content/zygote/zygote_linux.cc - chromium/src - Git at Google

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 #include "content/zygote/zygote_linux.h"

 #include <fcntl.h>
 #include <string.h>
 #include <sys/socket.h>
 #include <sys/types.h>
 #include <sys/wait.h>

 #include "base/command_line.h"
 #include "base/files/file_util.h"
 #include "base/linux_util.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/scoped_vector.h"
 #include "base/pickle.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/posix/global_descriptors.h"
 #include "base/posix/unix_domain_socket_linux.h"
 #include "base/process/kill.h"
 #include "base/process/launch.h"
 #include "base/process/process.h"
 #include "base/process/process_handle.h"
 #include "base/trace_event/trace_event.h"
 #include "content/common/child_process_sandbox_support_impl_linux.h"
 #include "content/common/sandbox_linux/sandbox_linux.h"
 #include "content/common/set_process_title.h"
 #include "content/common/zygote_commands_linux.h"
 #include "content/public/common/content_descriptors.h"
 #include "content/public/common/result_codes.h"
 #include "content/public/common/sandbox_linux.h"
 #include "content/public/common/send_zygote_child_ping_linux.h"
 #include "content/public/common/zygote_fork_delegate_linux.h"
 #include "ipc/ipc_channel.h"
 #include "ipc/ipc_switches.h"

 // See http://code.google.com/p/chromium/wiki/LinuxZygote

 namespace content {

 namespace {

 // NOP function. See below where this handler is installed.
 void SIGCHLDHandler(int signal) {
 }

 int LookUpFd(const base::GlobalDescriptors::Mapping& fd_mapping, uint32_t key) {
   for (size_t index = 0; index < fd_mapping.size(); ++index) {
     if (fd_mapping[index].key == key)
       return fd_mapping[index].fd;
   }
   return -1;
 }

 void CreatePipe(base::ScopedFD* read_pipe, base::ScopedFD* write_pipe) {
   int raw_pipe[2];
   PCHECK(0 == pipe(raw_pipe));
   read_pipe->reset(raw_pipe[0]);
   write_pipe->reset(raw_pipe[1]);
 }

 void KillAndReap(pid_t pid, ZygoteForkDelegate* helper) {
   if (helper) {
     // Helper children may be forked in another PID namespace, so |pid| might
     // be meaningless to us; or we just might not be able to directly send it
     // signals.  So we can't kill it.
     // Additionally, we're not its parent, so we can't reap it anyway.
     // TODO(mdempsky): Extend the ZygoteForkDelegate API to handle this.
     LOG(WARNING) << "Unable to kill or reap helper children";
     return;
   }

   // Kill the child process in case it's not already dead, so we can safely
   // perform a blocking wait.
   PCHECK(0 == kill(pid, SIGKILL));
   PCHECK(pid == HANDLE_EINTR(waitpid(pid, NULL, 0)));
 }

 }  // namespace

 Zygote::Zygote(int sandbox_flags, ScopedVector<ZygoteForkDelegate> helpers,
                const std::vector<base::ProcessHandle>& extra_children,
                const std::vector<int>& extra_fds)
     : sandbox_flags_(sandbox_flags),
       helpers_(helpers.Pass()),
       initial_uma_index_(0),
       extra_children_(extra_children),
       extra_fds_(extra_fds) {}

 Zygote::~Zygote() {
 }

 bool Zygote::ProcessRequests() {
   // A SOCK_SEQPACKET socket is installed in fd 3. We get commands from the
   // browser on it.
   // A SOCK_DGRAM is installed in fd 5. This is the sandbox IPC channel.
   // See http://code.google.com/p/chromium/wiki/LinuxSandboxIPC

   // We need to accept SIGCHLD, even though our handler is a no-op because
   // otherwise we cannot wait on children. (According to POSIX 2001.)
   struct sigaction action;
   memset(&action, 0, sizeof(action));
   action.sa_handler = &SIGCHLDHandler;
   CHECK(sigaction(SIGCHLD, &action, NULL) == 0);

   if (UsingSUIDSandbox() || UsingNSSandbox()) {
     // Let the ZygoteHost know we are ready to go.
     // The receiving code is in content/browser/zygote_host_linux.cc.
     bool r = UnixDomainSocket::SendMsg(kZygoteSocketPairFd,
                                        kZygoteHelloMessage,
                                        sizeof(kZygoteHelloMessage),
                                        std::vector<int>());
 #if defined(OS_CHROMEOS)
     LOG_IF(WARNING, !r) << "Sending zygote magic failed";
     // Exit normally on chromeos because session manager may send SIGTERM
     // right after the process starts and it may fail to send zygote magic
     // number to browser process.
     if (!r)
       _exit(RESULT_CODE_NORMAL_EXIT);
 #else
     CHECK(r) << "Sending zygote magic failed";
 #endif
   }

   for (;;) {
     // This function call can return multiple times, once per fork().
     if (HandleRequestFromBrowser(kZygoteSocketPairFd))
       return true;
   }
 }

 bool Zygote::GetProcessInfo(base::ProcessHandle pid,
                             ZygoteProcessInfo* process_info) {
   DCHECK(process_info);
   const ZygoteProcessMap::const_iterator it = process_info_map_.find(pid);
   if (it == process_info_map_.end()) {
     return false;
   }
   *process_info = it->second;
   return true;
 }

