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* 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
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include "ueventd.h"
#include <ctype.h>
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <set>
#include <thread>
#include <android-base/chrono_utils.h>
#include <android-base/logging.h>
#include <android-base/properties.h>
#include <selinux/android.h>
#include <selinux/selinux.h>
#include "devices.h"
#include "firmware_handler.h"
#include "log.h"
#include "selinux.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 {
ColdBoot(UeventListener& uevent_listener, DeviceHandler& device_handler)
: uevent_listener_(uevent_listener),
num_handler_subprocesses_(std::thread::hardware_concurrency() ?: 4) {}
void Run();
void UeventHandlerMain(unsigned int process_num, unsigned int total_processes);
void RegenerateUevents();
void ForkSubProcesses();
void DoRestoreCon();
void WaitForSubProcesses();
UeventListener& uevent_listener_;
DeviceHandler& device_handler_;
unsigned int num_handler_subprocesses_;
std::vector<Uevent> uevent_queue_;
std::set<pid_t> subprocess_pids_;
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];
void ColdBoot::RegenerateUevents() {
uevent_listener_.RegenerateUevents([this](const Uevent& 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_);
void ColdBoot::DoRestoreCon() {
selinux_android_restorecon("/sys", SELINUX_ANDROID_RESTORECON_RECURSE);
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 recovery.
// 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";
auto it = std::find(subprocess_pids_.begin(), subprocess_pids_.end(), pid);
if (it == subprocess_pids_.end()) continue;
if (WIFEXITED(status)) {
} 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;
LOG(INFO) << "Coldboot took " << cold_boot_timer.duration().count() / 1000.0f << " seconds";
DeviceHandler CreateDeviceHandler() {
Parser parser;
std::vector<Subsystem> subsystems;
parser.AddSectionParser("subsystem", std::make_unique<SubsystemParser>(&subsystems));
using namespace std::placeholders;
std::vector<SysfsPermissions> sysfs_permissions;
std::vector<Permissions> dev_permissions;
std::bind(ParsePermissionsLine, _1, &sysfs_permissions, nullptr));
std::bind(ParsePermissionsLine, _1, nullptr, &dev_permissions));
* keep the current product name base configuration so
* we remain backwards compatible and allow it to override
* everything
* TODO: cleanup platform ueventd.rc to remove vendor specific
* device node entries (b/34968103)
std::string hardware = android::base::GetProperty("ro.hardware", "");
parser.ParseConfig("/ueventd." + hardware + ".rc");
return DeviceHandler(std::move(dev_permissions), std::move(sysfs_permissions),
std::move(subsystems), true);
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.
LOG(INFO) << "ueventd started!";
DeviceHandler device_handler = CreateDeviceHandler();
UeventListener uevent_listener;
if (access(COLDBOOT_DONE, F_OK) != 0) {
ColdBoot cold_boot(uevent_listener, device_handler);
// We use waitpid() in ColdBoot, so we can't ignore SIGCHLD until now.
// 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) {
uevent_listener.Poll([&device_handler](const Uevent& uevent) {
return ListenerAction::kContinue;
return 0;
} // namespace init
} // namespace android