Revert "[platform] Remove {PageAllocator::kReadWriteExecute}."

This reverts commit bf19e60c.

Reason for revert: Two issues discovered with W^X in V8's 6.5 branch (see v8:7272 and chromium:793428). Still need a way to disable the feature.

Original change's description:
> [platform] Remove {PageAllocator::kReadWriteExecute}.
> 
> Now that write-protection of code memory is enabled everywhere and V8 is
> fully W^X compliant, we can remove the permission mode in question.
> 
> R=​hpayer@chromium.org
> BUG=v8:6792
> 
> Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
> Change-Id: I80fe95ac6bb0e2d1ad6d993154ce45d492d941be
> Reviewed-on: https://chromium-review.googlesource.com/866855
> Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Reviewed-by: Bill Budge <bbudge@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50770}

TBR=bbudge@chromium.org,mstarzinger@chromium.org,hpayer@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:6792
Change-Id: If4a205497ac83084a4092560363affb13b391462
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/883461Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50834}

include/v8-platform.h

@@ -200,11 +200,13 @@ class PageAllocator {
   virtual void* GetRandomMmapAddr() = 0;
 
   /**
-   * Memory permissions. Note that V8 is W^X compliant.
+   * Memory permissions.
    */
   enum Permission {
     kNoAccess,
     kReadWrite,
+    // TODO(hpayer): Remove this flag. Memory should never be rwx.
+    kReadWriteExecute,
     kReadExecute
   };
 

src/base/page-allocator.cc

@@ -17,6 +17,8 @@ STATIC_ASSERT_ENUM(PageAllocator::kNoAccess,
                    base::OS::MemoryPermission::kNoAccess);
 STATIC_ASSERT_ENUM(PageAllocator::kReadWrite,
                    base::OS::MemoryPermission::kReadWrite);
+STATIC_ASSERT_ENUM(PageAllocator::kReadWriteExecute,
+                   base::OS::MemoryPermission::kReadWriteExecute);
 STATIC_ASSERT_ENUM(PageAllocator::kReadExecute,
                    base::OS::MemoryPermission::kReadExecute);
 

src/base/platform/platform-cygwin.cc

@@ -36,6 +36,8 @@ DWORD GetProtectionFromMemoryPermission(OS::MemoryPermission access) {
       return PAGE_NOACCESS;
     case OS::MemoryPermission::kReadWrite:
       return PAGE_READWRITE;
+    case OS::MemoryPermission::kReadWriteExecute:
+      return PAGE_EXECUTE_READWRITE;
     case OS::MemoryPermission::kReadExecute:
       return PAGE_EXECUTE_READ;
   }

src/base/platform/platform-fuchsia.cc

@@ -21,6 +21,9 @@ uint32_t GetProtectionFromMemoryPermission(OS::MemoryPermission access) {
       return 0;  // no permissions
     case OS::MemoryPermission::kReadWrite:
       return ZX_VM_FLAG_PERM_READ | ZX_VM_FLAG_PERM_WRITE;
+    case OS::MemoryPermission::kReadWriteExecute:
+      return ZX_VM_FLAG_PERM_READ | ZX_VM_FLAG_PERM_WRITE |
+             ZX_VM_FLAG_PERM_EXECUTE;
     case OS::MemoryPermission::kReadExecute:
       return ZX_VM_FLAG_PERM_READ | ZX_VM_FLAG_PERM_EXECUTE;
   }

src/base/platform/platform-posix.cc

@@ -109,6 +109,8 @@ int GetProtectionFromMemoryPermission(OS::MemoryPermission access) {
       return PROT_NONE;
     case OS::MemoryPermission::kReadWrite:
       return PROT_READ | PROT_WRITE;
+    case OS::MemoryPermission::kReadWriteExecute:
+      return PROT_READ | PROT_WRITE | PROT_EXEC;
     case OS::MemoryPermission::kReadExecute:
       return PROT_READ | PROT_EXEC;
   }

src/base/platform/platform-win32.cc

@@ -747,6 +747,8 @@ DWORD GetProtectionFromMemoryPermission(OS::MemoryPermission access) {
       return PAGE_NOACCESS;
     case OS::MemoryPermission::kReadWrite:
       return PAGE_READWRITE;
+    case OS::MemoryPermission::kReadWriteExecute:
+      return PAGE_EXECUTE_READWRITE;
     case OS::MemoryPermission::kReadExecute:
       return PAGE_EXECUTE_READ;
   }

src/base/platform/platform.h

@@ -156,12 +156,12 @@ class V8_BASE_EXPORT OS {
   static PRINTF_FORMAT(1, 0) void VPrintError(const char* format, va_list args);
 
   // Memory permissions. These should be kept in sync with the ones in
-  // v8::PageAllocator. Note that there is on purpose no combination of
-  // the "write" and "execute" permission, because V8 is W^X compliant.
-  // Avoid introducing such a combination as embedders might rely on it.
+  // v8::PageAllocator.
   enum class MemoryPermission {
     kNoAccess,
     kReadWrite,
+    // TODO(hpayer): Remove this flag. Memory should never be rwx.
+    kReadWriteExecute,
     kReadExecute
   };
 

test/cctest/cctest.h

@@ -573,8 +573,8 @@ static inline uint8_t* AllocateAssemblerBuffer(
     size_t requested = v8::internal::AssemblerBase::kMinimalBufferSize) {
   size_t page_size = v8::internal::AllocatePageSize();
   size_t alloc_size = RoundUp(requested, page_size);
-  void* result = v8::internal::AllocatePages(nullptr, alloc_size, page_size,
-                                             v8::PageAllocator::kReadWrite);
+  void* result = v8::internal::AllocatePages(
+      nullptr, alloc_size, page_size, v8::PageAllocator::kReadWriteExecute);
   CHECK(result);
   *allocated = alloc_size;
   return static_cast<uint8_t*>(result);

test/unittests/allocation-unittest.cc

@@ -111,7 +111,7 @@ sigjmp_buf MemoryAllocationPermissionsTest::continuation_;
 TEST_F(MemoryAllocationPermissionsTest, DoTest) {
   TestPermissions(PageAllocator::Permission::kNoAccess, false, false);
   TestPermissions(PageAllocator::Permission::kReadWrite, true, true);
-  TestPermissions(PageAllocator::Permission::kReadExecute, true, false);
+  TestPermissions(PageAllocator::Permission::kReadWriteExecute, true, true);
 }
 #endif  // V8_OS_POSIX