[PATCH 2/2] ntdll: Report SegDs to be identical to SegSs in x86_64 exception handlers.

[Zebediah Figura]

Based on a patch by Dávid Török.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=47970
Signed-off-by: Zebediah Figura <z.figura12 at gmail.com>
---
Actually, the behaviour of segment registers on x86_64 Windows is more
complicated:

* By default, %ss == %ds == %es == %gs == 0x2b; %cs == 0x33; %fs == 0x53.

* NtGetContextThread() does not capture the actual values, but instead returns
  the default values.

* All system calls restore all segments to their default values, including
  NtContinue().

* RtlRestoreContext() also restores all segments to their default values.

* SegDs, SegEs, SegFs, SegGs all restore default values in exception handlers;
  SegSs returns the actual value of %ss.

* RtlCaptureContext() does capture the actual values.

I did not test:

* whether %cs is correctly captured or restored in any functions;

* whether segment registers are restored from exception handlers (either to
  the default values or to the values set in the context).

We could, to a point, emulate the above, but it gets tricky. According to the
Intel 64 and IA-32 Architectures Software Developer's Manual:

* %es, %ds, %ss can point to any segment—the entire contents of the descriptor
  table are effectively ignored (if I'm reading Volume 3 §3.4.4 right). However,
  they have to point to a *valid* segment (cf. Volume 1 §3.4.2.1).

* %cs is more restricted—the attributes in the descriptor table are also
  validated (Volume 3 §5.2.1).

* I don't even know about %fs and %gs; the manual is too vague and confusing.

 dlls/ntdll/tests/exception.c    | 2 +-
 dlls/ntdll/unix/signal_x86_64.c | 8 +++-----
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/dlls/ntdll/tests/exception.c b/dlls/ntdll/tests/exception.c
index e2a8071168d..90af9de1bba 100644
--- a/dlls/ntdll/tests/exception.c
+++ b/dlls/ntdll/tests/exception.c
@@ -3000,7 +3000,7 @@ static DWORD WINAPI handler( EXCEPTION_RECORD *rec, ULONG64 frame,
         "%u: Unexpected exception address %p/%p\n", entry,
         rec->ExceptionAddress, (char*)context->Rip );
 
-    todo_wine ok( context->SegDs == context->SegSs,
+    ok( context->SegDs == context->SegSs,
         "%u: ds %#x does not match ss %#x\n", entry, context->SegDs, context->SegSs );
 
     if (except->status == STATUS_BREAKPOINT && is_wow64)
diff --git a/dlls/ntdll/unix/signal_x86_64.c b/dlls/ntdll/unix/signal_x86_64.c
index 7dab5fbf4ed..814f2824129 100644
--- a/dlls/ntdll/unix/signal_x86_64.c
+++ b/dlls/ntdll/unix/signal_x86_64.c
@@ -1500,11 +1500,6 @@ static void save_context( struct xcontext *xcontext, const ucontext_t *sigcontex
     context->SegFs  = FS_sig(sigcontext);
     context->SegGs  = GS_sig(sigcontext);
     context->EFlags = EFL_sig(sigcontext);
-#ifdef DS_sig
-    context->SegDs  = DS_sig(sigcontext);
-#else
-    __asm__("movw %%ds,%0" : "=m" (context->SegDs));
-#endif
 #ifdef ES_sig
     context->SegEs  = ES_sig(sigcontext);
 #else
@@ -1515,6 +1510,9 @@ static void save_context( struct xcontext *xcontext, const ucontext_t *sigcontex
 #else
     __asm__("movw %%ss,%0" : "=m" (context->SegSs));
 #endif
+   /* Legends of Runeterra depends on having SegDs == SegSs in an exception
+    * handler. */
+    context->SegDs  = context->SegSs;
     context->Dr0    = amd64_thread_data()->dr0;
     context->Dr1    = amd64_thread_data()->dr1;
     context->Dr2    = amd64_thread_data()->dr2;
-- 
2.30.2