init/ueventd.cpp - LeafOS-Project/android_system_core - Gitiles

 /*
  * Copyright (C) 2010 The Android Open Source Project
  *
  * 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 "ueventd.h"

 #include <android/api-level.h>
 #include <ctype.h>
 #include <dirent.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
 #include <unistd.h>

 #include <set>
 #include <thread>

 #include <android-base/chrono_utils.h>
 #include <android-base/logging.h>
 #include <android-base/properties.h>
 #include <fstab/fstab.h>
 #include <selinux/android.h>
 #include <selinux/selinux.h>

 #include "devices.h"
 #include "firmware_handler.h"
 #include "modalias_handler.h"
 #include "selabel.h"
 #include "selinux.h"
 #include "uevent_handler.h"
 #include "uevent_listener.h"
 #include "ueventd_parser.h"
 #include "util.h"

 // At a high level, ueventd listens for uevent messages generated by the kernel through a netlink
 // socket.  When ueventd receives such a message it handles it by taking appropriate actions,
 // which can typically be creating a device node in /dev, setting file permissions, setting selinux
 // labels, etc.
 // Ueventd also handles loading of firmware that the kernel requests, and creates symlinks for block
 // and character devices.

 // When ueventd starts, it regenerates uevents for all currently registered devices by traversing
 // /sys and writing 'add' to each 'uevent' file that it finds.  This causes the kernel to generate
 // and resend uevent messages for all of the currently registered devices.  This is done, because
 // ueventd would not have been running when these devices were registered and therefore was unable
 // to receive their uevent messages and handle them appropriately.  This process is known as
 // 'cold boot'.

 // 'init' currently waits synchronously on the cold boot process of ueventd before it continues
 // its boot process.  For this reason, cold boot should be as quick as possible.  One way to achieve
 // a speed up here is to parallelize the handling of ueventd messages, which consume the bulk of the
 // time during cold boot.

 // Handling of uevent messages has two unique properties:
 // 1) It can be done in isolation; it doesn't need to read or write any status once it is started.
 // 2) It uses setegid() and setfscreatecon() so either care (aka locking) must be taken to ensure
 //    that no file system operations are done while the uevent process has an abnormal egid or
 //    fscreatecon or this handling must happen in a separate process.
 // Given the above two properties, it is best to fork() subprocesses to handle the uevents.  This
 // reduces the overhead and complexity that would be required in a solution with threads and locks.
 // In testing, a racy multithreaded solution has the same performance as the fork() solution, so
 // there is no reason to deal with the complexity of the former.

 // One other important caveat during the boot process is the handling of SELinux restorecon.
 // Since many devices have child devices, calling selinux_android_restorecon() recursively for each
 // device when its uevent is handled, results in multiple restorecon operations being done on a
 // given file.  It is more efficient to simply do restorecon recursively on /sys during cold boot,
 // than to do restorecon on each device as its uevent is handled.  This only applies to cold boot;
 // once that has completed, restorecon is done for each device as its uevent is handled.

 // With all of the above considered, the cold boot process has the below steps:
 // 1) ueventd regenerates uevents by doing the /sys traversal and listens to the netlink socket for
 //    the generated uevents.  It writes these uevents into a queue represented by a vector.
 //
 // 2) ueventd forks 'n' separate uevent handler subprocesses and has each of them to handle the
 //    uevents in the queue based on a starting offset (their process number) and a stride (the total
 //    number of processes).  Note that no IPC happens at this point and only const functions from
 //    DeviceHandler should be called from this context.
 //
 // 3) In parallel to the subprocesses handling the uevents, the main thread of ueventd calls
 //    selinux_android_restorecon() recursively on /sys/class, /sys/block, and /sys/devices.
 //
 // 4) Once the restorecon operation finishes, the main thread calls waitpid() to wait for all
 //    subprocess handlers to complete and exit.  Once this happens, it marks coldboot as having
 //    completed.
 //
 // At this point, ueventd is single threaded, poll()'s and then handles any future uevents.

