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[Samba.git] / lib / util / tfork.c

/*
   fork on steroids to avoid SIGCHLD and waitpid

   Copyright (C) Stefan Metzmacher 2010
   Copyright (C) Ralph Boehme 2017

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "replace.h"
#include "system/wait.h"
#include "system/filesys.h"
#include "system/network.h"
#include "lib/util/samba_util.h"
#include "lib/util/sys_rw.h"
#include "lib/util/tfork.h"
#include "lib/util/debug.h"

#ifdef HAVE_PTHREAD
#include <pthread.h>
#endif

#ifdef NDEBUG
#undef NDEBUG
#endif
#include <assert.h>

/*
 * This is how the process hierarchy looks like:
 *
 *   +----------+
 *   |  caller  |
 *   +----------+
 *         |
 *       fork
 *         |
 *         v
 *   +----------+
 *   |  waiter  |
 *   +----------+
 *         |
 *       fork
 *         |
 *         v
 *   +----------+
 *   |  worker  |
 *   +----------+
 */

#ifdef HAVE_VALGRIND_HELGRIND_H
#include <valgrind/helgrind.h>
#endif
#ifndef ANNOTATE_BENIGN_RACE_SIZED
#define ANNOTATE_BENIGN_RACE_SIZED(obj, size, description)
#endif

#define TFORK_ANNOTATE_BENIGN_RACE(obj)                                 \
        ANNOTATE_BENIGN_RACE_SIZED(                                     \
                (obj), sizeof(*(obj)),                                  \
                "no race, serialized by tfork_[un]install_sigchld_handler");

/*
 * The resulting (private) state per tfork_create() call, returned as a opaque
 * handle to the caller.
 */
struct tfork {
        /*
         * This is returned to the caller with tfork_event_fd()
         */
        int event_fd;

        /*
         * This is used in the caller by tfork_status() to read the worker exit
         * status and to tell the waiter to exit by closing the fd.
         */
        int status_fd;

        pid_t waiter_pid;
        pid_t worker_pid;
};

/*
 * Internal per-thread state maintained while inside tfork.
 */
struct tfork_state {
        pid_t waiter_pid;
        int waiter_errno;

        pid_t worker_pid;
};

/*
 * A global state that synchronizes access to handling SIGCHLD and waiting for
 * childs.
 */
struct tfork_signal_state {
        bool available;

#ifdef HAVE_PTHREAD
        pthread_cond_t cond;
        pthread_mutex_t mutex;
#endif

        /*
         * pid of the waiter child. This points at waiter_pid in either struct
         * tfork or struct tfork_state, depending on who called
         * tfork_install_sigchld_handler().
         *
         * When tfork_install_sigchld_handler() is called the waiter_pid is
         * still -1 and only set later after fork(), that's why this is must be
         * a pointer. The signal handler checks this.
         */
        pid_t *pid;

        struct sigaction oldact;
        sigset_t oldset;
};

static struct tfork_signal_state signal_state;

#ifdef HAVE_PTHREAD
static pthread_once_t tfork_global_is_initialized = PTHREAD_ONCE_INIT;
static pthread_key_t tfork_global_key;
#else
static struct tfork_state *global_state;
#endif

static void tfork_sigchld_handler(int signum, siginfo_t *si, void *p);

#ifdef HAVE_PTHREAD
static void tfork_global_destructor(void *state)
{
        anonymous_shared_free(state);
}
#endif

static int tfork_acquire_sighandling(void)
{
        int ret = 0;

#ifdef HAVE_PTHREAD
        ret = pthread_mutex_lock(&signal_state.mutex);
        if (ret != 0) {
                return ret;
        }

        while (!signal_state.available) {
                ret = pthread_cond_wait(&signal_state.cond,
                                        &signal_state.mutex);
                if (ret != 0) {
                        return ret;
                }
        }

        signal_state.available = false;

        ret = pthread_mutex_unlock(&signal_state.mutex);
        if (ret != 0) {
                return ret;
        }
#endif

        return ret;
}

static int tfork_release_sighandling(void)
{
        int ret = 0;

