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SubjectRe: Assignment of GDT entries
Albert Cahalan wrote:
> So if I grabbed the first two slots before glibc got to
> mess with them, glibc wouldn't break horribly?

glibc would be happy with anything it got; if you grabbed all 3 TLS
slots it would probably be upset.

> If I grabbed one slot and glibc grabbed another, Wine
> would be OK with the third instead of the second?

Presumably.

> So basically it's not allowed to just grab the 3rd slot?
Eh? You mean there's no "allocate and return TLS slot #N" operation?
No, but all the TLS slots should be interchangeable. Once you've got
your entry numbers and worked out your selector values, you can just use
them.

> 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?
The kernel reserves 3 slots in the GDT for usermode use, which are
per-thread. If you want more segment descriptors, you can always
allocate an LDT.

> Eeeeeeew. Well this was documented exactly nowhere.
> The man page is even vague about entry_number,

man set_thread_area has this as paragraph 2:

When set_thread_area() is passed an entry_number of -1, it uses a free
TLS entry. If set_thread_area() finds a free TLS entry, the value of
u_info->entry_number is set upon return to show which entry was
changed.

which seems pretty clear to me. A quick run with strace on any binary
shows this in action:

set_thread_area({entry_number:-1 -> 6, base_addr:0xb7fb06c0,
limit:1048575, seg_32bit:1, contents:0, read_exec_only:0,
limit_in_pages:1, seg_not_present:0, useable:1}) = 0

J
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