[libunwind] [SEH] Initialize _msContext with RtlCaptureContext

[mstorsjo]

Summary

When we initialize the UnwindCursor (unw_cursor_t) based on
an existing Registers object (unw_context_t), we only initialize
a subset of the class.

Fill the struct properly for the current thread with RtlCaptureContext,
followed by overwriting of the subset of registers that we do have
available in the Registers class.

One might think that it's enough to initialize specifically the
registers that we signal availability for with ContextFlags,
however in practice, that's not enough.

This fixes crashes when restoring the context via RtlRestoreContext
(via UnwindCursor::jumpto), via __unw_resume.

[cdavis5x]

I had hoped to avoid that duplicated work here. Clearly there was something I missed...

[mstorsjo]

In D147636#4249742, @cdavis5x wrote:

I had hoped to avoid that duplicated work here. Clearly there was something I missed...

Yep - I didn't quite pinpoint what goes wrong and where, but it's reproducible both on Wine and on native Windows. A (untested) mini repro of the issue might look something like this:

CONTEXT ctx;
RtlCaptureContext(&ctx);
static int Restored = 0;
if (!Restored) {
  Restored = 1;
  ctx.ContextFlags &= CONTEXT_CONTROL|CONTEXT_INTEGER|CONTEXT_FLOATING_POINT;
  ctx.SegCs = 0;
  RtlRestoreContext(&ctx);
}

One could think it's ok to clobber SegCs since ContextFlags doesn't have CONTEXT_SEGMENTS set - but it looks like it does matter nevertheless.

[mstorsjo]

Can someone give a libunwind group approval for this one? It's got an approval from @cdavis5x who originally authored the SEH codepaths here.

[MaskRay]

Rubber stamp :)

libunwind/src/UnwindCursor.hpp

UnwindCursor<A, R>::UnwindCursor(unw_context_t *context, A &as)
   static_assert((alignof(UnwindCursor<A, R>) <= alignof(unw_cursor_t)),
                 "UnwindCursor<> requires more alignment than unw_cursor_t");
   memset(&_info, 0, sizeof(_info));
   memset(&_histTable, 0, sizeof(_histTable));
   _dispContext.ContextRecord = &_msContext;
   _dispContext.HistoryTable = &_histTable;
   // Initialize MS context from ours.
   R r(context);
+  RtlCaptureContext(&_msContext);
   _msContext.ContextFlags = CONTEXT_CONTROL|CONTEXT_INTEGER|CONTEXT_FLOATING_POINT;
 #if defined(_LIBUNWIND_TARGET_X86_64)
   _msContext.Rax = r.getRegister(UNW_X86_64_RAX);
   _msContext.Rcx = r.getRegister(UNW_X86_64_RCX);
   _msContext.Rdx = r.getRegister(UNW_X86_64_RDX);
   _msContext.Rbx = r.getRegister(UNW_X86_64_RBX);
   _msContext.Rsp = r.getRegister(UNW_X86_64_RSP);
   _msContext.Rbp = r.getRegister(UNW_X86_64_RBP);