Ensure debug-mode apps are always attached on iOS. (#10186)

ci/licenses_golden/licenses_flutter

@@ -456,6 +456,8 @@ FILE: ../../../flutter/runtime/dart_vm_unittests.cc
 FILE: ../../../flutter/runtime/embedder_resources.cc
 FILE: ../../../flutter/runtime/embedder_resources.h
 FILE: ../../../flutter/runtime/fixtures/runtime_test.dart
+FILE: ../../../flutter/runtime/ptrace_ios.cc
+FILE: ../../../flutter/runtime/ptrace_ios.h
 FILE: ../../../flutter/runtime/runtime_controller.cc
 FILE: ../../../flutter/runtime/runtime_controller.h
 FILE: ../../../flutter/runtime/runtime_delegate.cc

common/settings.h

@@ -88,6 +88,7 @@ struct Settings {
   std::vector<std::string> dart_entrypoint_args;
 
   // Isolate settings
+  bool enable_checked_mode = false;
   bool start_paused = false;
   bool trace_skia = false;
   bool trace_startup = false;

runtime/BUILD.gn

@@ -58,6 +58,8 @@ source_set("runtime") {
     "dart_vm_lifecycle.h",
     "embedder_resources.cc",
     "embedder_resources.h",
+    "ptrace_ios.cc",
+    "ptrace_ios.h",
     "runtime_controller.cc",
     "runtime_controller.h",
     "runtime_delegate.cc",

runtime/dart_vm.cc

@@ -24,6 +24,7 @@
 #include "flutter/lib/ui/dart_ui.h"
 #include "flutter/runtime/dart_isolate.h"
 #include "flutter/runtime/dart_service_isolate.h"
+#include "flutter/runtime/ptrace_ios.h"
 #include "flutter/runtime/start_up.h"
 #include "third_party/dart/runtime/include/bin/dart_io_api.h"
 #include "third_party/tonic/converter/dart_converter.h"
@@ -307,11 +308,15 @@ DartVM::DartVM(std::shared_ptr<const DartVMData> vm_data,
   }
 #endif  // !OS_FUCHSIA
 
-#if FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG
+#if (FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG)
+#if !OS_IOS || TARGET_OS_SIMULATOR
   // Debug mode uses the JIT, disable code page write protection to avoid
   // memory page protection changes before and after every compilation.
   PushBackAll(&args, kDartWriteProtectCodeArgs,
               fml::size(kDartWriteProtectCodeArgs));
+#else
+  EnsureDebuggedIOS(settings_);
+#endif
 #endif
 