 bool Zygote::UsingSUIDSandbox() const {
   return sandbox_flags_ & kSandboxLinuxSUID;
 }

 bool Zygote::UsingNSSandbox() const {
   return sandbox_flags_ & kSandboxLinuxUserNS;
 }

 bool Zygote::HandleRequestFromBrowser(int fd) {
   ScopedVector<base::ScopedFD> fds;
   char buf[kZygoteMaxMessageLength];
   const ssize_t len = UnixDomainSocket::RecvMsg(fd, buf, sizeof(buf), &fds);

   if (len == 0 || (len == -1 && errno == ECONNRESET)) {
     // EOF from the browser. We should die.
     // TODO(earthdok): call __sanititizer_cov_dump() here to obtain code
     // coverage  for the Zygote. Currently it's not possible because of
     // confusion over who is responsible for closing the file descriptor.
     for (std::vector<int>::iterator it = extra_fds_.begin();
          it < extra_fds_.end(); ++it) {
       PCHECK(0 == IGNORE_EINTR(close(*it)));
     }
 #if !defined(SANITIZER_COVERAGE)
     // TODO(earthdok): add watchdog thread before using this in builds not
     // using sanitizer coverage.
     CHECK(extra_children_.empty());
 #endif
     for (std::vector<base::ProcessHandle>::iterator it =
              extra_children_.begin();
          it < extra_children_.end(); ++it) {
       PCHECK(*it == HANDLE_EINTR(waitpid(*it, NULL, 0)));
     }
     _exit(0);
     return false;
   }

   if (len == -1) {
     PLOG(ERROR) << "Error reading message from browser";
     return false;
   }

   Pickle pickle(buf, len);
   PickleIterator iter(pickle);

   int kind;
   if (iter.ReadInt(&kind)) {
     switch (kind) {
       case kZygoteCommandFork:
         // This function call can return multiple times, once per fork().
         return HandleForkRequest(fd, iter, fds.Pass());

       case kZygoteCommandReap:
         if (!fds.empty())
           break;
         HandleReapRequest(fd, iter);
         return false;
       case kZygoteCommandGetTerminationStatus:
         if (!fds.empty())
           break;
         HandleGetTerminationStatus(fd, iter);
         return false;
       case kZygoteCommandGetSandboxStatus:
         HandleGetSandboxStatus(fd, iter);
         return false;
       case kZygoteCommandForkRealPID:
         // This shouldn't happen in practice, but some failure paths in
         // HandleForkRequest (e.g., if ReadArgsAndFork fails during depickling)
         // could leave this command pending on the socket.
         LOG(ERROR) << "Unexpected real PID message from browser";
         NOTREACHED();
         return false;
       default:
         NOTREACHED();
         break;
     }
   }

   LOG(WARNING) << "Error parsing message from browser";
   return false;
 }

 // TODO(jln): remove callers to this broken API. See crbug.com/274855.
 void Zygote::HandleReapRequest(int fd,
                                PickleIterator iter) {
   base::ProcessId child;

   if (!iter.ReadInt(&child)) {
     LOG(WARNING) << "Error parsing reap request from browser";
     return;
   }

   ZygoteProcessInfo child_info;
   if (!GetProcessInfo(child, &child_info)) {
     LOG(ERROR) << "Child not found!";
     NOTREACHED();
     return;
   }

   if (!child_info.started_from_helper) {
     // Do not call base::EnsureProcessTerminated() under ThreadSanitizer, as it
     // spawns a separate thread which may live until the call to fork() in the
     // zygote. As a result, ThreadSanitizer will report an error and almost
     // disable race detection in the child process.
     // Not calling EnsureProcessTerminated() may result in zombie processes
     // sticking around. This will only happen during testing, so we can live
     // with this for now.
 #if !defined(THREAD_SANITIZER)
     // TODO(jln): this old code is completely broken. See crbug.com/274855.
     base::EnsureProcessTerminated(base::Process(child_info.internal_pid));
 #else
     LOG(WARNING) << "Zygote process omitting a call to "
         << "base::EnsureProcessTerminated() for child pid " << child
         << " under ThreadSanitizer. See http://crbug.com/274855.";
 #endif
   } else {
     // For processes from the helper, send a GetTerminationStatus request
     // with known_dead set to true.
     // This is not perfect, as the process may be killed instantly, but is
     // better than ignoring the request.
     base::TerminationStatus status;
     int exit_code;
     bool got_termination_status =
         GetTerminationStatus(child, true /* known_dead */, &status, &exit_code);
     DCHECK(got_termination_status);
   }
   process_info_map_.erase(child);
 }

 bool Zygote::GetTerminationStatus(base::ProcessHandle real_pid,
                                   bool known_dead,
                                   base::TerminationStatus* status,
                                   int* exit_code) {

