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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 <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 {
ColdBoot(UeventListener& uevent_listener,
std::vector<std::unique_ptr<UeventHandler>>& uevent_handlers,
bool enable_parallel_restorecon)
: uevent_listener_(uevent_listener),
num_handler_subprocesses_(std::thread::hardware_concurrency() ?: 4),
enable_parallel_restorecon_(enable_parallel_restorecon) {}
void Run();
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_;
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_) {
void ColdBoot::RestoreConHandler(unsigned int process_num, unsigned int total_processes) {
for (unsigned int i = process_num; i < restorecon_queue_.size(); i += total_processes) {
auto& dir = restorecon_queue_[i];
selinux_android_restorecon(dir.c_str(), SELINUX_ANDROID_RESTORECON_RECURSE);
void ColdBoot::GenerateRestoreCon(const std::string& directory) {
std::unique_ptr<DIR, decltype(&closedir)> dir(opendir(directory.c_str()), &closedir);
if (!dir) 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;
if (fullpath != "/sys/devices") {
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_);
if (enable_parallel_restorecon_) {
RestoreConHandler(i, num_handler_subprocesses_);
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";
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;
if (enable_parallel_restorecon_) {
selinux_android_restorecon("/sys", 0);
selinux_android_restorecon("/sys/devices", 0);
// takes long time for /sys/devices, parallelize it
if (!enable_parallel_restorecon_) {
selinux_android_restorecon("/sys", SELINUX_ANDROID_RESTORECON_RECURSE);
android::base::SetProperty(kColdBootDoneProp, "true");
LOG(INFO) << "Coldboot took " << cold_boot_timer.duration().count() / 1000.0f << " seconds";
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.
android::base::InitLogging(argv, &android::base::KernelLogger);
LOG(INFO) << "ueventd started!";
std::vector<std::unique_ptr<UeventHandler>> uevent_handlers;
auto ueventd_configuration = ParseConfig("/system/etc/ueventd.rc");
std::move(ueventd_configuration.subsystems), android::fs_mgr::GetBootDevices(), true));
if (ueventd_configuration.enable_modalias_handling) {
std::vector<std::string> base_paths = {"/odm/lib/modules", "/vendor/lib/modules"};
UeventListener uevent_listener(ueventd_configuration.uevent_socket_rcvbuf_size);
if (!android::base::GetBoolProperty(kColdBootDoneProp, false)) {
ColdBoot cold_boot(uevent_listener, uevent_handlers,
for (auto& uevent_handler : uevent_handlers) {
// 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) {
// Restore prio before main loop
setpriority(PRIO_PROCESS, 0, 0);
uevent_listener.Poll([&uevent_handlers](const Uevent& uevent) {
for (auto& uevent_handler : uevent_handlers) {
return ListenerAction::kContinue;
return 0;
} // namespace init
} // namespace android