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Commit 1325ec8c authored by Ryan Prichard's avatar Ryan Prichard
Browse files

init: use signalfd to catch SIGCHLD 

Previously, if init received too many SIGCHLD signals, then the write to
signal_write_fd could fail with EAGAIN. The handler tried to log the
EAGAIN error, and init deadlocked if the interrupted init process had
already acquired a logging-related lock.

Bug: b/77867680
Test: manual
Change-Id: Ief0b5e94d8517827a5a7d03773391ba3ba9447c4
parent af15fbf9

init/init.cpp

+50 −27
Original line number Diff line number Diff line
@@ -79,7 +79,7 @@ static char qemu[32]; 
std::string default_console = "/dev/console";



static int epoll_fd = -1;

static int sigterm_signal_fd = -1;

static int signal_fd = -1;



static std::unique_ptr<Timer> waiting_for_prop(nullptr);

static std::string wait_prop_name;
@@ -492,14 +492,7 @@ static void InstallRebootSignalHandlers() { 
    sigaction(SIGTRAP, &action, nullptr);

}



static void HandleSigtermSignal() {

    signalfd_siginfo siginfo;

    ssize_t bytes_read = TEMP_FAILURE_RETRY(read(sigterm_signal_fd, &siginfo, sizeof(siginfo)));

    if (bytes_read != sizeof(siginfo)) {

        PLOG(ERROR) << "Failed to read siginfo from sigterm_signal_fd";

        return;

    }



static void HandleSigtermSignal(const signalfd_siginfo& siginfo) {

    if (siginfo.ssi_pid != 0) {

        // Drop any userspace SIGTERM requests.

        LOG(DEBUG) << "Ignoring SIGTERM from pid " << siginfo.ssi_pid;
@@ -509,37 +502,73 @@ static void HandleSigtermSignal() { 
    HandlePowerctlMessage("shutdown,container");

}



static void UnblockSigterm() {

static void HandleSignalFd() {

    signalfd_siginfo siginfo;

    ssize_t bytes_read = TEMP_FAILURE_RETRY(read(signal_fd, &siginfo, sizeof(siginfo)));

    if (bytes_read != sizeof(siginfo)) {

        PLOG(ERROR) << "Failed to read siginfo from signal_fd";

        return;

    }



    switch (siginfo.ssi_signo) {

        case SIGCHLD:

            ReapAnyOutstandingChildren();

            break;

        case SIGTERM:

            HandleSigtermSignal(siginfo);

            break;

        default:

            PLOG(ERROR) << "signal_fd: received unexpected signal " << siginfo.ssi_signo;

            break;

    }

}



static void UnblockSignals() {

    const struct sigaction act { .sa_handler = SIG_DFL };

    sigaction(SIGCHLD, &act, nullptr);



    sigset_t mask;

    sigemptyset(&mask);

    sigaddset(&mask, SIGCHLD);

    sigaddset(&mask, SIGTERM);



    if (sigprocmask(SIG_UNBLOCK, &mask, nullptr) == -1) {

        PLOG(FATAL) << "failed to unblock SIGTERM for PID " << getpid();

        PLOG(FATAL) << "failed to unblock signals for PID " << getpid();

    }

}



static void InstallSigtermHandler() {

static void InstallSignalFdHandler() {

    // Applying SA_NOCLDSTOP to a defaulted SIGCHLD handler prevents the signalfd from receiving

    // SIGCHLD when a child process stops or continues (b/77867680#comment9).

    const struct sigaction act { .sa_handler = SIG_DFL, .sa_flags = SA_NOCLDSTOP };

    sigaction(SIGCHLD, &act, nullptr);



    sigset_t mask;

    sigemptyset(&mask);

    sigaddset(&mask, SIGCHLD);



    if (!IsRebootCapable()) {

        // If init does not have the CAP_SYS_BOOT capability, it is running in a container.

        // In that case, receiving SIGTERM will cause the system to shut down.

        sigaddset(&mask, SIGTERM);

    }



    if (sigprocmask(SIG_BLOCK, &mask, nullptr) == -1) {

        PLOG(FATAL) << "failed to block SIGTERM";

        PLOG(FATAL) << "failed to block signals";

    }



    // Register a handler to unblock SIGTERM in the child processes.

    const int result = pthread_atfork(nullptr, nullptr, &UnblockSigterm);

    // Register a handler to unblock signals in the child processes.

    const int result = pthread_atfork(nullptr, nullptr, &UnblockSignals);

    if (result != 0) {

        LOG(FATAL) << "Failed to register a fork handler: " << strerror(result);

    }



    sigterm_signal_fd = signalfd(-1, &mask, SFD_CLOEXEC);

    if (sigterm_signal_fd == -1) {

        PLOG(FATAL) << "failed to create signalfd for SIGTERM";

    signal_fd = signalfd(-1, &mask, SFD_CLOEXEC);

    if (signal_fd == -1) {

        PLOG(FATAL) << "failed to create signalfd";

    }



    register_epoll_handler(sigterm_signal_fd, HandleSigtermSignal);

    register_epoll_handler(signal_fd, HandleSignalFd);

}



int main(int argc, char** argv) {
@@ -682,13 +711,7 @@ int main(int argc, char** argv) { 
        PLOG(FATAL) << "epoll_create1 failed";

    }



    sigchld_handler_init();



    if (!IsRebootCapable()) {

        // If init does not have the CAP_SYS_BOOT capability, it is running in a container.

        // In that case, receiving SIGTERM will cause the system to shut down.

        InstallSigtermHandler();

    }

    InstallSignalFdHandler();



    property_load_boot_defaults();

    export_oem_lock_status();

init/sigchld_handler.cpp

+0 −39
Original line number Diff line number Diff line
@@ -39,9 +39,6 @@ using android::base::make_scope_guard; 
namespace android {

namespace init {



static int signal_write_fd = -1;

static int signal_read_fd = -1;



static bool ReapOneProcess() {

    siginfo_t siginfo = {};

    // This returns a zombie pid or informs us that there are no zombies left to be reaped.
@@ -102,46 +99,10 @@ static bool ReapOneProcess() { 
    return true;

}



static void handle_signal() {

    // Clear outstanding requests.

    char buf[32];

    read(signal_read_fd, buf, sizeof(buf));



    ReapAnyOutstandingChildren();

}



static void SIGCHLD_handler(int) {

    if (TEMP_FAILURE_RETRY(write(signal_write_fd, "1", 1)) == -1) {

        PLOG(ERROR) << "write(signal_write_fd) failed";

    }

}



void ReapAnyOutstandingChildren() {

    while (ReapOneProcess()) {

    }

}



void sigchld_handler_init() {

    // Create a signalling mechanism for SIGCHLD.

    int s[2];

    if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0, s) == -1) {

        PLOG(FATAL) << "socketpair failed in sigchld_handler_init";

    }



    signal_write_fd = s[0];

    signal_read_fd = s[1];



    // Write to signal_write_fd if we catch SIGCHLD.

    struct sigaction act;

    memset(&act, 0, sizeof(act));

    act.sa_handler = SIGCHLD_handler;

    act.sa_flags = SA_NOCLDSTOP;

    sigaction(SIGCHLD, &act, 0);



    ReapAnyOutstandingChildren();



    register_epoll_handler(signal_read_fd, handle_signal);

}



}  // namespace init

}  // namespace android

init/sigchld_handler.h

+0 −2
Original line number Diff line number Diff line
@@ -22,8 +22,6 @@ namespace init { 


void ReapAnyOutstandingChildren();



void sigchld_handler_init(void);



}  // namespace init

}  // namespace android