 // Lastly, it should be noted that uevents that occur during the coldboot process are handled
 // without issue after the coldboot process completes.  This is because the uevent listener is
 // paused while the uevent handler and restorecon actions take place.  Once coldboot completes,
 // the uevent listener resumes in polling mode and will handle the uevents that occurred during
 // coldboot.

 namespace android {
 namespace init {

 class ColdBoot {
   public:
     ColdBoot(UeventListener& uevent_listener,
              std::vector<std::unique_ptr<UeventHandler>>& uevent_handlers,
              bool enable_parallel_restorecon,
              std::vector<std::string> parallel_restorecon_queue)
         : uevent_listener_(uevent_listener),
           uevent_handlers_(uevent_handlers),
           num_handler_subprocesses_(std::thread::hardware_concurrency() ?: 4),
           enable_parallel_restorecon_(enable_parallel_restorecon),
           parallel_restorecon_queue_(parallel_restorecon_queue) {}

     void Run();

   private:
     void UeventHandlerMain(unsigned int process_num, unsigned int total_processes);
     void RegenerateUevents();
     void ForkSubProcesses();
     void WaitForSubProcesses();
     void RestoreConHandler(unsigned int process_num, unsigned int total_processes);
     void GenerateRestoreCon(const std::string& directory);

     UeventListener& uevent_listener_;
     std::vector<std::unique_ptr<UeventHandler>>& uevent_handlers_;

     unsigned int num_handler_subprocesses_;
     bool enable_parallel_restorecon_;

     std::vector<Uevent> uevent_queue_;

     std::set<pid_t> subprocess_pids_;

     std::vector<std::string> restorecon_queue_;

     std::vector<std::string> parallel_restorecon_queue_;
 };

 void ColdBoot::UeventHandlerMain(unsigned int process_num, unsigned int total_processes) {
     for (unsigned int i = process_num; i < uevent_queue_.size(); i += total_processes) {
         auto& uevent = uevent_queue_[i];

         for (auto& uevent_handler : uevent_handlers_) {
             uevent_handler->HandleUevent(uevent);
         }
     }
 }

 void ColdBoot::RestoreConHandler(unsigned int process_num, unsigned int total_processes) {
     android::base::Timer t_process;

     for (unsigned int i = process_num; i < restorecon_queue_.size(); i += total_processes) {
         android::base::Timer t;
         auto& dir = restorecon_queue_[i];

         selinux_android_restorecon(dir.c_str(), SELINUX_ANDROID_RESTORECON_RECURSE);

         //Mark a dir restorecon operation for 50ms,
         //Maybe you can add this dir to the ueventd.rc script to parallel processing
         if (t.duration() > 50ms) {
             LOG(INFO) << "took " << t.duration().count() <<"ms restorecon '"
                         << dir.c_str() << "' on process '" << process_num  <<"'";
         }
     }

     //Calculate process restorecon time
     LOG(VERBOSE) << "took " << t_process.duration().count() << "ms on process '"
                 << process_num  << "'";
 }

 void ColdBoot::GenerateRestoreCon(const std::string& directory) {
     std::unique_ptr<DIR, decltype(&closedir)> dir(opendir(directory.c_str()), &closedir);

     if (!dir) {
         PLOG(WARNING) << "opendir " << directory.c_str();
         return;
     }

     struct dirent* dent;
     while ((dent = readdir(dir.get())) != NULL) {
         if (strcmp(dent->d_name, ".") == 0 || strcmp(dent->d_name, "..") == 0) continue;

         struct stat st;
         if (fstatat(dirfd(dir.get()), dent->d_name, &st, 0) == -1) continue;

         if (S_ISDIR(st.st_mode)) {
             std::string fullpath = directory + "/" + dent->d_name;
             auto parallel_restorecon =
                 std::find(parallel_restorecon_queue_.begin(),
                     parallel_restorecon_queue_.end(), fullpath);
             if (parallel_restorecon == parallel_restorecon_queue_.end()) {
                 restorecon_queue_.emplace_back(fullpath);
             }
         }
     }
 }

 void ColdBoot::RegenerateUevents() {
     uevent_listener_.RegenerateUevents([this](const Uevent& uevent) {
         uevent_queue_.emplace_back(uevent);
         return ListenerAction::kContinue;
     });
 }

 void ColdBoot::ForkSubProcesses() {
     for (unsigned int i = 0; i < num_handler_subprocesses_; ++i) {
         auto pid = fork();
         if (pid < 0) {
             PLOG(FATAL) << "fork() failed!";
         }

         if (pid == 0) {
             UeventHandlerMain(i, num_handler_subprocesses_);
             if (enable_parallel_restorecon_) {
                 RestoreConHandler(i, num_handler_subprocesses_);
             }
             _exit(EXIT_SUCCESS);
         }

