[wasm] Add new API for enabling trap handler

The new API supersedes the old `RegisterDefaultSignalHandler` and flag
combination. Now the embedder must explicitly call
`EnableWebAssemblyTrapHandler` to activate the trap handler and optionally
install the default signal handler. The old flag is now used only by D8 to
decide whether to call this function.

Bug: v8:5277

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I05fbb2138138bfc95b14361aabd712db84789b4a
Reviewed-on: https://chromium-review.googlesource.com/963179
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52081}

include/v8.h

@@ -8037,7 +8037,16 @@ class V8_EXPORT V8 {
    * Enable the default signal handler rather than using one provided by the
    * embedder.
    */
-  static bool RegisterDefaultSignalHandler();
+  V8_DEPRECATE_SOON("Use EnableWebAssemblyTrapHandler",
+                    static bool RegisterDefaultSignalHandler());
+
+  /**
+   * Activate trap-based bounds checking for WebAssembly.
+   *
+   * \param use_v8_signal_handler Whether V8 should install its own signal
+   * handler or rely on the embedder's.
+   */
+  static bool EnableWebAssemblyTrapHandler(bool use_v8_signal_handler);
 
  private:
   V8();

src/api.cc

@@ -6105,6 +6105,10 @@ bool V8::RegisterDefaultSignalHandler() {
   return v8::internal::trap_handler::RegisterDefaultSignalHandler();
 }
 
+bool V8::EnableWebAssemblyTrapHandler(bool use_v8_signal_handler) {
+  return v8::internal::trap_handler::EnableTrapHandler(use_v8_signal_handler);
+}
+
 void v8::V8::SetEntropySource(EntropySource entropy_source) {
   base::RandomNumberGenerator::SetEntropySource(entropy_source);
 }

src/d8.cc

@@ -3354,8 +3354,9 @@ int Shell::Main(int argc, char* argv[]) {
     create_params.add_histogram_sample_callback = AddHistogramSample;
   }
 
-  if (i::trap_handler::IsTrapHandlerEnabled()) {
-    if (!v8::V8::RegisterDefaultSignalHandler()) {
+  if (V8_TRAP_HANDLER_SUPPORTED && i::FLAG_wasm_trap_handler) {
+    constexpr bool use_default_signal_handler = true;
+    if (!v8::V8::EnableWebAssemblyTrapHandler(use_default_signal_handler)) {
       fprintf(stderr, "Could not register signal handler");
       exit(1);
     }

src/trap-handler/handler-outside.cc

@@ -45,8 +45,8 @@ namespace v8 {
 namespace internal {
 namespace trap_handler {
 
-const size_t kInitialCodeObjectSize = 1024;
-const size_t kCodeObjectGrowthFactor = 2;
+constexpr size_t kInitialCodeObjectSize = 1024;
+constexpr size_t kCodeObjectGrowthFactor = 2;
 
 constexpr size_t HandlerDataSize(size_t num_protected_instructions) {
   return offsetof(CodeProtectionInfo, instructions) +
@@ -275,6 +275,20 @@ size_t GetRecoveredTrapCount() {
   return gRecoveredTrapCount.load(std::memory_order_relaxed);
 }
 
+bool g_is_trap_handler_enabled;
+
+bool EnableTrapHandler(bool use_v8_signal_handler) {
+  if (!V8_TRAP_HANDLER_SUPPORTED) {
+    return false;
+  }
+  if (use_v8_signal_handler) {
+    g_is_trap_handler_enabled = RegisterDefaultSignalHandler();
+    return g_is_trap_handler_enabled;
+  }
+  g_is_trap_handler_enabled = true;
+  return true;
+}
+
 }  // namespace trap_handler
 }  // namespace internal
 }  // namespace v8

src/trap-handler/trap-handler.h

@@ -66,8 +66,17 @@ void ReleaseHandlerData(int index);
 #define THREAD_LOCAL __thread
 #endif
 
+extern bool g_is_trap_handler_enabled;
+// Enables trap handling for WebAssembly bounds checks.
+//
+// use_v8_signal_handler indicates that V8 should install its own signal handler
+// rather than relying on the embedder to do it.
+bool EnableTrapHandler(bool use_v8_signal_handler);
+
 inline bool IsTrapHandlerEnabled() {
-  return FLAG_wasm_trap_handler && V8_TRAP_HANDLER_SUPPORTED;
+  DCHECK_IMPLIES(g_is_trap_handler_enabled, V8_TRAP_HANDLER_SUPPORTED);
+  return (V8_TRAP_HANDLER_SUPPORTED && FLAG_wasm_trap_handler) ||
+         g_is_trap_handler_enabled;
 }
 
 extern THREAD_LOCAL int g_thread_in_wasm_code;

test/cctest/cctest.cc

@@ -269,8 +269,9 @@ int main(int argc, char* argv[]) {
   v8::V8::Initialize();
   v8::V8::InitializeExternalStartupData(argv[0]);
 
-  if (i::trap_handler::IsTrapHandlerEnabled()) {
-    v8::V8::RegisterDefaultSignalHandler();
+  if (V8_TRAP_HANDLER_SUPPORTED && i::FLAG_wasm_trap_handler) {
+    constexpr bool use_default_signal_handler = true;
+    CHECK(v8::V8::EnableWebAssemblyTrapHandler(use_default_signal_handler));
   }
 
   CcTest::set_array_buffer_allocator(

test/unittests/wasm/trap-handler-unittest.cc

@@ -13,7 +13,7 @@
 
 namespace {
 
-#if V8_OS_POSIX
+#if V8_TRAP_HANDLER_SUPPORTED
 
 void CrashOnPurpose() { *reinterpret_cast<volatile int*>(42); }
 
@@ -52,7 +52,8 @@ sigjmp_buf SignalHandlerFallbackTest::continuation_;
 TEST_F(SignalHandlerFallbackTest, DoTest) {
   const int save_sigs = 1;
   if (!sigsetjmp(continuation_, save_sigs)) {
-    v8::V8::RegisterDefaultSignalHandler();
+    constexpr bool use_default_signal_handler = true;
+    CHECK(v8::V8::EnableWebAssemblyTrapHandler(use_default_signal_handler));
     CrashOnPurpose();
     FAIL();
   } else {