Don't try to read return address from the stack of another thread on Win64.

runtime/vm/profiler.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/utils.h"
 
 #include "vm/allocation.h"
 #include "vm/atomic.h"
 #include "vm/code_patcher.h"
 #include "vm/isolate.h"
@@ skipping 1658 matching lines @@
 
 
 static void SetPCMarkerIfSafe(Sample* sample) {
   ASSERT(sample != NULL);
 
   uword* fp = reinterpret_cast<uword*>(sample->fp());
   uword* sp = reinterpret_cast<uword*>(sample->sp());
 
   // If FP == SP, the pc marker hasn't been pushed.
   if (fp > sp) {
-#if defined(TARGET_OS_WINDOWS)

[Vyacheslav Egorov (Google), 2014/08/08 12:47:54]

I don't think this code was doing anything.

Stack is growing downwards hence GUARD page can only be reasoned to be below
last accessed SP (and thus FP --- if FP is really FP not some random stack
pointer).

If FP is at the beggining of the page FP+8 is on the same page not on the next
page. Hence if FP is really FP then FP+8 can't be on the guard page (it was
already written to).

Overall logic behind this check is unclear to me.

I would understand this guard if we were reading [FP-8] here. In any case this
is not sufficient to protect us from hitting guard page given how freely Win64
compiler uses RBP.

[Cutch, 2014/08/08 13:24:53]

Your analysis is correct, this logic isn't entirely correct- but, this code was
successfully avoiding a similar flaky crash on Win32. 

-    // If the fp is at the beginning of a page, it may be unsafe to access
-    // the pc marker, because we are reading it from a different thread on
-    // Windows. The next page may be a guard page.
-    const intptr_t kPageMask = VirtualMemory::PageSize() - 1;
-    if ((sample->fp() & kPageMask) == 0) {
-      return;
-    }
-#endif
     const uword pc_marker = *(fp + kPcMarkerSlotFromFp);
     sample->set_pc_marker(pc_marker);
   }
 }
 
 
 // Given an exit frame, walk the Dart stack.
 class ProfilerDartExitStackWalker : public ValueObject {
  public:
   ProfilerDartExitStackWalker(Isolate* isolate, Sample* sample)
@@ skipping 342 matching lines @@
   ASSERT(counters != NULL);
   counters->Increment(isolate->vm_tag());
 
   // Setup sample.
   Sample* sample = sample_buffer->ReserveSample();
   sample->Init(isolate, OS::GetCurrentTimeMicros(), state.tid);
   sample->set_vm_tag(isolate->vm_tag());
   sample->set_user_tag(isolate->user_tag());
   sample->set_sp(state.sp);
   sample->set_fp(state.fp);
+#if !(defined(TARGET_OS_WINDOWS) && defined(HOST_ARCH_X64))
+  // It is never safe to read other thread's stack unless
+  // other thread is inside Dart code.
   SetPCMarkerIfSafe(sample);

[Cutch, 2014/08/08 13:24:53]

This should either be:

#if !defined(TARGET_OS_WINDOWS)
#endif

or keep the check in SetPCMarkerIfSafe

+#endif
 
   // Walk the call stack.
   if (FLAG_profile_vm) {
     // Always walk the native stack collecting both native and Dart frames.
     ProfilerNativeStackWalker stackWalker(sample,
                                           stack_lower,
                                           stack_upper,
                                           state.pc,
                                           state.fp,
                                           state.sp);
@@ skipping 26 matching lines @@
                                             stack_upper,
                                             state.pc,
                                             state.fp,
                                             state.sp);
       stackWalker.walk();
     }
   }
 }
 
 }  // namespace dart