Use NtCurrentTeb() in GetStackStart() to fix 64-bit Wine on macOS

When running 64-bit Windows binaries on macOS using Wine, there is a
conflict between macOS's use of GS to point to pthread thread-specific
data, and Windows' use of GS to point to the TEB.

Apple has reserved some TSD slots for use by Wine to store commonly-used
TEB members (such as 0x30, the 'Self' pointer to the TEB).
But, other direct GS accesses by Windows programs (such as to
'StackBase') will return macOS pthread data rather than the TEB member.
This was causing 64-bit Chrome to crash on macOS under Wine.

Using NtCurrentTeb() gets the 'Self' pointer first, then dereferences
it to access the correct 'StackBase', fixing the crash.
This turns GetStackStart() from one instruction into two.
Crashpad also uses NtCurrentTeb().

The 32-bit change isn't needed, but is just for consistency.

Bug: 1121842
Change-Id: Id794030eb22b292530865f940bef8f0705bfd542
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380425
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802648}

AUTHORS

@@ -1154,6 +1154,7 @@ Canonical Limited <*@canonical.com>
 Cloudflare, Inc. <*@cloudflare.com>
 CloudMosa, Inc. <*@cloudmosa.com>
 Code Aurora Forum <*@codeaurora.org>
+CodeWeavers, Inc. <*@codeweavers.com>
 Collabora Limited <*@collabora.com>
 Comodo CA Limited
 CoSMo Software pvt ltd <*@cosmosoftware.io>

third_party/blink/renderer/platform/wtf/stack_util.cc

@@ -132,15 +132,16 @@ void* GetStackStart() {
   return pthread_get_stackaddr_np(pthread_self());
 #elif defined(OS_WIN) && defined(COMPILER_MSVC)
 // On Windows stack limits for the current thread are available in
-// the thread information block (TIB). Its fields can be accessed through
-// FS segment register on x86 and GS segment register on x86_64.
+// the thread information block (TIB).
 // On Windows ARM64, stack limits could be retrieved by calling
 // GetCurrentThreadStackLimits. This API doesn't work on x86 and x86_64 here
 // because it requires Windows 8+.
 #if defined(ARCH_CPU_X86_64)
-  return reinterpret_cast<void*>(__readgsqword(offsetof(NT_TIB64, StackBase)));
+  return reinterpret_cast<void*>(
+      reinterpret_cast<NT_TIB64*>(NtCurrentTeb())->StackBase);
 #elif defined(ARCH_CPU_X86)
-  return reinterpret_cast<void*>(__readfsdword(offsetof(NT_TIB, StackBase)));
+  return reinterpret_cast<void*>(
+      reinterpret_cast<NT_TIB*>(NtCurrentTeb())->StackBase);
 #elif defined(ARCH_CPU_ARM64)
   ULONG_PTR lowLimit, highLimit;
   ::GetCurrentThreadStackLimits(&lowLimit, &highLimit);