init/ueventd.cpp

/*
 * 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 <thread>

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

#include "coldboot.h"
#include "devices.h"
#include "firmware_handler.h"
#include "modalias_handler.h"
#include "selabel.h"
#include "selinux.h"
#include "uevent_dependency_graph.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) Messages can be handled in isolation when they do not depend on another. A device event
//    depends on another when the device is the same, parent, or child of another device which
//    should be handled first by another event. e.g. An add event must be handled first before
//    removal. A device foo must be added before foo/bar.
// 2) The cold boot is unlikely to have events that depend on another in a critical manner.
// Therefore, ueventd handles uevent message in parallel. We provide two options for the
// parallel handling:
// 1) 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).
// 2) Coldboot threadpool: ueventd uses a threadpool to handle uevents.
// The first option is the default option. The second option can be enabled by a feature flag
// RELEASE_UEVENTD_COLDBOOT_THREADPOOL. The second option provides the better performance since it
// distributes the uevent handling workload evenly across all the threads, while the first option
// only assigns equal numbers of uevents to each process and does not take the actual workload of
// each of them into consideration.

// 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.
//
// 2 (When threadpool is disabled)) 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.

// 2 (When threadpool is enabled)) ueventd uses a threadpool to handle uevents. The threadpool
//    has 'n' threads and each thread is responsible for handling available uevents in the queue.
//
// 3) If enable_parallel_restorecon is false, the main thread of ueventd calls
//    selinux_android_restorecon() recursively on /sys in parallel to the subprocesses or threadpool
//    handling the uevents.
//
// 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 {

static constexpr bool kLogUeventDuration = false;

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

    return ParseConfig({"/system/etc/ueventd.rc"});
}

void main_loop(const UeventListener& uevent_listener,
               const std::vector<std::shared_ptr<UeventHandler>>& uevent_handlers) {
    uevent_listener.Poll([&uevent_handlers](const Uevent& uevent) {
        android::base::Timer t;
        for (auto& uevent_handler : uevent_handlers) {
            uevent_handler->HandleUevent(uevent);
        }
        if (kLogUeventDuration) {
            LOG(INFO) << uevent << " took " << t.duration().count() << "ms";
        }
        return ListenerAction::kContinue;
    });
}

void parallel_main_loop(const UeventListener& uevent_listener,
                        const std::vector<std::shared_ptr<UeventHandler>>& uevent_handlers,
                        size_t num_threads) {
    LOG(INFO) << "parallel main loop is enabled with " << num_threads << " threads";

    std::vector<std::thread> threads;
    UeventDependencyGraph graph;

    for (unsigned int i = 0; i < num_threads; i++) {
        threads.emplace_back([&graph, &uevent_handlers] {
            while (true) {
                auto uevent = graph.WaitDependencyFreeEvent();
                android::base::Timer t;
                for (auto& uevent_handler : uevent_handlers) {
                    uevent_handler->HandleUevent(uevent);
                }
                if (kLogUeventDuration) {
                    LOG(INFO) << uevent << " took " << t.duration().count() << "ms";
                }
                graph.MarkEventCompleted(uevent.seqnum);
            }
        });
    }

    uevent_listener.Poll([&graph](const Uevent& uevent) {
        graph.Add(uevent);
        return ListenerAction::kContinue;
    });
}

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::shared_ptr<UeventHandler>> uevent_handlers;

    auto ueventd_configuration = GetConfiguration();

    UeventListener uevent_listener(ueventd_configuration.uevent_socket_rcvbuf_size);

    // Right after making DeviceHandler, replay all events looking for which
    // block device has the boot partition. This lets us make symlinks
    // for all of the other partitions on the same disk. Note that by the time
    // we get here we know that the boot partition has already shown up (if
    // we're looking for it) so just regenerating events is enough to know
    // we'll see it.
    std::shared_ptr<DeviceHandler> device_handler = std::make_shared<DeviceHandler>(
            std::move(ueventd_configuration.dev_permissions),
            std::move(ueventd_configuration.sysfs_permissions),
            std::move(ueventd_configuration.drivers), std::move(ueventd_configuration.subsystems),
            android::fs_mgr::GetBootDevices(), android::fs_mgr::GetBootPartUuid(), true);
    uevent_listener.RegenerateUevents([&](const Uevent& uevent) -> ListenerAction {
        bool uuid_check_done = device_handler->CheckUeventForBootPartUuid(uevent);
        return uuid_check_done ? ListenerAction::kStop : ListenerAction::kContinue;
    });

    if (ueventd_configuration.enable_modalias_handling) {
        std::vector<std::string> base_paths = {"/odm/lib/modules", "/vendor/lib/modules"};
        uevent_handlers.emplace_back(std::make_shared<ModaliasHandler>(base_paths));
    }
    uevent_handlers.emplace_back(std::move(device_handler));
    uevent_handlers.emplace_back(std::make_shared<FirmwareHandler>(
            std::move(ueventd_configuration.firmware_directories),
            std::move(ueventd_configuration.external_firmware_handlers),
            /*serial_handler_after_cold_boot=*/false));

    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);

    if (ueventd_configuration.enable_parallel_ueventd_main_loop) {
        size_t num_threads =
                std::thread::hardware_concurrency() != 0 ? std::thread::hardware_concurrency() : 4;
        if (ueventd_configuration.parallel_main_loop_max_workers.has_value()) {
            num_threads = std::min(num_threads,
                                   ueventd_configuration.parallel_main_loop_max_workers.value());
        }
        parallel_main_loop(uevent_listener, uevent_handlers, num_threads);
    } else {
        main_loop(uevent_listener, uevent_handlers);
    }

    LOG(ERROR) << "main loop exited unexpectedly";
    return EXIT_FAILURE;
}

}  // namespace init
}  // namespace android