   ZygoteProcessInfo child_info;
   if (!GetProcessInfo(real_pid, &child_info)) {
     LOG(ERROR) << "Zygote::GetTerminationStatus for unknown PID "
                << real_pid;
     NOTREACHED();
     return false;
   }
   // We know about |real_pid|.
   const base::ProcessHandle child = child_info.internal_pid;
   if (child_info.started_from_helper) {
     if (!child_info.started_from_helper->GetTerminationStatus(
             child, known_dead, status, exit_code)) {
       return false;
     }
   } else {
     // Handle the request directly.
     if (known_dead) {
       *status = base::GetKnownDeadTerminationStatus(child, exit_code);
     } else {
       // We don't know if the process is dying, so get its status but don't
       // wait.
       *status = base::GetTerminationStatus(child, exit_code);
     }
   }
   // Successfully got a status for |real_pid|.
   if (*status != base::TERMINATION_STATUS_STILL_RUNNING) {
     // Time to forget about this process.
     process_info_map_.erase(real_pid);
   }
   return true;
 }

 void Zygote::HandleGetTerminationStatus(int fd,
                                         PickleIterator iter) {
   bool known_dead;
   base::ProcessHandle child_requested;

   if (!iter.ReadBool(&known_dead) || !iter.ReadInt(&child_requested)) {
     LOG(WARNING) << "Error parsing GetTerminationStatus request "
                  << "from browser";
     return;
   }

   base::TerminationStatus status;
   int exit_code;

   bool got_termination_status =
       GetTerminationStatus(child_requested, known_dead, &status, &exit_code);
   if (!got_termination_status) {
     // Assume that if we can't find the child in the sandbox, then
     // it terminated normally.
     NOTREACHED();
     status = base::TERMINATION_STATUS_NORMAL_TERMINATION;
     exit_code = RESULT_CODE_NORMAL_EXIT;
   }

   Pickle write_pickle;
   write_pickle.WriteInt(static_cast<int>(status));
   write_pickle.WriteInt(exit_code);
   ssize_t written =
       HANDLE_EINTR(write(fd, write_pickle.data(), write_pickle.size()));
   if (written != static_cast<ssize_t>(write_pickle.size()))
     PLOG(ERROR) << "write";
 }

 int Zygote::ForkWithRealPid(const std::string& process_type,
                             const base::GlobalDescriptors::Mapping& fd_mapping,
                             const std::string& channel_id,
                             base::ScopedFD pid_oracle,
                             std::string* uma_name,
                             int* uma_sample,
                             int* uma_boundary_value) {
   ZygoteForkDelegate* helper = NULL;
   for (ScopedVector<ZygoteForkDelegate>::iterator i = helpers_.begin();
        i != helpers_.end();
        ++i) {
     if ((*i)->CanHelp(process_type, uma_name, uma_sample, uma_boundary_value)) {
       helper = *i;
       break;
     }
   }

   base::ScopedFD read_pipe, write_pipe;
   base::ProcessId pid = 0;
   if (helper) {
     int ipc_channel_fd = LookUpFd(fd_mapping, kPrimaryIPCChannel);
     if (ipc_channel_fd < 0) {
       DLOG(ERROR) << "Failed to find kPrimaryIPCChannel in FD mapping";
       return -1;
     }
     std::vector<int> fds;
     fds.push_back(ipc_channel_fd);  // kBrowserFDIndex
     fds.push_back(pid_oracle.get());  // kPIDOracleFDIndex
     pid = helper->Fork(process_type, fds, channel_id);

     // Helpers should never return in the child process.
     CHECK_NE(pid, 0);
   } else {
     CreatePipe(&read_pipe, &write_pipe);
     // This is roughly equivalent to a fork(). We are using ForkWithFlags mainly
     // to give it some more diverse test coverage.
     pid = base::ForkWithFlags(SIGCHLD, nullptr, nullptr);
   }

   if (pid == 0) {
     // In the child process.
     write_pipe.reset();

     // Ping the PID oracle socket so the browser can find our PID.
     CHECK(SendZygoteChildPing(pid_oracle.get()));

     // Now read back our real PID from the zygote.
     base::ProcessId real_pid;
     if (!base::ReadFromFD(read_pipe.get(),
                           reinterpret_cast<char*>(&real_pid),
                           sizeof(real_pid))) {
       LOG(FATAL) << "Failed to synchronise with parent zygote process";
     }
     if (real_pid <= 0) {
       LOG(FATAL) << "Invalid pid from parent zygote";
     }
 #if defined(OS_LINUX)
     // Sandboxed processes need to send the global, non-namespaced PID when
     // setting up an IPC channel to their parent.
     IPC::Channel::SetGlobalPid(real_pid);
     // Force the real PID so chrome event data have a PID that corresponds
     // to system trace event data.
     base::trace_event::TraceLog::GetInstance()->SetProcessID(
         static_cast<int>(real_pid));
 #endif
     return 0;
   }

   // In the parent process.
   read_pipe.reset();
   pid_oracle.reset();

   // Always receive a real PID from the zygote host, though it might
   // be invalid (see below).
   base::ProcessId real_pid;
   {
     ScopedVector<base::ScopedFD> recv_fds;
     char buf[kZygoteMaxMessageLength];
     const ssize_t len = UnixDomainSocket::RecvMsg(
         kZygoteSocketPairFd, buf, sizeof(buf), &recv_fds);
     CHECK_GT(len, 0);
     CHECK(recv_fds.empty());

     Pickle pickle(buf, len);
     PickleIterator iter(pickle);

     int kind;
     CHECK(iter.ReadInt(&kind));
     CHECK(kind == kZygoteCommandForkRealPID);
     CHECK(iter.ReadInt(&real_pid));
   }

   // Fork failed.
   if (pid < 0) {
     return -1;
   }

   // If we successfully forked a child, but it crashed without sending
   // a message to the browser, the browser won't have found its PID.
   if (real_pid < 0) {
     KillAndReap(pid, helper);
     return -1;
   }

