It's been a while since I worked on this and got sidetracked by other
stuff, so I didn't completely finish the implementation yet.


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