[wasm] Adjust the traphandler implementation for MacOS

In the chrome-side implementation I currently use the default
trap handlers of V8, see https://crrev.com/c/1290955

Bug: chromium:906565

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I74c5a18c479ad1c69303d104ad4f040de436c4e7
Reviewed-on: https://chromium-review.googlesource.com/c/1282960
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57636}

BUILD.gn

@@ -1534,7 +1534,7 @@ v8_header_set("v8_headers") {
     "include/v8config.h",
   ]
 
-  if (is_linux) {
+  if (is_linux || is_mac) {
     sources += [ "include/v8-wasm-trap-handler-posix.h" ]
   }
 
@@ -2759,7 +2759,7 @@ v8_source_set("v8_base") {
       "src/x64/simulator-x64.h",
       "src/x64/sse-instr.h",
     ]
-    if (is_linux) {
+    if (is_linux || is_mac) {
       sources += [
         "src/trap-handler/handler-inside-posix.cc",
         "src/trap-handler/handler-inside-posix.h",

src/api.cc

@@ -102,7 +102,7 @@
 #include "src/wasm/wasm-result.h"
 #include "src/wasm/wasm-serialization.h"
 
-#ifdef V8_OS_POSIX
+#if V8_OS_LINUX || V8_OS_MACOSX
 #include <signal.h>
 #include "src/trap-handler/handler-inside-posix.h"
 #endif
@@ -5873,12 +5873,12 @@ bool v8::V8::Initialize() {
   return true;
 }
 
-#if V8_OS_POSIX
+#if V8_OS_LINUX || V8_OS_MACOSX
 bool TryHandleWebAssemblyTrapPosix(int sig_code, siginfo_t* info,
                                    void* context) {
-#if V8_OS_LINUX && V8_TARGET_ARCH_X64 && !V8_OS_ANDROID
+#if V8_TARGET_ARCH_X64 && !V8_OS_ANDROID
   return i::trap_handler::TryHandleSignal(sig_code, info, context);
-#else  // V8_OS_LINUX && V8_TARGET_ARCH_X64 && !V8_OS_ANDROID
+#else
   return false;
 #endif
 }

src/trap-handler/DEPS

@@ -14,6 +14,10 @@ specific_include_rules = {
     "+src/globals.h",
     "+src/flags.h",
   ],
+  "handler-inside-posix.h": [
+    # To access V8_OS_LINUX. This file is already included in build_config.h.
+    "+include/v8config.h",
+  ],
   "handler-inside-win.h": [
     "+src/base/macros.h",
   ]

src/trap-handler/handler-inside-posix.cc

@@ -26,6 +26,13 @@
 #include "src/trap-handler/handler-inside-posix.h"
 
 #include <signal.h>
+
+#ifdef V8_OS_LINUX
+#include <ucontext.h>
+#elif V8_OS_MACOSX
+#include <sys/ucontext.h>
+#endif
+
 #include <stddef.h>
 #include <stdlib.h>
 
@@ -37,6 +44,9 @@ namespace internal {
 namespace trap_handler {
 
 bool IsKernelGeneratedSignal(siginfo_t* info) {
+  // On macOS, only `info->si_code > 0` is relevant, because macOS leaves
+  // si_code at its default of 0 for signals that don’t originate in hardware.
+  // The other conditions are only relevant for Linux.
   return info->si_code > 0 && info->si_code != SI_USER &&
          info->si_code != SI_QUEUE && info->si_code != SI_TIMER &&
          info->si_code != SI_ASYNCIO && info->si_code != SI_MESGQ;
@@ -63,7 +73,8 @@ class SigUnmaskStack {
 
 bool TryHandleSignal(int signum, siginfo_t* info, void* context) {
   // Bail out early in case we got called for the wrong kind of signal.
-  if (signum != SIGSEGV) {
+
+  if (signum != kOobSignal) {
     return false;
   }
 