         subprocess_pids_.emplace(pid);
     }
 }

 void ColdBoot::WaitForSubProcesses() {
     // Treat subprocesses that crash or get stuck the same as if ueventd itself has crashed or gets
     // stuck.
     //
     // When a subprocess crashes, we fatally abort from ueventd.  init will restart ueventd when
     // init reaps it, and the cold boot process will start again.  If this continues to fail, then
     // since ueventd is marked as a critical service, init will reboot to bootloader.
     //
     // When a subprocess gets stuck, keep ueventd spinning waiting for it.  init has a timeout for
     // cold boot and will reboot to the bootloader if ueventd does not complete in time.
     while (!subprocess_pids_.empty()) {
         int status;
         pid_t pid = TEMP_FAILURE_RETRY(waitpid(-1, &status, 0));
         if (pid == -1) {
             PLOG(ERROR) << "waitpid() failed";
             continue;
         }

         auto it = std::find(subprocess_pids_.begin(), subprocess_pids_.end(), pid);
         if (it == subprocess_pids_.end()) continue;

         if (WIFEXITED(status)) {
             if (WEXITSTATUS(status) == EXIT_SUCCESS) {
                 subprocess_pids_.erase(it);
             } else {
                 LOG(FATAL) << "subprocess exited with status " << WEXITSTATUS(status);
             }
         } else if (WIFSIGNALED(status)) {
             LOG(FATAL) << "subprocess killed by signal " << WTERMSIG(status);
         }
     }
 }

 void ColdBoot::Run() {
     android::base::Timer cold_boot_timer;

     RegenerateUevents();

     if (enable_parallel_restorecon_) {
         if (parallel_restorecon_queue_.empty()) {
             parallel_restorecon_queue_.emplace_back("/sys");
             // takes long time for /sys/devices, parallelize it
             parallel_restorecon_queue_.emplace_back("/sys/devices");
             LOG(INFO) << "Parallel processing directory is not set, set the default";
         }
         for (const auto& dir : parallel_restorecon_queue_) {
             selinux_android_restorecon(dir.c_str(), 0);
             GenerateRestoreCon(dir);
         }
     }

     ForkSubProcesses();

     if (!enable_parallel_restorecon_) {
         selinux_android_restorecon("/sys", SELINUX_ANDROID_RESTORECON_RECURSE);
     }

     WaitForSubProcesses();

     android::base::SetProperty(kColdBootDoneProp, "true");
     LOG(INFO) << "Coldboot took " << cold_boot_timer.duration().count() / 1000.0f << " seconds";
 }

 static UeventdConfiguration GetConfiguration() {
     if (IsMicrodroid()) {
         return ParseConfig({"/system/etc/ueventd.rc", "/vendor/etc/ueventd.rc"});
     }

     auto hardware = android::base::GetProperty("ro.hardware", "");

     struct LegacyPathInfo {
         std::string legacy_path;
         std::string preferred;
     };
     std::vector<LegacyPathInfo> legacy_paths{
             {"/vendor/ueventd.rc", "/vendor/etc/ueventd.rc"},
             {"/odm/ueventd.rc", "/odm/etc/ueventd.rc"},
             {"/ueventd." + hardware + ".rc", "another ueventd.rc file"}};

     std::vector<std::string> canonical{"/system/etc/ueventd.rc"};

     if (android::base::GetIntProperty("ro.product.first_api_level", 10000) < __ANDROID_API_T__) {
         // TODO: Remove these legacy paths once Android S is no longer supported.
         for (const auto& info : legacy_paths) {
             canonical.push_back(info.legacy_path);
         }
     } else {
         // Warn if newer device is using legacy paths.
         for (const auto& info : legacy_paths) {
             if (access(info.legacy_path.c_str(), F_OK) == 0) {
                 LOG(FATAL_WITHOUT_ABORT)
                         << "Legacy ueventd configuration file detected and will not be parsed: "
                         << info.legacy_path << ". Please move your configuration to "
                         << info.preferred << " instead.";
             }
         }
     }

     return ParseConfig(canonical);
 }

 int ueventd_main(int argc, char** argv) {
     /*
      * init sets the umask to 077 for forked processes. We need to
      * create files with exact permissions, without modification by
      * the umask.
      */
     umask(000);

     android::base::InitLogging(argv, &android::base::KernelLogger);

     LOG(INFO) << "ueventd started!";

