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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