[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}

include/v8-platform.h

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

src/base/page-allocator.cc

@@ -17,8 +17,6 @@ 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

@@ -36,8 +36,6 @@ 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

@@ -21,9 +21,6 @@ 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

@@ -109,8 +109,6 @@ 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

@@ -747,8 +747,6 @@ 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.
+  // 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.
   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::kReadWriteExecute);
+  void* result = v8::internal::AllocatePages(nullptr, alloc_size, page_size,
+                                             v8::PageAllocator::kReadWrite);
   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::kReadWriteExecute, true, true);
+  TestPermissions(PageAllocator::Permission::kReadExecute, true, false);
 }
 #endif  // V8_OS_POSIX