     SelinuxSetupKernelLogging();
     SelabelInitialize();

     std::vector<std::unique_ptr<UeventHandler>> uevent_handlers;

     auto ueventd_configuration = GetConfiguration();

     uevent_handlers.emplace_back(std::make_unique<DeviceHandler>(
             std::move(ueventd_configuration.dev_permissions),
             std::move(ueventd_configuration.sysfs_permissions),
             std::move(ueventd_configuration.subsystems), android::fs_mgr::GetBootDevices(), true));
     uevent_handlers.emplace_back(std::make_unique<FirmwareHandler>(
             std::move(ueventd_configuration.firmware_directories),
             std::move(ueventd_configuration.external_firmware_handlers)));

     if (ueventd_configuration.enable_modalias_handling) {
         std::vector<std::string> base_paths = {"/odm/lib/modules", "/vendor/lib/modules"};
         uevent_handlers.emplace_back(std::make_unique<ModaliasHandler>(base_paths));
     }
     UeventListener uevent_listener(ueventd_configuration.uevent_socket_rcvbuf_size);

     if (!android::base::GetBoolProperty(kColdBootDoneProp, false)) {
         ColdBoot cold_boot(uevent_listener, uevent_handlers,
                            ueventd_configuration.enable_parallel_restorecon,
                            ueventd_configuration.parallel_restorecon_dirs);
         cold_boot.Run();
     }

     for (auto& uevent_handler : uevent_handlers) {
         uevent_handler->ColdbootDone();
     }

     // We use waitpid() in ColdBoot, so we can't ignore SIGCHLD until now.
     signal(SIGCHLD, SIG_IGN);
     // Reap and pending children that exited between the last call to waitpid() and setting SIG_IGN
     // for SIGCHLD above.
     while (waitpid(-1, nullptr, WNOHANG) > 0) {
     }

     // Restore prio before main loop
     setpriority(PRIO_PROCESS, 0, 0);
     uevent_listener.Poll([&uevent_handlers](const Uevent& uevent) {
         for (auto& uevent_handler : uevent_handlers) {
             uevent_handler->HandleUevent(uevent);
         }
         return ListenerAction::kContinue;
     });

     return 0;
 }

 }  // namespace init
 }  // namespace android
	/*
	* Copyright (C) 2010 The Android Open Source Project
	*
	* 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 "ueventd.h"

	#include <android/api-level.h>
	#include <ctype.h>
	#include <dirent.h>
	#include <fcntl.h>
	#include <signal.h>
	#include <stdio.h>
	#include <stdlib.h>
	#include <string.h>
	#include <sys/stat.h>
	#include <sys/wait.h>
	#include <unistd.h>

	#include <set>
	#include <thread>

	#include <android-base/chrono_utils.h>
	#include <android-base/logging.h>
	#include <android-base/properties.h>
	#include <fstab/fstab.h>
	#include <selinux/android.h>
	#include <selinux/selinux.h>

	#include "devices.h"
	#include "firmware_handler.h"
	#include "modalias_handler.h"
	#include "selabel.h"
	#include "selinux.h"
	#include "uevent_handler.h"
	#include "uevent_listener.h"
	#include "ueventd_parser.h"
	#include "util.h"

	// At a high level, ueventd listens for uevent messages generated by the kernel through a netlink
	// socket. When ueventd receives such a message it handles it by taking appropriate actions,
	// which can typically be creating a device node in /dev, setting file permissions, setting selinux
	// labels, etc.
	// Ueventd also handles loading of firmware that the kernel requests, and creates symlinks for block
	// and character devices.

	// When ueventd starts, it regenerates uevents for all currently registered devices by traversing
	// /sys and writing 'add' to each 'uevent' file that it finds. This causes the kernel to generate
	// and resend uevent messages for all of the currently registered devices. This is done, because
	// ueventd would not have been running when these devices were registered and therefore was unable
	// to receive their uevent messages and handle them appropriately. This process is known as
	// 'cold boot'.

	// 'init' currently waits synchronously on the cold boot process of ueventd before it continues
	// its boot process. For this reason, cold boot should be as quick as possible. One way to achieve
	// a speed up here is to parallelize the handling of ueventd messages, which consume the bulk of the
	// time during cold boot.