#ifdef HAVE_PTHREAD
        ret = pthread_mutex_lock(&signal_state.mutex);
        if (ret != 0) {
                return ret;
        }

        signal_state.available = true;

        ret = pthread_cond_signal(&signal_state.cond);
        if (ret != 0) {
                pthread_mutex_unlock(&signal_state.mutex);
                return ret;
        }

        ret = pthread_mutex_unlock(&signal_state.mutex);
        if (ret != 0) {
                return ret;
        }
#endif

        return ret;
}

#ifdef HAVE_PTHREAD
static void tfork_atfork_prepare(void)
{
        int ret;

        ret = pthread_mutex_lock(&signal_state.mutex);
        assert(ret == 0);
}

static void tfork_atfork_parent(void)
{
        int ret;

        ret = pthread_mutex_unlock(&signal_state.mutex);
        assert(ret == 0);
}
#endif

static void tfork_atfork_child(void)
{
        int ret;

#ifdef HAVE_PTHREAD
        ret = pthread_mutex_unlock(&signal_state.mutex);
        assert(ret == 0);

        ret = pthread_key_delete(tfork_global_key);
        assert(ret == 0);

        ret = pthread_key_create(&tfork_global_key, tfork_global_destructor);
        assert(ret == 0);

        /*
         * There's no data race on the cond variable from the signal state, we
         * are writing here, but there are no readers yet. Some data race
         * detection tools report a race, but the readers are in the parent
         * process.
         */
        TFORK_ANNOTATE_BENIGN_RACE(&signal_state.cond);

        /*
         * There's no way to destroy a condition variable if there are waiters,
         * pthread_cond_destroy() will return EBUSY. Just zero out memory and
         * then initialize again. This is not backed by POSIX but should be ok.
         */
        ZERO_STRUCT(signal_state.cond);
        ret = pthread_cond_init(&signal_state.cond, NULL);
        assert(ret == 0);
#endif

        if (signal_state.pid != NULL) {

                ret = sigaction(SIGCHLD, &signal_state.oldact, NULL);
                assert(ret == 0);

#ifdef HAVE_PTHREAD
                ret = pthread_sigmask(SIG_SETMASK, &signal_state.oldset, NULL);
#else
                ret = sigprocmask(SIG_SETMASK, &signal_state.oldset, NULL);
                assert(ret == 0);
#endif

                signal_state.pid = NULL;
        }

        signal_state.available = true;
}

static void tfork_global_initialize(void)
{
#ifdef HAVE_PTHREAD
        int ret;

        pthread_atfork(tfork_atfork_prepare,
                       tfork_atfork_parent,
                       tfork_atfork_child);

        ret = pthread_key_create(&tfork_global_key, tfork_global_destructor);
        assert(ret == 0);

        ret = pthread_mutex_init(&signal_state.mutex, NULL);
        assert(ret == 0);

        ret = pthread_cond_init(&signal_state.cond, NULL);
        assert(ret == 0);

        /*
         * In a threaded process there's no data race on t->waiter_pid as
         * we're serializing globally via tfork_acquire_sighandling() and
         * tfork_release_sighandling().
         */
        TFORK_ANNOTATE_BENIGN_RACE(&signal_state.pid);
#endif

        signal_state.available = true;
}

static struct tfork_state *tfork_global_get(void)
{
        struct tfork_state *state = NULL;
#ifdef HAVE_PTHREAD
        int ret;
#endif

#ifdef HAVE_PTHREAD
        state = (struct tfork_state *)pthread_getspecific(tfork_global_key);
#else
        state = global_state;
#endif
        if (state != NULL) {
                return state;
        }

        state = (struct tfork_state *)anonymous_shared_allocate(
                sizeof(struct tfork_state));
        if (state == NULL) {
                return NULL;
        }

#ifdef HAVE_PTHREAD
        ret = pthread_setspecific(tfork_global_key, state);
        if (ret != 0) {
                anonymous_shared_free(state);
                return NULL;
        }
#endif
        return state;
}

static void tfork_global_free(void)
{
        struct tfork_state *state = NULL;
#ifdef HAVE_PTHREAD
        int ret;
#endif

#ifdef HAVE_PTHREAD
        state = (struct tfork_state *)pthread_getspecific(tfork_global_key);
#else
        state = global_state;
#endif
        if (state == NULL) {
                return;
        }

#ifdef HAVE_PTHREAD
        ret = pthread_setspecific(tfork_global_key, NULL);
        if (ret != 0) {
                return;
        }
#endif
        anonymous_shared_free(state);
}