   // If we're not using a helper, send the PID back to the child process.
   if (!helper) {
     ssize_t written =
         HANDLE_EINTR(write(write_pipe.get(), &real_pid, sizeof(real_pid)));
     if (written != sizeof(real_pid)) {
       KillAndReap(pid, helper);
       return -1;
     }
   }

   // Now set-up this process to be tracked by the Zygote.
   if (process_info_map_.find(real_pid) != process_info_map_.end()) {
     LOG(ERROR) << "Already tracking PID " << real_pid;
     NOTREACHED();
   }
   process_info_map_[real_pid].internal_pid = pid;
   process_info_map_[real_pid].started_from_helper = helper;

   return real_pid;
 }

 base::ProcessId Zygote::ReadArgsAndFork(PickleIterator iter,
                                         ScopedVector<base::ScopedFD> fds,
                                         std::string* uma_name,
                                         int* uma_sample,
                                         int* uma_boundary_value) {
   std::vector<std::string> args;
   int argc = 0;
   int numfds = 0;
   base::GlobalDescriptors::Mapping mapping;
   std::string process_type;
   std::string channel_id;
   const std::string channel_id_prefix = std::string("--")
       + switches::kProcessChannelID + std::string("=");

   if (!iter.ReadString(&process_type))
     return -1;
   if (!iter.ReadInt(&argc))
     return -1;

   for (int i = 0; i < argc; ++i) {
     std::string arg;
     if (!iter.ReadString(&arg))
       return -1;
     args.push_back(arg);
     if (arg.compare(0, channel_id_prefix.length(), channel_id_prefix) == 0)
       channel_id = arg.substr(channel_id_prefix.length());
   }

   if (!iter.ReadInt(&numfds))
     return -1;
   if (numfds != static_cast<int>(fds.size()))
     return -1;

   // First FD is the PID oracle socket.
   if (fds.size() < 1)
     return -1;
   base::ScopedFD pid_oracle(fds[0]->Pass());

   // Remaining FDs are for the global descriptor mapping.
   for (int i = 1; i < numfds; ++i) {
     base::GlobalDescriptors::Key key;
     if (!iter.ReadUInt32(&key))
       return -1;
     mapping.push_back(base::GlobalDescriptors::Descriptor(key, fds[i]->get()));
   }

   mapping.push_back(base::GlobalDescriptors::Descriptor(
       static_cast<uint32_t>(kSandboxIPCChannel), GetSandboxFD()));

   // Returns twice, once per process.
   base::ProcessId child_pid = ForkWithRealPid(process_type,
                                               mapping,
                                               channel_id,
                                               pid_oracle.Pass(),
                                               uma_name,
                                               uma_sample,
                                               uma_boundary_value);
   if (!child_pid) {
     // This is the child process.

     // Our socket from the browser.
     PCHECK(0 == IGNORE_EINTR(close(kZygoteSocketPairFd)));

     // Pass ownership of file descriptors from fds to GlobalDescriptors.
     for (ScopedVector<base::ScopedFD>::iterator i = fds.begin(); i != fds.end();
          ++i)
       ignore_result((*i)->release());
     base::GlobalDescriptors::GetInstance()->Reset(mapping);

     // Reset the process-wide command line to our new command line.
     base::CommandLine::Reset();
     base::CommandLine::Init(0, NULL);
     base::CommandLine::ForCurrentProcess()->InitFromArgv(args);

     // Update the process title. The argv was already cached by the call to
     // SetProcessTitleFromCommandLine in ChromeMain, so we can pass NULL here
     // (we don't have the original argv at this point).
     SetProcessTitleFromCommandLine(NULL);
   } else if (child_pid < 0) {
     LOG(ERROR) << "Zygote could not fork: process_type " << process_type
         << " numfds " << numfds << " child_pid " << child_pid;
   }
   return child_pid;
 }

 bool Zygote::HandleForkRequest(int fd,
                                PickleIterator iter,
                                ScopedVector<base::ScopedFD> fds) {
   std::string uma_name;
   int uma_sample;
   int uma_boundary_value;
   base::ProcessId child_pid = ReadArgsAndFork(
       iter, fds.Pass(), &uma_name, &uma_sample, &uma_boundary_value);
   if (child_pid == 0)
     return true;
   // If there's no UMA report for this particular fork, then check if any
   // helpers have an initial UMA report for us to send instead.
   while (uma_name.empty() && initial_uma_index_ < helpers_.size()) {
     helpers_[initial_uma_index_++]->InitialUMA(
         &uma_name, &uma_sample, &uma_boundary_value);
   }
   // Must always send reply, as ZygoteHost blocks while waiting for it.
   Pickle reply_pickle;
   reply_pickle.WriteInt(child_pid);
   reply_pickle.WriteString(uma_name);
   if (!uma_name.empty()) {
     reply_pickle.WriteInt(uma_sample);
     reply_pickle.WriteInt(uma_boundary_value);
   }
   if (HANDLE_EINTR(write(fd, reply_pickle.data(), reply_pickle.size())) !=
       static_cast<ssize_t> (reply_pickle.size()))
     PLOG(ERROR) << "write";
   return false;
 }

 bool Zygote::HandleGetSandboxStatus(int fd,
                                     PickleIterator iter) {
   if (HANDLE_EINTR(write(fd, &sandbox_flags_, sizeof(sandbox_flags_))) !=
                    sizeof(sandbox_flags_)) {
     PLOG(ERROR) << "write";
   }

   return false;
 }

 }  // namespace content
	// Copyright (c) 2012 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	#include "content/zygote/zygote_linux.h"