	// Handling of uevent messages has two unique properties:
	// 1) It can be done in isolation; it doesn't need to read or write any status once it is started.
	// 2) It uses setegid() and setfscreatecon() so either care (aka locking) must be taken to ensure
	// that no file system operations are done while the uevent process has an abnormal egid or
	// fscreatecon or this handling must happen in a separate process.
	// Given the above two properties, it is best to fork() subprocesses to handle the uevents. This
	// reduces the overhead and complexity that would be required in a solution with threads and locks.
	// In testing, a racy multithreaded solution has the same performance as the fork() solution, so
	// there is no reason to deal with the complexity of the former.

	// One other important caveat during the boot process is the handling of SELinux restorecon.
	// Since many devices have child devices, calling selinux_android_restorecon() recursively for each
	// device when its uevent is handled, results in multiple restorecon operations being done on a
	// given file. It is more efficient to simply do restorecon recursively on /sys during cold boot,
	// than to do restorecon on each device as its uevent is handled. This only applies to cold boot;
	// once that has completed, restorecon is done for each device as its uevent is handled.

	// With all of the above considered, the cold boot process has the below steps:
	// 1) ueventd regenerates uevents by doing the /sys traversal and listens to the netlink socket for
	// the generated uevents. It writes these uevents into a queue represented by a vector.
	//
	// 2) ueventd forks 'n' separate uevent handler subprocesses and has each of them to handle the
	// uevents in the queue based on a starting offset (their process number) and a stride (the total
	// number of processes). Note that no IPC happens at this point and only const functions from
	// DeviceHandler should be called from this context.
	//
	// 3) In parallel to the subprocesses handling the uevents, the main thread of ueventd calls
	// selinux_android_restorecon() recursively on /sys/class, /sys/block, and /sys/devices.
	//
	// 4) Once the restorecon operation finishes, the main thread calls waitpid() to wait for all
	// subprocess handlers to complete and exit. Once this happens, it marks coldboot as having
	// completed.
	//
	// At this point, ueventd is single threaded, poll()'s and then handles any future uevents.

	// Lastly, it should be noted that uevents that occur during the coldboot process are handled
	// without issue after the coldboot process completes. This is because the uevent listener is
	// paused while the uevent handler and restorecon actions take place. Once coldboot completes,
	// the uevent listener resumes in polling mode and will handle the uevents that occurred during
	// coldboot.

	namespace android {
	namespace init {

	class ColdBoot {
	public:
	ColdBoot(UeventListener& uevent_listener,
	std::vector<std::unique_ptr<UeventHandler>>& uevent_handlers,
	bool enable_parallel_restorecon,
	std::vector<std::string> parallel_restorecon_queue)
	: uevent_listener_(uevent_listener),
	uevent_handlers_(uevent_handlers),
	num_handler_subprocesses_(std::thread::hardware_concurrency() ?: 4),
	enable_parallel_restorecon_(enable_parallel_restorecon),
	parallel_restorecon_queue_(parallel_restorecon_queue) {}

	void Run();

	private:
	void UeventHandlerMain(unsigned int process_num, unsigned int total_processes);
	void RegenerateUevents();
	void ForkSubProcesses();
	void WaitForSubProcesses();
	void RestoreConHandler(unsigned int process_num, unsigned int total_processes);
	void GenerateRestoreCon(const std::string& directory);

	UeventListener& uevent_listener_;
	std::vector<std::unique_ptr<UeventHandler>>& uevent_handlers_;

	unsigned int num_handler_subprocesses_;
	bool enable_parallel_restorecon_;

	std::vector<Uevent> uevent_queue_;

	std::set<pid_t> subprocess_pids_;

	std::vector<std::string> restorecon_queue_;

	std::vector<std::string> parallel_restorecon_queue_;
	};

	void ColdBoot::UeventHandlerMain(unsigned int process_num, unsigned int total_processes) {
	for (unsigned int i = process_num; i < uevent_queue_.size(); i += total_processes) {
	auto& uevent = uevent_queue_[i];

	for (auto& uevent_handler : uevent_handlers_) {
	uevent_handler->HandleUevent(uevent);
	}
	}
	}

	void ColdBoot::RestoreConHandler(unsigned int process_num, unsigned int total_processes) {
	android::base::Timer t_process;

	for (unsigned int i = process_num; i < restorecon_queue_.size(); i += total_processes) {
	android::base::Timer t;
	auto& dir = restorecon_queue_[i];

	selinux_android_restorecon(dir.c_str(), SELINUX_ANDROID_RESTORECON_RECURSE);

