Experimental 64 bit OS X packages ready for testing

Michael Müller michael at fds-team.de
Sat Apr 9 15:17:56 CDT 2016

Hi Ken,

I've attached the hack as attachment, but let me explain the theory
first. ;)

After checking a couple of 64-bit applications (VLC, Teamspeak 3, ...),
they were mostly using a single TEB field, which is %%gs:0x30
(Teb->Tib.Self). My theory is that they use the NtCurrentTeb() inline
function from the MS header files. As you are aware, just changing the
%gs segment is not possible, but in order to get these applications
working we just need to set %%gs:0x30 correctly.

I looked through the last public pthread implementation on OS X and saw
that %%gs:0x30 collides with the 6th TLS field. In fact the first 256
fields are reserved for system use, but there is some documentation
available how those fields are used
As you can see the fields 1-9 are reserved for dyld, but only the fields
1,2,3 and 8 are currently in use. The fields 4-7 are reserved for future
usage. I therefore looked through the dyld source code to see if those
fields might be in use now. To my surprise current versions of dyld
don't seem to use hardcoded TLS indices any more. For example field 8
was reserved for Unwind_SjLj and when you take a look at the current
source code
(http://opensource.apple.com/source/dyld/dyld-360.18/src/dyldAPIs.cpp -
function at line 1261) you see that the key is dynamically created using
pthread_key_create which only returns fields > 256. Moreover, all the
code uses pthread_setspecific which doesn't allow to set fields < 10.
There is an exception, when a library requests  TLS storage, the values
are directly changed using the gs segment, but even in this case the
keys are dynamically allocated using pthread_key_create. So far I have
found no evidence (neither source code or tests) that those fields are
actually used or that the pthread implementation changed in this regard.
I therefore assume that overwriting this single field should cause no
harm and I gave the hack a try.

Anyway, I only spent like an hour yesterday to investigate this and I my
have overlooked something. You are of course also welcome to do your own
investigations. When my theory holds, we might also be able to optimize
Wines NtCurrentTeb() again by using %%gs:0x30 instead of using the
pthread_getspecific function.


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