	#include <fcntl.h>
	#include <string.h>
	#include <sys/socket.h>
	#include <sys/types.h>
	#include <sys/wait.h>

	#include "base/command_line.h"
	#include "base/files/file_util.h"
	#include "base/linux_util.h"
	#include "base/logging.h"
	#include "base/macros.h"
	#include "base/memory/scoped_vector.h"
	#include "base/pickle.h"
	#include "base/posix/eintr_wrapper.h"
	#include "base/posix/global_descriptors.h"
	#include "base/posix/unix_domain_socket_linux.h"
	#include "base/process/kill.h"
	#include "base/process/launch.h"
	#include "base/process/process.h"
	#include "base/process/process_handle.h"
	#include "base/trace_event/trace_event.h"
	#include "content/common/child_process_sandbox_support_impl_linux.h"
	#include "content/common/sandbox_linux/sandbox_linux.h"
	#include "content/common/set_process_title.h"
	#include "content/common/zygote_commands_linux.h"
	#include "content/public/common/content_descriptors.h"
	#include "content/public/common/result_codes.h"
	#include "content/public/common/sandbox_linux.h"
	#include "content/public/common/send_zygote_child_ping_linux.h"
	#include "content/public/common/zygote_fork_delegate_linux.h"
	#include "ipc/ipc_channel.h"
	#include "ipc/ipc_switches.h"

	// See http://code.google.com/p/chromium/wiki/LinuxZygote

	namespace content {

	namespace {

	// NOP function. See below where this handler is installed.
	void SIGCHLDHandler(int signal) {
	}

	int LookUpFd(const base::GlobalDescriptors::Mapping& fd_mapping, uint32_t key) {
	for (size_t index = 0; index < fd_mapping.size(); ++index) {
	if (fd_mapping[index].key == key)
	return fd_mapping[index].fd;
	}
	return -1;
	}

	void CreatePipe(base::ScopedFD* read_pipe, base::ScopedFD* write_pipe) {
	int raw_pipe[2];
	PCHECK(0 == pipe(raw_pipe));
	read_pipe->reset(raw_pipe[0]);
	write_pipe->reset(raw_pipe[1]);
	}

	void KillAndReap(pid_t pid, ZygoteForkDelegate* helper) {
	if (helper) {
	// Helper children may be forked in another PID namespace, so \|pid\| might
	// be meaningless to us; or we just might not be able to directly send it
	// signals. So we can't kill it.
	// Additionally, we're not its parent, so we can't reap it anyway.
	// TODO(mdempsky): Extend the ZygoteForkDelegate API to handle this.
	LOG(WARNING) << "Unable to kill or reap helper children";
	return;
	}

	// Kill the child process in case it's not already dead, so we can safely
	// perform a blocking wait.
	PCHECK(0 == kill(pid, SIGKILL));
	PCHECK(pid == HANDLE_EINTR(waitpid(pid, NULL, 0)));
	}

	} // namespace

	Zygote::Zygote(int sandbox_flags, ScopedVector<ZygoteForkDelegate> helpers,
	const std::vector<base::ProcessHandle>& extra_children,
	const std::vector<int>& extra_fds)
	: sandbox_flags_(sandbox_flags),
	helpers_(helpers.Pass()),
	initial_uma_index_(0),
	extra_children_(extra_children),
	extra_fds_(extra_fds) {}

	Zygote::~Zygote() {
	}

	bool Zygote::ProcessRequests() {
	// A SOCK_SEQPACKET socket is installed in fd 3. We get commands from the
	// browser on it.
	// A SOCK_DGRAM is installed in fd 5. This is the sandbox IPC channel.
	// See http://code.google.com/p/chromium/wiki/LinuxSandboxIPC

	// We need to accept SIGCHLD, even though our handler is a no-op because
	// otherwise we cannot wait on children. (According to POSIX 2001.)
	struct sigaction action;
	memset(&action, 0, sizeof(action));
	action.sa_handler = &SIGCHLDHandler;
	CHECK(sigaction(SIGCHLD, &action, NULL) == 0);

	if (UsingSUIDSandbox() \|\| UsingNSSandbox()) {
	// Let the ZygoteHost know we are ready to go.
	// The receiving code is in content/browser/zygote_host_linux.cc.
	bool r = UnixDomainSocket::SendMsg(kZygoteSocketPairFd,
	kZygoteHelloMessage,
	sizeof(kZygoteHelloMessage),
	std::vector<int>());
	#if defined(OS_CHROMEOS)
	LOG_IF(WARNING, !r) << "Sending zygote magic failed";
	// Exit normally on chromeos because session manager may send SIGTERM
	// right after the process starts and it may fail to send zygote magic
	// number to browser process.
	if (!r)
	_exit(RESULT_CODE_NORMAL_EXIT);
	#else
	CHECK(r) << "Sending zygote magic failed";
	#endif
	}

	for (;;) {
	// This function call can return multiple times, once per fork().
	if (HandleRequestFromBrowser(kZygoteSocketPairFd))
	return true;
	}
	}

	bool Zygote::GetProcessInfo(base::ProcessHandle pid,
	ZygoteProcessInfo* process_info) {
	DCHECK(process_info);
	const ZygoteProcessMap::const_iterator it = process_info_map_.find(pid);
	if (it == process_info_map_.end()) {
	return false;
	}
	*process_info = it->second;
	return true;
	}

