Fix PartitionAlloc randomization on 32-bit systems

third_party/WebKit/Source/wtf/AddressSpaceRandomization.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "wtf/AddressSpaceRandomization.h"
 
 #include "wtf/PageAllocator.h"
 #include "wtf/SpinLock.h"
 
@@ skipping 68 matching lines @@
 
 static struct ranctx s_ranctx;
 
 }
 
 // Calculates a random preferred mapping address. In calculating an
 // address, we balance good ASLR against not fragmenting the address
 // space too badly.
 void* getRandomPageBase()
 {
+#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)

[Alexander Potapenko, 2015/10/13 16:02:42]

You also need "if !defined(MEMORY_TOOL_REPLACES_ALLOCATOR)" around the anonymous
namespace above.

[jschuh, 2015/10/13 17:44:13]

I dropped this approach and just added a custom range as we discussed over chat.

+    return nullptr;
+#else
     uintptr_t random;
     random = static_cast<uintptr_t>(ranval(&s_ranctx));
 #if CPU(X86_64)
     random <<= 32UL;
     random |= static_cast<uintptr_t>(ranval(&s_ranctx));
     // This address mask gives a low liklihood of address space collisions.
     // We handle the situation gracefully if there is a collision.
 #if OS(WIN)
     // 64-bit Windows has a bizarrely small 8TB user address space.
     // Allocates in the 1-5TB region.
     // TODO(cevans): I think Win 8.1 has 47-bits like Linux.
     random &= 0x3ffffffffffUL;
     random += 0x10000000000UL;
 #else
     // Linux and OS X support the full 47-bit user space of x64 processors.
     random &= 0x3fffffffffffUL;
 #endif
 #elif CPU(ARM64)
     // ARM64 on Linux has 39-bit user space.
     random &= 0x3fffffffffUL;
     random += 0x1000000000UL;
 #else // !CPU(X86_64) && !CPU(ARM64)
+#if OS(WIN)
+    // On win32 host systems the randomization plus huge alignment causes
+    // excessive fragmentation. Plus most of these systems lack ASLR, so the
+    // randomization isn't buying anything. In that case we just skip it.
+    // TODO(jschuh): Just dump the randomization when HE-ASLR is present.
+    static BOOL isWow64 = -1;
+    if (isWow64 == -1 && !IsWow64Process(GetCurrentProcess(), &isWow64))
+        isWow64 = FALSE;
+    if (!isWow64)
+        return nullptr;
+#endif // OS(WIN)
     // This is a good range on Windows, Linux and Mac.
     // Allocates in the 0.5-1.5GB region.
     random &= 0x3fffffff;
     random += 0x20000000;
 #endif // CPU(X86_64)
     random &= kPageAllocationGranularityBaseMask;
     return reinterpret_cast<void*>(random);
+#endif // !MEMORY_TOOL_REPLACES_ALLOCATOR
 }
 
 }