Re: Assignment of GDT entries

[Albert Cahalan]

On 9/14/06, Zachary Amsden <zach@xxxxxxxxxx> wrote:
> Albert Cahalan wrote:

> > So basically it's not allowed to just grab the 3rd slot?
>
> You can, but you should be prepared for it to fail as well.

Without knowing details of the kernel's GDT, how?

> > What if I want to find out what is already in use?
> > Am I supposed to iterate over all 8191 possible
> > GDT entries? How do I even tell how many slots
> > are available without using them all up?
>
> There are only 32 possible GDT entries in 32-bit i386 Linux, and only
> three of them are usable for userspace.  You can't find out which slots
> are in use, but you can cause one to be allocated and returned to you.
> This seems like a perfectly reasonable API to me, why do you think it is
> so ugly?

Eh, "returned to you" doesn't work for me. I need to
figure out what other code (not written by me) uses.

I may need to "borrow" a slot if all three slots are in
use. Without using evil knowledge of the GDT, how
am I to do that? I don't know what slots might have
been allocated by other libraries.
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