	//Mark a dir restorecon operation for 50ms,
	//Maybe you can add this dir to the ueventd.rc script to parallel processing
	if (t.duration() > 50ms) {
	LOG(INFO) << "took " << t.duration().count() <<"ms restorecon '"
	<< dir.c_str() << "' on process '" << process_num <<"'";
	}
	}

	//Calculate process restorecon time
	LOG(VERBOSE) << "took " << t_process.duration().count() << "ms on process '"
	<< process_num << "'";
	}

	void ColdBoot::GenerateRestoreCon(const std::string& directory) {
	std::unique_ptr<DIR, decltype(&closedir)> dir(opendir(directory.c_str()), &closedir);

	if (!dir) {
	PLOG(WARNING) << "opendir " << directory.c_str();
	return;
	}

	struct dirent* dent;
	while ((dent = readdir(dir.get())) != NULL) {
	if (strcmp(dent->d_name, ".") == 0 \|\| strcmp(dent->d_name, "..") == 0) continue;

	struct stat st;
	if (fstatat(dirfd(dir.get()), dent->d_name, &st, 0) == -1) continue;

	if (S_ISDIR(st.st_mode)) {
	std::string fullpath = directory + "/" + dent->d_name;
	auto parallel_restorecon =
	std::find(parallel_restorecon_queue_.begin(),
	parallel_restorecon_queue_.end(), fullpath);
	if (parallel_restorecon == parallel_restorecon_queue_.end()) {
	restorecon_queue_.emplace_back(fullpath);
	}
	}
	}
	}

	void ColdBoot::RegenerateUevents() {
	uevent_listener_.RegenerateUevents([this](const Uevent& uevent) {
	uevent_queue_.emplace_back(uevent);
	return ListenerAction::kContinue;
	});
	}

	void ColdBoot::ForkSubProcesses() {
	for (unsigned int i = 0; i < num_handler_subprocesses_; ++i) {
	auto pid = fork();
	if (pid < 0) {
	PLOG(FATAL) << "fork() failed!";
	}

	if (pid == 0) {
	UeventHandlerMain(i, num_handler_subprocesses_);
	if (enable_parallel_restorecon_) {
	RestoreConHandler(i, num_handler_subprocesses_);
	}
	_exit(EXIT_SUCCESS);
	}

	subprocess_pids_.emplace(pid);
	}
	}

	void ColdBoot::WaitForSubProcesses() {
	// Treat subprocesses that crash or get stuck the same as if ueventd itself has crashed or gets
	// stuck.
	//
	// When a subprocess crashes, we fatally abort from ueventd. init will restart ueventd when
	// init reaps it, and the cold boot process will start again. If this continues to fail, then
	// since ueventd is marked as a critical service, init will reboot to bootloader.
	//
	// When a subprocess gets stuck, keep ueventd spinning waiting for it. init has a timeout for
	// cold boot and will reboot to the bootloader if ueventd does not complete in time.
	while (!subprocess_pids_.empty()) {
	int status;
	pid_t pid = TEMP_FAILURE_RETRY(waitpid(-1, &status, 0));
	if (pid == -1) {
	PLOG(ERROR) << "waitpid() failed";
	continue;
	}

	auto it = std::find(subprocess_pids_.begin(), subprocess_pids_.end(), pid);
	if (it == subprocess_pids_.end()) continue;

	if (WIFEXITED(status)) {
	if (WEXITSTATUS(status) == EXIT_SUCCESS) {
	subprocess_pids_.erase(it);
	} else {
	LOG(FATAL) << "subprocess exited with status " << WEXITSTATUS(status);
	}
	} else if (WIFSIGNALED(status)) {
	LOG(FATAL) << "subprocess killed by signal " << WTERMSIG(status);
	}
	}
	}

	void ColdBoot::Run() {
	android::base::Timer cold_boot_timer;

	RegenerateUevents();

	if (enable_parallel_restorecon_) {
	if (parallel_restorecon_queue_.empty()) {
	parallel_restorecon_queue_.emplace_back("/sys");
	// takes long time for /sys/devices, parallelize it
	parallel_restorecon_queue_.emplace_back("/sys/devices");
	LOG(INFO) << "Parallel processing directory is not set, set the default";
	}
	for (const auto& dir : parallel_restorecon_queue_) {
	selinux_android_restorecon(dir.c_str(), 0);
	GenerateRestoreCon(dir);
	}
	}

