Albert Cahalan wrote
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 don't understand. Why do you need to figure that out? You need a
selector, you ask for one, and you get assigned one. It is that
simple. You can't figure out what other code uses, and the kernel has
no way to tell you, because that is an application level allocation
problem, not a kernel responsibility. The kernel has no visibility into
userspace intentions regarding segment usage.
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.
What kind of libraries are you using? Unless this is really, really,
special purpose, they are going to allocate at most one, and that is
only if you use TLS libraries.
If all three slots are in use (i.e. your allocation fails), you'll have
to allocate an LDT selector, just like wine:
void wine_ldt_init_fs( unsigned short sel, const LDT_ENTRY *entry )
{
if ((sel & ~3) == (global_fs_sel & ~3))
{
#ifdef __linux__
struct modify_ldt_s ldt_info;
int ret;
ldt_info.entry_number = sel >> 3;
fill_modify_ldt_struct( &ldt_info, entry );
if ((ret = set_thread_area( &ldt_info ) < 0)) perror(
"set_thread_area" );
#elif defined(__APPLE__)
int ret = thread_set_user_ldt( wine_ldt_get_base(entry),
wine_ldt_get_limit(entry), 0 );
if (ret == -1) perror( "thread_set_user_ldt" );
else assert( ret == global_fs_sel );
#endif /* __APPLE__ */
}
else /* LDT selector */
{
internal_set_entry( sel, entry ); <---- just like this
}
wine_set_fs( sel );
}
Zach
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