64-bit TEB on WoW64

[Andrew Wesie]

In bug 47198 (https://bugs.winehq.org/show_bug.cgi?id=47198), we found
that League of Legends is trying to access the 64-bit TEB if it
detects that it is running under a 64-bit kernel. It does this by
checking if the %cs segment selector matches a WoW64 environment, and
if it does, it will then access %gs:0x60.

On WoW64, %gs points to the 64-bit TEB and %gs:0x60 is a pointer to
the 64-bit PEB. Under Wine on Linux, however, %gs is reserved for
glibc and %gs:0x60 contains a list pointer that we cannot corrupt
without crashing. The current solution in the referenced bug is to
patch glibc to reserve some space for Wine; thankfully that portion of
the TEB is glibc internal and can be reserved without rebuilding other
libraries.

So, my question is how to move forward. I see a few options:

1. Require users to have a patched glibc.
1a) Ask glibc nicely to upstream this patch.
1b) Lutris and downstream packagers can ship a compatible glibc with
their Wine builds if they choose.
2. Similar to the aarch64 issue with x18, find a way to save and
restore %gs at the boundary between PE modules and native modules.
3. Find some way to trap accesses to %gs:0x60 without using hardware
breakpoints and without destroying performance.

As an aside, I realize that it is impossible for Wine to perfectly
emulate a Windows environment and at some point it may be impossible
to continue to support apps that choose to apply aggressive
anti-debugging / obfuscation.