	bool Zygote::UsingSUIDSandbox() const {
	return sandbox_flags_ & kSandboxLinuxSUID;
	}

	bool Zygote::UsingNSSandbox() const {
	return sandbox_flags_ & kSandboxLinuxUserNS;
	}

	bool Zygote::HandleRequestFromBrowser(int fd) {
	ScopedVector<base::ScopedFD> fds;
	char buf[kZygoteMaxMessageLength];
	const ssize_t len = UnixDomainSocket::RecvMsg(fd, buf, sizeof(buf), &fds);

	if (len == 0 \|\| (len == -1 && errno == ECONNRESET)) {
	// EOF from the browser. We should die.
	// TODO(earthdok): call __sanititizer_cov_dump() here to obtain code
	// coverage for the Zygote. Currently it's not possible because of
	// confusion over who is responsible for closing the file descriptor.
	for (std::vector<int>::iterator it = extra_fds_.begin();
	it < extra_fds_.end(); ++it) {
	PCHECK(0 == IGNORE_EINTR(close(*it)));
	}
	#if !defined(SANITIZER_COVERAGE)
	// TODO(earthdok): add watchdog thread before using this in builds not
	// using sanitizer coverage.
	CHECK(extra_children_.empty());
	#endif
	for (std::vector<base::ProcessHandle>::iterator it =
	extra_children_.begin();
	it < extra_children_.end(); ++it) {
	PCHECK(it == HANDLE_EINTR(waitpid(it, NULL, 0)));
	}
	_exit(0);
	return false;
	}

	if (len == -1) {
	PLOG(ERROR) << "Error reading message from browser";
	return false;
	}

	Pickle pickle(buf, len);
	PickleIterator iter(pickle);

	int kind;
	if (iter.ReadInt(&kind)) {
	switch (kind) {
	case kZygoteCommandFork:
	// This function call can return multiple times, once per fork().
	return HandleForkRequest(fd, iter, fds.Pass());

	case kZygoteCommandReap:
	if (!fds.empty())
	break;
	HandleReapRequest(fd, iter);
	return false;
	case kZygoteCommandGetTerminationStatus:
	if (!fds.empty())
	break;
	HandleGetTerminationStatus(fd, iter);
	return false;
	case kZygoteCommandGetSandboxStatus:
	HandleGetSandboxStatus(fd, iter);
	return false;
	case kZygoteCommandForkRealPID:
	// This shouldn't happen in practice, but some failure paths in
	// HandleForkRequest (e.g., if ReadArgsAndFork fails during depickling)
	// could leave this command pending on the socket.
	LOG(ERROR) << "Unexpected real PID message from browser";
	NOTREACHED();
	return false;
	default:
	NOTREACHED();
	break;
	}
	}

	LOG(WARNING) << "Error parsing message from browser";
	return false;
	}

	// TODO(jln): remove callers to this broken API. See crbug.com/274855.
	void Zygote::HandleReapRequest(int fd,
	PickleIterator iter) {
	base::ProcessId child;

	if (!iter.ReadInt(&child)) {
	LOG(WARNING) << "Error parsing reap request from browser";
	return;
	}

	ZygoteProcessInfo child_info;
	if (!GetProcessInfo(child, &child_info)) {
	LOG(ERROR) << "Child not found!";
	NOTREACHED();
	return;
	}

	if (!child_info.started_from_helper) {
	// Do not call base::EnsureProcessTerminated() under ThreadSanitizer, as it
	// spawns a separate thread which may live until the call to fork() in the
	// zygote. As a result, ThreadSanitizer will report an error and almost
	// disable race detection in the child process.
	// Not calling EnsureProcessTerminated() may result in zombie processes
	// sticking around. This will only happen during testing, so we can live
	// with this for now.
	#if !defined(THREAD_SANITIZER)
	// TODO(jln): this old code is completely broken. See crbug.com/274855.
	base::EnsureProcessTerminated(base::Process(child_info.internal_pid));
	#else
	LOG(WARNING) << "Zygote process omitting a call to "
	<< "base::EnsureProcessTerminated() for child pid " << child
	<< " under ThreadSanitizer. See http://crbug.com/274855.";
	#endif
	} else {
	// For processes from the helper, send a GetTerminationStatus request
	// with known_dead set to true.
	// This is not perfect, as the process may be killed instantly, but is
	// better than ignoring the request.
	base::TerminationStatus status;
	int exit_code;
	bool got_termination_status =
	GetTerminationStatus(child, true /* known_dead */, &status, &exit_code);
	DCHECK(got_termination_status);
	}
	process_info_map_.erase(child);
	}

	bool Zygote::GetTerminationStatus(base::ProcessHandle real_pid,
	bool known_dead,
	base::TerminationStatus* status,
	int* exit_code) {