   if (enable_asserts) {

runtime/ptrace_ios.cc

@@ -0,0 +1,103 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// It is __imperative__ that the functions in this file are __not__ included in
+// release or profile builds.
+//
+// They call into the "private" ptrace() API to ensure that the current process
+// is being ptrace()-d. Only debug builds rely on ptrace(), and the ptrace() API
+// is not allowed for use in the App Store, so we must exclude it from profile-
+// and release-builds.
+//
+// When an app is launched from a host workstation (e.g. via Xcode or
+// "ios-deploy"), the process is already ptrace()-d by debugserver. However,
+// when an app is launched from the home screen, it is not, so for debug builds
+// we initialize the ptrace() relationship via PT_TRACE_ME if necessary.
+//
+// Please see the following documents for more details:
+//   - go/decommissioning-dbc
+//   - go/decommissioning-dbc-engine
+//   - go/decommissioning-dbc-tools
+#include "flutter/common/settings.h"
+#include "flutter/fml/build_config.h"  // For OS_IOS.
+
+#if OS_IOS && (FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG)
+
+// These headers should only be needed in debug mode.
+#include <sys/sysctl.h>
+#include <sys/types.h>
+
+#define PT_TRACE_ME 0
+#define PT_SIGEXC 12
+extern "C" int ptrace(int request, pid_t pid, caddr_t addr, int data);
+
+static bool DebuggedIOS(const flutter::Settings& vm_settings) {
+  // Only the Flutter CLI passes "--enable-checked-mode". Therefore, if the flag
+  // is present, we have been launched by "ios-deploy" via "debugserver".
+  //
+  // We choose this flag because it is always passed to launch debug builds.
+  if (vm_settings.enable_checked_mode) {
+    return true;
+  }
+
+  // Use "sysctl()" to check if we're currently being debugged (e.g. by Xcode).
+  // We could also check "getppid() != 1" (launchd), but this is more direct.
+  const pid_t self = getpid();
+  int mib[5] = {CTL_KERN, KERN_PROC, KERN_PROC_PID, self, 0};
+
+  auto proc = std::make_unique<struct kinfo_proc>();
+  size_t proc_size = sizeof(struct kinfo_proc);
+  if (sysctl(mib, 4, proc.get(), &proc_size, nullptr, 0) < 0) {
+    FML_LOG(ERROR) << "Could not execute sysctl() to get current process info: "
+                   << strerror(errno);
+    return false;
+  }
+
+  return proc->kp_proc.p_flag & P_TRACED;
+}
+
+void EnsureDebuggedIOS(const flutter::Settings& vm_settings) {
+  if (DebuggedIOS(vm_settings)) {
+    return;
+  }
+
+  if (ptrace(PT_TRACE_ME, 0, nullptr, 0) == -1) {
+    FML_LOG(ERROR) << "Could not call ptrace(PT_TRACE_ME): " << strerror(errno);
+    // No use trying PT_SIGEXC -- it's only needed if PT_TRACE_ME succeeds.
+    return;
+  }
+  if (ptrace(PT_SIGEXC, 0, nullptr, 0) == -1) {
+    FML_LOG(ERROR) << "Could not call ptrace(PT_SIGEXC): " << strerror(errno);
+  }
+
+  // The previous operation causes this process to not be reaped after it
+  // terminates (even if PT_SIGEXC fails). Issue a warning to the console every
+  // (approximiately) maxproc/10 leaks. See the links above for an explanation
+  // of this issue.
+  size_t maxproc = 0;
+  size_t maxproc_size = sizeof(size_t);
+  const int sysctl_result =
+      sysctlbyname("kern.maxproc", &maxproc, &maxproc_size, nullptr, 0);
+  if (sysctl_result < 0) {
+    FML_LOG(ERROR)
+        << "Could not execute sysctl() to determine process count limit: "
+        << strerror(errno);
+  }
+
+  const char* warning =
+      "Launching a debug-mode app from the home screen may cause problems.\n"
+      "Please compile a profile-/release-build, launch your app via \"flutter "
+      "run\", or see https://github.com/flutter/flutter/wiki/"
+      "PID-leak-in-iOS-debug-builds-launched-from-home-screen for details.";
+
+  if (vm_settings.verbose_logging  // used for testing and also informative
+      || sysctl_result < 0         // could not determine maximum process count
+      || maxproc / 10 == 0         // avoid division (%) by 0
+      || getpid() % (maxproc / 10) == 0)  // warning every ~maxproc/10 leaks
+  {
+    FML_LOG(ERROR) << warning;
+  }
+}
+
+#endif  // OS_IOS && (FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG)

runtime/ptrace_ios.h

@@ -0,0 +1,18 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_RUNTIME_PTRACE_IOS_H_
+#define FLUTTER_RUNTIME_PTRACE_IOS_H_
+
+#include "flutter/common/settings.h"
+
+#if OS_IOS && (FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG)
+
+// Ensure that the current process is or was ptrace()-d at some point in its
+// life. Can only be used within debug builds for iOS.
+void EnsureDebuggedIOS(const flutter::Settings& vm_settings);
+
+#endif
+
+#endif  // FLUTTER_RUNTIME_PTRACE_IOS_H_

shell/common/switches.cc

@@ -223,6 +223,9 @@ Settings SettingsFromCommandLine(const fml::CommandLine& command_line) {
   settings.start_paused =
       command_line.HasOption(FlagForSwitch(Switch::StartPaused));
 
+  settings.enable_checked_mode =
+      command_line.HasOption(FlagForSwitch(Switch::EnableCheckedMode));
+
   settings.enable_dart_profiling =
       command_line.HasOption(FlagForSwitch(Switch::EnableDartProfiling));
 

shell/common/switches.h

@@ -115,6 +115,7 @@ DEF_SWITCH(DisableServiceAuthCodes,
 DEF_SWITCH(StartPaused,
            "start-paused",
            "Start the application paused in the Dart debugger.")
+DEF_SWITCH(EnableCheckedMode, "enable-checked-mode", "Enable checked mode.")
 DEF_SWITCH(TraceStartup,
            "trace-startup",
            "Trace early application lifecycle. Automatically switches to an "