[fuchsia] VmexResource improvement tweaks

- Adds a GN flag to enable the feature
- Adds facets to manifest used by d8/unittests
- Adds some DCHECKS
- Uses zx_handle_t type to avoid global initialization/destructor

Bug: v8:11232
Change-Id: Ibd7766abefbf8c213393cf6365c34f9ff4e6ed7d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3420828Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78886}

BUILD.gn

@@ -357,6 +357,13 @@ declare_args() {
   # Change code emission and runtime features to be CET shadow-stack compliant
   # (incomplete and experimental).
   v8_enable_cet_shadow_stack = false
+
+  # Get VMEX priviledge at startup.
+  # It allows to run V8 without "deprecated-ambient-replace-as-executable".
+  # Sets -DV8_USE_VMEX_RESOURCE.
+  # TODO(victorgomes): Remove this flag once Chormium no longer needs
+  # the deprecated feature.
+  v8_fuchsia_use_vmex_resource = is_fuchsia && !build_with_chromium
 }
 
 # Derived defaults.
@@ -550,6 +557,10 @@ if (v8_enable_single_generation == true) {
 assert(!v8_enable_conservative_stack_scanning || v8_enable_single_generation,
        "Conservative stack scanning requires single generation")
 
+if (v8_fuchsia_use_vmex_resource) {
+  assert(target_os == "fuchsia", "VMEX resource only available on Fuchsia")
+}
+
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
 
@@ -976,6 +987,9 @@ config("features") {
   if (v8_advanced_bigint_algorithms) {
     defines += [ "V8_ADVANCED_BIGINT_ALGORITHMS" ]
   }
+  if (v8_fuchsia_use_vmex_resource) {
+    defines += [ "V8_USE_VMEX_RESOURCE" ]
+  }
 }
 
 config("toolchain") {

gni/v8.cmx

 {
+  "facets": {
+    "fuchsia.test": {
+      "system-services": [
+        "fuchsia.kernel.VmexResource"
+      ]
+    }
+  },
   "sandbox": {
     "dev": [
       "null",

src/base/platform/platform-fuchsia.cc

@@ -20,21 +20,25 @@ namespace base {
 
 namespace {
 
-static zx::resource g_vmex_resource;
+static zx_handle_t g_vmex_resource = ZX_HANDLE_INVALID;
 
 static void* g_root_vmar_base = nullptr;
 
-void SetGlobalVmexResource() {
-  fuchsia::kernel::VmexResourceSyncPtr vmex_resource;
-  auto path = std::string("/svc/") + fuchsia::kernel::VmexResource::Name_;
+#ifdef V8_USE_VMEX_RESOURCE
+void SetVmexResource() {
+  DCHECK_EQ(g_vmex_resource, ZX_HANDLE_INVALID);
+  zx::resource vmex_resource;
+  fuchsia::kernel::VmexResourceSyncPtr vmex_resource_svc;
   zx_status_t status = fdio_service_connect(
-      path.data(), vmex_resource.NewRequest().TakeChannel().release());
-  if (status != ZX_OK) {
-    g_vmex_resource = zx::resource();
-  } else {
-    vmex_resource->Get(&g_vmex_resource);
-  }
+      "/svc/fuchsia.kernel.VmexResource",
+      vmex_resource_svc.NewRequest().TakeChannel().release());
+  DCHECK_EQ(status, ZX_OK);
+  status = vmex_resource_svc->Get(&vmex_resource);
+  DCHECK_EQ(status, ZX_OK);
+  DCHECK(vmex_resource.is_valid());
+  g_vmex_resource = vmex_resource.release();
 }
+#endif
 
 zx_vm_option_t GetProtectionFromMemoryPermission(OS::MemoryPermission access) {
   switch (access) {
@@ -103,7 +107,8 @@ void* AllocateInternal(const zx::vmar& vmar, void* vmar_base, size_t page_size,
   // to be marked as executable in the future.
   // TOOD(https://crbug.com/v8/8899): Only call this when we know that the
   // region will need to be marked as executable in the future.
-  if (vmo.replace_as_executable(g_vmex_resource, &vmo) != ZX_OK) {
+  zx::unowned_resource vmex(g_vmex_resource);
+  if (vmo.replace_as_executable(*vmex, &vmo) != ZX_OK) {
     return nullptr;
   }
 
@@ -225,7 +230,9 @@ void OS::Initialize(bool hard_abort, const char* const gc_fake_mmap) {
   CHECK_EQ(ZX_OK, status);
   g_root_vmar_base = reinterpret_cast<void*>(info.base);
 
-  SetGlobalVmexResource();
+#ifdef V8_USE_VMEX_RESOURCE
+  SetVmexResource();
+#endif
 }
 
 // static