	ZygoteProcessInfo child_info;
	if (!GetProcessInfo(real_pid, &child_info)) {
	LOG(ERROR) << "Zygote::GetTerminationStatus for unknown PID "
	<< real_pid;
	NOTREACHED();
	return false;
	}
	// We know about \|real_pid\|.
	const base::ProcessHandle child = child_info.internal_pid;
	if (child_info.started_from_helper) {
	if (!child_info.started_from_helper->GetTerminationStatus(
	child, known_dead, status, exit_code)) {
	return false;
	}
	} else {
	// Handle the request directly.
	if (known_dead) {
	*status = base::GetKnownDeadTerminationStatus(child, exit_code);
	} else {
	// We don't know if the process is dying, so get its status but don't
	// wait.
	*status = base::GetTerminationStatus(child, exit_code);
	}
	}
	// Successfully got a status for \|real_pid\|.
	if (*status != base::TERMINATION_STATUS_STILL_RUNNING) {
	// Time to forget about this process.
	process_info_map_.erase(real_pid);
	}
	return true;
	}

	void Zygote::HandleGetTerminationStatus(int fd,
	PickleIterator iter) {
	bool known_dead;
	base::ProcessHandle child_requested;

	if (!iter.ReadBool(&known_dead) \|\| !iter.ReadInt(&child_requested)) {
	LOG(WARNING) << "Error parsing GetTerminationStatus request "
	<< "from browser";
	return;
	}

	base::TerminationStatus status;
	int exit_code;

	bool got_termination_status =
	GetTerminationStatus(child_requested, known_dead, &status, &exit_code);
	if (!got_termination_status) {
	// Assume that if we can't find the child in the sandbox, then
	// it terminated normally.
	NOTREACHED();
	status = base::TERMINATION_STATUS_NORMAL_TERMINATION;
	exit_code = RESULT_CODE_NORMAL_EXIT;
	}

	Pickle write_pickle;
	write_pickle.WriteInt(static_cast<int>(status));
	write_pickle.WriteInt(exit_code);
	ssize_t written =
	HANDLE_EINTR(write(fd, write_pickle.data(), write_pickle.size()));
	if (written != static_cast<ssize_t>(write_pickle.size()))
	PLOG(ERROR) << "write";
	}

	int Zygote::ForkWithRealPid(const std::string& process_type,
	const base::GlobalDescriptors::Mapping& fd_mapping,
	const std::string& channel_id,
	base::ScopedFD pid_oracle,
	std::string* uma_name,
	int* uma_sample,
	int* uma_boundary_value) {
	ZygoteForkDelegate* helper = NULL;
	for (ScopedVector<ZygoteForkDelegate>::iterator i = helpers_.begin();
	i != helpers_.end();
	++i) {
	if ((*i)->CanHelp(process_type, uma_name, uma_sample, uma_boundary_value)) {
	helper = *i;
	break;
	}
	}

	base::ScopedFD read_pipe, write_pipe;
	base::ProcessId pid = 0;
	if (helper) {
	int ipc_channel_fd = LookUpFd(fd_mapping, kPrimaryIPCChannel);
	if (ipc_channel_fd < 0) {
	DLOG(ERROR) << "Failed to find kPrimaryIPCChannel in FD mapping";
	return -1;
	}
	std::vector<int> fds;
	fds.push_back(ipc_channel_fd); // kBrowserFDIndex
	fds.push_back(pid_oracle.get()); // kPIDOracleFDIndex
	pid = helper->Fork(process_type, fds, channel_id);

	// Helpers should never return in the child process.
	CHECK_NE(pid, 0);
	} else {
	CreatePipe(&read_pipe, &write_pipe);
	// This is roughly equivalent to a fork(). We are using ForkWithFlags mainly
	// to give it some more diverse test coverage.
	pid = base::ForkWithFlags(SIGCHLD, nullptr, nullptr);
	}

	if (pid == 0) {
	// In the child process.
	write_pipe.reset();

	// Ping the PID oracle socket so the browser can find our PID.
	CHECK(SendZygoteChildPing(pid_oracle.get()));

	// Now read back our real PID from the zygote.
	base::ProcessId real_pid;
	if (!base::ReadFromFD(read_pipe.get(),
	reinterpret_cast<char*>(&real_pid),
	sizeof(real_pid))) {
	LOG(FATAL) << "Failed to synchronise with parent zygote process";
	}
	if (real_pid <= 0) {
	LOG(FATAL) << "Invalid pid from parent zygote";
	}
	#if defined(OS_LINUX)
	// Sandboxed processes need to send the global, non-namespaced PID when
	// setting up an IPC channel to their parent.
	IPC::Channel::SetGlobalPid(real_pid);
	// Force the real PID so chrome event data have a PID that corresponds
	// to system trace event data.
	base::trace_event::TraceLog::GetInstance()->SetProcessID(
	static_cast<int>(real_pid));
	#endif
	return 0;
	}

	// In the parent process.
	read_pipe.reset();
	pid_oracle.reset();

	// Always receive a real PID from the zygote host, though it might
	// be invalid (see below).
	base::ProcessId real_pid;
	{
	ScopedVector<base::ScopedFD> recv_fds;
	char buf[kZygoteMaxMessageLength];
	const ssize_t len = UnixDomainSocket::RecvMsg(
	kZygoteSocketPairFd, buf, sizeof(buf), &recv_fds);
	CHECK_GT(len, 0);
	CHECK(recv_fds.empty());

	Pickle pickle(buf, len);
	PickleIterator iter(pickle);

	int kind;
	CHECK(iter.ReadInt(&kind));
	CHECK(kind == kZygoteCommandForkRealPID);
	CHECK(iter.ReadInt(&real_pid));
	}

	// Fork failed.
	if (pid < 0) {
	return -1;
	}

	// If we successfully forked a child, but it crashed without sending
	// a message to the browser, the browser won't have found its PID.
	if (real_pid < 0) {
	KillAndReap(pid, helper);
	return -1;
	}

