Mac handler work-in-progress. Not for check-in.

client/crashpad_client_mac.cc

Index: client/crashpad_client_mac.cc
diff --git a/client/crashpad_client_mac.cc b/client/crashpad_client_mac.cc
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+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "client/crashpad_client.h"
+
+#include <mach/mach.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "base/logging.h"
+#include "base/posix/eintr_wrapper.h"
+#include "base/strings/stringprintf.h"
+#include "client/crashpad_client.h"
+#include "util/mach/child_port_handshake.h"
+#include "util/mach/exception_ports.h"
+#include "util/mach/mach_extensions.h"
+#include "util/posix/close_multiple.h"
+
+namespace crashpad {
+
+CrashpadClient::CrashpadClient()
+    : exception_port_() {
+}
+
+CrashpadClient::~CrashpadClient() {
+}
+
+bool CrashpadClient::StartHandler(
+    const base::FilePath& handler,
+    const std::vector<std::string>& handler_arguments) {
+  DCHECK_EQ(exception_port_, kMachPortNull);
+
+  // Set up the arguments for execve() first. These aren’t needed until execve()
+  // is called, but it’s dangerous to do this in a child process after fork().
+  ChildPortHandshake child_port_handshake;
+  int handshake_fd = child_port_handshake.ReadPipeFD();
+  std::string handshake_fd_arg =
+      base::StringPrintf("--handshake-fd=%d", handshake_fd);
+
+  const std::string& handler_s = handler.value();
+  const char* const handler_c = handler_s.c_str();
+
+  // Use handler as argv[0], followed by handler_arguments, handshake_fd_arg,
+  // and a nullptr terminator.
+  std::vector<const char*> argv(1, handler_c);
+  argv.reserve(1 + handler_arguments.size() + 1 + 1);
+  for (const std::string& handler_argument : handler_arguments) {
+    argv.push_back(handler_argument.c_str());
+  }
+  argv.push_back(handshake_fd_arg.c_str());
+  argv.push_back(nullptr);
+
+  // Double-fork(). The three processes involved are parent, child, and
+  // grandchild. The grandchild will become the handler process. The child exits
+  // immediately after spawning the grandchild, so the grandchild becomes an
+  // orphan and its parent process ID becomes 1. This relieves the parent and
+  // child of the responsibility for reaping the grandchild with waitpid() or
+  // similar. The handler process is expected to outlive the parent process, so
+  // the parent shouldn’t be concerned with reaping it. This approach means that
+  // accidental early termination of the handler process will not result in a
+  // zombie process.
+  pid_t pid = fork();
+  if (pid < 0) {
+    PLOG(ERROR) << "fork";
+    return false;
+  }
+
+  if (pid == 0) {
+    // Child process.
+
+    // Call setsid(), creating a new process group and a new session, both led
+    // by this process. The new process group has no controlling terminal. This
+    // disconnects it from signals generated by the parent process’ terminal.
+    //
+    // setsid() is done in the child instead of the grandchild so that the
+    // grandchild will not be a session leader. If it were a session leader, an
+    // accidental open() of a terminal device without O_NOCTTY would make that
+    // terminal the controlling terminal.
+    //
+    // It’s not desirable for the handler to have a controlling terminale. The
+    // handler monitors clients on its own and manages its own lifetime, exiting
+    // when it loses all clients and when it deems it appropraite to do so. It
+    // may serve clients in different process groups or sessions than its
+    // original client, and receiving signals intended for its original client’s
+    // process group could be harmful in that case.
+    PCHECK(setsid() != -1) << "setsid";
+
+    pid = fork();
+    if (pid < 0) {
+      PLOG(FATAL) << "fork";
+    }
+
+    if (pid > 0) {
+      // Child process.
+
+      // _exit() instead of exit(), because fork() was called.
+      _exit(EXIT_SUCCESS);
+    }
+
+    // Grandchild process.