@@ -94,11 +105,18 @@ bool TryHandleSignal(int signum, siginfo_t* info, void* context) {
     SigUnmaskStack unmask(sigs);
 
     ucontext_t* uc = reinterpret_cast<ucontext_t*>(context);
-    uintptr_t fault_addr = uc->uc_mcontext.gregs[REG_RIP];
+#if V8_OS_LINUX
+    auto* context_rip = &uc->uc_mcontext.gregs[REG_RIP];
+#elif V8_OS_MACOSX
+    auto* context_rip = &uc->uc_mcontext->__ss.__rip;
+#else
+#error Unsupported platform
+#endif
+    uintptr_t fault_addr = *context_rip;
     uintptr_t landing_pad = 0;
     if (TryFindLandingPad(fault_addr, &landing_pad)) {
       // Tell the caller to return to the landing pad.
-      uc->uc_mcontext.gregs[REG_RIP] = landing_pad;
+      *context_rip = landing_pad;
       // We will return to wasm code, so restore the g_thread_in_wasm_code flag.
       g_thread_in_wasm_code = true;
       return true;

src/trap-handler/handler-inside-posix.h

@@ -6,11 +6,20 @@
 #define V8_TRAP_HANDLER_HANDLER_INSIDE_POSIX_H_
 
 #include <signal.h>
+#include "include/v8config.h"
 
 namespace v8 {
 namespace internal {
 namespace trap_handler {
 
+#if V8_OS_LINUX
+constexpr int kOobSignal = SIGSEGV;
+#elif V8_OS_MACOSX
+constexpr int kOobSignal = SIGBUS;
+#else
+#error Posix trap handlers are only supported on Linux and MacOSX.
+#endif
+
 void HandleSignal(int signum, siginfo_t* info, void* context);
 
 bool TryHandleSignal(int signum, siginfo_t* info, void* context);

src/trap-handler/handler-outside-posix.cc

@@ -48,7 +48,7 @@ bool RegisterDefaultTrapHandler() {
   // {sigaction} installs a new custom segfault handler. On success, it returns
   // 0. If we get a nonzero value, we report an error to the caller by returning
   // false.
-  if (sigaction(SIGSEGV, &action, &g_old_handler) != 0) {
+  if (sigaction(kOobSignal, &action, &g_old_handler) != 0) {
     return false;
   }
 
@@ -61,7 +61,7 @@ bool RegisterDefaultTrapHandler() {
     defined(THREAD_SANITIZER) || defined(LEAK_SANITIZER) ||    \
     defined(UNDEFINED_SANITIZER)
   struct sigaction installed_handler;
-  CHECK_EQ(sigaction(SIGSEGV, NULL, &installed_handler), 0);
+  CHECK_EQ(sigaction(kOobSignal, NULL, &installed_handler), 0);
   // If the installed handler does not point to HandleSignal, then
   // allow_user_segv_handler is 0.
   if (installed_handler.sa_sigaction != HandleSignal) {
@@ -78,7 +78,7 @@ bool RegisterDefaultTrapHandler() {
 
 void RemoveTrapHandler() {
   if (g_is_default_signal_handler_registered) {
-    if (sigaction(SIGSEGV, &g_old_handler, nullptr) == 0) {
+    if (sigaction(kOobSignal, &g_old_handler, nullptr) == 0) {
       g_is_default_signal_handler_registered = false;
     }
   }

src/trap-handler/trap-handler.h

@@ -21,6 +21,8 @@ namespace trap_handler {
 #define V8_TRAP_HANDLER_SUPPORTED true
 #elif V8_TARGET_ARCH_X64 && V8_OS_WIN
 #define V8_TRAP_HANDLER_SUPPORTED true
+#elif V8_TARGET_ARCH_X64 && V8_OS_MACOSX
+#define V8_TRAP_HANDLER_SUPPORTED true
 #else
 #define V8_TRAP_HANDLER_SUPPORTED false
 #endif