	// If we're not using a helper, send the PID back to the child process.
	if (!helper) {
	ssize_t written =
	HANDLE_EINTR(write(write_pipe.get(), &real_pid, sizeof(real_pid)));
	if (written != sizeof(real_pid)) {
	KillAndReap(pid, helper);
	return -1;
	}
	}

	// Now set-up this process to be tracked by the Zygote.
	if (process_info_map_.find(real_pid) != process_info_map_.end()) {
	LOG(ERROR) << "Already tracking PID " << real_pid;
	NOTREACHED();
	}
	process_info_map_[real_pid].internal_pid = pid;
	process_info_map_[real_pid].started_from_helper = helper;

	return real_pid;
	}

	base::ProcessId Zygote::ReadArgsAndFork(PickleIterator iter,
	ScopedVector<base::ScopedFD> fds,
	std::string* uma_name,
	int* uma_sample,
	int* uma_boundary_value) {
	std::vector<std::string> args;
	int argc = 0;
	int numfds = 0;
	base::GlobalDescriptors::Mapping mapping;
	std::string process_type;
	std::string channel_id;
	const std::string channel_id_prefix = std::string("--")
	+ switches::kProcessChannelID + std::string("=");

	if (!iter.ReadString(&process_type))
	return -1;
	if (!iter.ReadInt(&argc))
	return -1;

	for (int i = 0; i < argc; ++i) {
	std::string arg;
	if (!iter.ReadString(&arg))
	return -1;
	args.push_back(arg);
	if (arg.compare(0, channel_id_prefix.length(), channel_id_prefix) == 0)
	channel_id = arg.substr(channel_id_prefix.length());
	}

	if (!iter.ReadInt(&numfds))
	return -1;
	if (numfds != static_cast<int>(fds.size()))
	return -1;

	// First FD is the PID oracle socket.
	if (fds.size() < 1)
	return -1;
	base::ScopedFD pid_oracle(fds[0]->Pass());

	// Remaining FDs are for the global descriptor mapping.
	for (int i = 1; i < numfds; ++i) {
	base::GlobalDescriptors::Key key;
	if (!iter.ReadUInt32(&key))
	return -1;
	mapping.push_back(base::GlobalDescriptors::Descriptor(key, fds[i]->get()));
	}

	mapping.push_back(base::GlobalDescriptors::Descriptor(
	static_cast<uint32_t>(kSandboxIPCChannel), GetSandboxFD()));

	// Returns twice, once per process.
	base::ProcessId child_pid = ForkWithRealPid(process_type,
	mapping,
	channel_id,
	pid_oracle.Pass(),
	uma_name,
	uma_sample,
	uma_boundary_value);
	if (!child_pid) {
	// This is the child process.

	// Our socket from the browser.
	PCHECK(0 == IGNORE_EINTR(close(kZygoteSocketPairFd)));

	// Pass ownership of file descriptors from fds to GlobalDescriptors.
	for (ScopedVector<base::ScopedFD>::iterator i = fds.begin(); i != fds.end();
	++i)
	ignore_result((*i)->release());
	base::GlobalDescriptors::GetInstance()->Reset(mapping);

	// Reset the process-wide command line to our new command line.
	base::CommandLine::Reset();
	base::CommandLine::Init(0, NULL);
	base::CommandLine::ForCurrentProcess()->InitFromArgv(args);

	// Update the process title. The argv was already cached by the call to
	// SetProcessTitleFromCommandLine in ChromeMain, so we can pass NULL here
	// (we don't have the original argv at this point).
	SetProcessTitleFromCommandLine(NULL);
	} else if (child_pid < 0) {
	LOG(ERROR) << "Zygote could not fork: process_type " << process_type
	<< " numfds " << numfds << " child_pid " << child_pid;
	}
	return child_pid;
	}

	bool Zygote::HandleForkRequest(int fd,
	PickleIterator iter,
	ScopedVector<base::ScopedFD> fds) {
	std::string uma_name;
	int uma_sample;
	int uma_boundary_value;
	base::ProcessId child_pid = ReadArgsAndFork(
	iter, fds.Pass(), &uma_name, &uma_sample, &uma_boundary_value);
	if (child_pid == 0)
	return true;
	// If there's no UMA report for this particular fork, then check if any
	// helpers have an initial UMA report for us to send instead.
	while (uma_name.empty() && initial_uma_index_ < helpers_.size()) {
	helpers_[initial_uma_index_++]->InitialUMA(
	&uma_name, &uma_sample, &uma_boundary_value);
	}
	// Must always send reply, as ZygoteHost blocks while waiting for it.
	Pickle reply_pickle;
	reply_pickle.WriteInt(child_pid);
	reply_pickle.WriteString(uma_name);
	if (!uma_name.empty()) {
	reply_pickle.WriteInt(uma_sample);
	reply_pickle.WriteInt(uma_boundary_value);
	}
	if (HANDLE_EINTR(write(fd, reply_pickle.data(), reply_pickle.size())) !=
	static_cast<ssize_t> (reply_pickle.size()))
	PLOG(ERROR) << "write";
	return false;
	}

	bool Zygote::HandleGetSandboxStatus(int fd,
	PickleIterator iter) {
	if (HANDLE_EINTR(write(fd, &sandbox_flags_, sizeof(sandbox_flags_))) !=
	sizeof(sandbox_flags_)) {
	PLOG(ERROR) << "write";
	}

	return false;
	}

	} // namespace content