+
+    CloseMultipleNowOrOnExec(STDERR_FILENO + 1, handshake_fd);
+
+    // &argv[0] is a pointer to a pointer to const char data, but because of how
+    // C (not C++) works, execvp() wants a pointer to a const pointer to char
+    // data. It modifies neither the data nor the pointers, so the const_cast is
+    // safe.
+    execvp(handler_c, const_cast<char* const*>(&argv[0]));
+    PLOG(FATAL) << "execvp " << handler_s;
+  }
+
+  // Parent process.
+
+  // waitpid() for the child, so that it does not become a zombie process. The
+  // child normally exits quickly.
+  int status;
+  pid_t wait_pid = HANDLE_EINTR(waitpid(pid, &status, 0));
+  PCHECK(wait_pid != -1) << "waitpid";
+  DCHECK_EQ(wait_pid, pid);
+
+  if (WIFSIGNALED(status)) {
+    LOG(WARNING) << "intermediate process: signal " << WTERMSIG(status);
+  } else if (!WIFEXITED(status)) {
+    DLOG(WARNING) << "intermediate process: unknown termination " << status;
+  } else if (WEXITSTATUS(status) != EXIT_SUCCESS) {
+    LOG(WARNING) << "intermediate process: exit status " << WEXITSTATUS(status);
+  }
+
+  // Rendezvous with the handler running in the grandchild process.
+  exception_port_.reset(child_port_handshake.RunServer());
+
+  return exception_port_ ? true : false;
+}
+
+bool CrashpadClient::UseHandler() {
+  DCHECK_NE(exception_port_, kMachPortNull);
+
+  // Set the exception handler for EXC_CRASH, EXC_RESOURCE, and EXC_GUARD.
+  //
+  // EXC_CRASH is how most crashes are received. Most other exception types such
+  // as EXC_BAD_ACCESS are delivered to a host-level exception handler in the
+  // kernel where they are converted to POSIX signals. See 10.9.5
+  // xnu-2422.115.4/bsd/uxkern/ux_exception.c catch_mach_exception_raise(). If a
+  // core-generating signal (triggered through this hardware mechanism or a
+  // software mechanism such as abort() sending SIGABRT) is unhandled and the
+  // process exits, the exception becomes EXC_CRASH. See 10.9.5
+  // xnu-2422.115.4/bsd/kern/kern_exit.c proc_prepareexit().
+  //
+  // EXC_RESOURCE and EXC_GUARD do not become signals or EXC_CRASH exceptions.
+  // The host-level exception handler in the kernel does not receive these
+  // exception types, and even if it did, it would not map them to signals.
+  // Instead, the first Mach service loaded by the root (process ID 1) launchd
+  // with a boolean “ExceptionServer” property in its job dictionary (regardless
+  // of its value) or with any subdictionary property will become the host-level
+  // exception handler for EXC_CRASH, EXC_RESOURCE, and EXC_GUARD. See 10.9.5
+  // launchd-842.92.1/src/core.c job_setup_exception_port(). Normally, this job
+  // is com.apple.ReportCrash.Root, the systemwide Apple Crash Reporter. Since
+  // it is impossible to receive EXC_RESOURCE and EXC_GUARD exceptions through
+  // the EXC_CRASH mechanism, an exception handler must be registered for them
+  // by name if it is to receive these exception types.
+  //
+  // EXC_MASK_RESOURCE and EXC_MASK_GUARD are not available on all systems, and
+  // the kernel will reject attempts to use them if it does not understand them,
+  // so AND them with ExcMaskAll(). EXC_MASK_CRASH is not present in
+  // ExcMaskAll() but is always supported. See the documentation for
+  // ExcMaskAll().
+  ExceptionPorts exception_ports(ExceptionPorts::kTargetTypeTask, TASK_NULL);
+  if (!exception_ports.SetExceptionPort(
+          EXC_MASK_CRASH |
+              ((EXC_MASK_RESOURCE | EXC_MASK_GUARD) & ExcMaskAll()),
+          exception_port_,
+          EXCEPTION_STATE_IDENTITY | MACH_EXCEPTION_CODES,
+          MACHINE_THREAD_STATE)) {
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace crashpad