	ForkSubProcesses();

	if (!enable_parallel_restorecon_) {
	selinux_android_restorecon("/sys", SELINUX_ANDROID_RESTORECON_RECURSE);
	}

	WaitForSubProcesses();

	android::base::SetProperty(kColdBootDoneProp, "true");
	LOG(INFO) << "Coldboot took " << cold_boot_timer.duration().count() / 1000.0f << " seconds";
	}

	static UeventdConfiguration GetConfiguration() {
	if (IsMicrodroid()) {
	return ParseConfig({"/system/etc/ueventd.rc", "/vendor/etc/ueventd.rc"});
	}

	auto hardware = android::base::GetProperty("ro.hardware", "");

	struct LegacyPathInfo {
	std::string legacy_path;
	std::string preferred;
	};
	std::vector<LegacyPathInfo> legacy_paths{
	{"/vendor/ueventd.rc", "/vendor/etc/ueventd.rc"},
	{"/odm/ueventd.rc", "/odm/etc/ueventd.rc"},
	{"/ueventd." + hardware + ".rc", "another ueventd.rc file"}};

	std::vector<std::string> canonical{"/system/etc/ueventd.rc"};

	if (android::base::GetIntProperty("ro.product.first_api_level", 10000) < __ANDROID_API_T__) {
	// TODO: Remove these legacy paths once Android S is no longer supported.
	for (const auto& info : legacy_paths) {
	canonical.push_back(info.legacy_path);
	}
	} else {
	// Warn if newer device is using legacy paths.
	for (const auto& info : legacy_paths) {
	if (access(info.legacy_path.c_str(), F_OK) == 0) {
	LOG(FATAL_WITHOUT_ABORT)
	<< "Legacy ueventd configuration file detected and will not be parsed: "
	<< info.legacy_path << ". Please move your configuration to "
	<< info.preferred << " instead.";
	}
	}
	}

	return ParseConfig(canonical);
	}

	int ueventd_main(int argc, char** argv) {
	/*
	* init sets the umask to 077 for forked processes. We need to
	* create files with exact permissions, without modification by
	* the umask.
	*/
	umask(000);

	android::base::InitLogging(argv, &android::base::KernelLogger);

	LOG(INFO) << "ueventd started!";

	SelinuxSetupKernelLogging();
	SelabelInitialize();

	std::vector<std::unique_ptr<UeventHandler>> uevent_handlers;

	auto ueventd_configuration = GetConfiguration();

	uevent_handlers.emplace_back(std::make_unique<DeviceHandler>(
	std::move(ueventd_configuration.dev_permissions),
	std::move(ueventd_configuration.sysfs_permissions),
	std::move(ueventd_configuration.subsystems), android::fs_mgr::GetBootDevices(), true));
	uevent_handlers.emplace_back(std::make_unique<FirmwareHandler>(
	std::move(ueventd_configuration.firmware_directories),
	std::move(ueventd_configuration.external_firmware_handlers)));

	if (ueventd_configuration.enable_modalias_handling) {
	std::vector<std::string> base_paths = {"/odm/lib/modules", "/vendor/lib/modules"};
	uevent_handlers.emplace_back(std::make_unique<ModaliasHandler>(base_paths));
	}
	UeventListener uevent_listener(ueventd_configuration.uevent_socket_rcvbuf_size);

	if (!android::base::GetBoolProperty(kColdBootDoneProp, false)) {
	ColdBoot cold_boot(uevent_listener, uevent_handlers,
	ueventd_configuration.enable_parallel_restorecon,
	ueventd_configuration.parallel_restorecon_dirs);
	cold_boot.Run();
	}

	for (auto& uevent_handler : uevent_handlers) {
	uevent_handler->ColdbootDone();
	}

	// We use waitpid() in ColdBoot, so we can't ignore SIGCHLD until now.
	signal(SIGCHLD, SIG_IGN);
	// Reap and pending children that exited between the last call to waitpid() and setting SIG_IGN
	// for SIGCHLD above.
	while (waitpid(-1, nullptr, WNOHANG) > 0) {
	}

	// Restore prio before main loop
	setpriority(PRIO_PROCESS, 0, 0);
	uevent_listener.Poll([&uevent_handlers](const Uevent& uevent) {
	for (auto& uevent_handler : uevent_handlers) {
	uevent_handler->HandleUevent(uevent);
	}
	return ListenerAction::kContinue;
	});

	return 0;
	}

	} // namespace init
	} // namespace android