/**
 * Only one thread at a time is allowed to handle SIGCHLD signals
 **/
static int tfork_install_sigchld_handler(pid_t *pid)
{
        int ret;
        struct sigaction act;
        sigset_t set;

        ret = tfork_acquire_sighandling();
        if (ret != 0) {
                return -1;
        }

        assert(signal_state.pid == NULL);
        signal_state.pid = pid;

        act = (struct sigaction) {
                .sa_sigaction = tfork_sigchld_handler,
                .sa_flags = SA_SIGINFO,
        };

        ret = sigaction(SIGCHLD, &act, &signal_state.oldact);
        if (ret != 0) {
                return -1;
        }

        sigemptyset(&set);
        sigaddset(&set, SIGCHLD);
#ifdef HAVE_PTHREAD
        ret = pthread_sigmask(SIG_UNBLOCK, &set, &signal_state.oldset);
#else
        ret = sigprocmask(SIG_UNBLOCK, &set, &signal_state.oldset);
#endif
        if (ret != 0) {
                return -1;
        }

        return 0;
}

static int tfork_uninstall_sigchld_handler(void)
{
        int ret;

        signal_state.pid = NULL;

        ret = sigaction(SIGCHLD, &signal_state.oldact, NULL);
        if (ret != 0) {
                return -1;
        }

#ifdef HAVE_PTHREAD
        ret = pthread_sigmask(SIG_SETMASK, &signal_state.oldset, NULL);
#else
        ret = sigprocmask(SIG_SETMASK, &signal_state.oldset, NULL);
#endif
        if (ret != 0) {
                return -1;
        }

        ret = tfork_release_sighandling();
        if (ret != 0) {
                return -1;
        }

        return 0;
}

static void tfork_sigchld_handler(int signum, siginfo_t *si, void *p)
{
        if ((signal_state.pid != NULL) &&
            (*signal_state.pid != -1) &&
            (si->si_pid == *signal_state.pid))
        {
                return;
        }

        /*
         * Not our child, forward to old handler
         */
        if (signal_state.oldact.sa_flags & SA_SIGINFO) {
                signal_state.oldact.sa_sigaction(signum, si, p);
                return;
        }

        if (signal_state.oldact.sa_handler == SIG_IGN) {
                return;
        }
        if (signal_state.oldact.sa_handler == SIG_DFL) {
                return;
        }
        signal_state.oldact.sa_handler(signum);
}

static pid_t tfork_start_waiter_and_worker(struct tfork_state *state,
                                           int *_event_fd,
                                           int *_status_fd)
{
        int p[2];
        int status_sp_caller_fd = -1;
        int status_sp_waiter_fd = -1;
        int event_pipe_caller_fd = -1;
        int event_pipe_waiter_fd = -1;
        int ready_pipe_caller_fd = -1;
        int ready_pipe_worker_fd = -1;
        ssize_t nwritten;
        ssize_t nread;
        pid_t pid;
        int status;
        int fd;
        char c;
        int ret;

        *_event_fd = -1;
        *_status_fd = -1;

        if (state == NULL) {
                return -1;
        }

        ret = socketpair(AF_UNIX, SOCK_STREAM, 0, p);
        if (ret != 0) {
                return -1;
        }
        set_close_on_exec(p[0]);
        set_close_on_exec(p[1]);
        status_sp_caller_fd = p[0];
        status_sp_waiter_fd = p[1];

        ret = pipe(p);
        if (ret != 0) {
                close(status_sp_caller_fd);
                close(status_sp_waiter_fd);
                return -1;
        }
        set_close_on_exec(p[0]);
        set_close_on_exec(p[1]);
        event_pipe_caller_fd = p[0];
        event_pipe_waiter_fd = p[1];


        ret = pipe(p);
        if (ret != 0) {
                close(status_sp_caller_fd);
                close(status_sp_waiter_fd);
                close(event_pipe_caller_fd);
                close(event_pipe_waiter_fd);
                return -1;
        }
        set_close_on_exec(p[0]);
        set_close_on_exec(p[1]);
        ready_pipe_worker_fd = p[0];
        ready_pipe_caller_fd = p[1];

        pid = fork();
        if (pid == -1) {
                close(status_sp_caller_fd);
                close(status_sp_waiter_fd);
                close(event_pipe_caller_fd);
                close(event_pipe_waiter_fd);
                close(ready_pipe_caller_fd);
                close(ready_pipe_worker_fd);
                return -1;
        }
        if (pid != 0) {
                /* The caller */

                /*
                 * In a threaded process there's no data race on
                 * state->waiter_pid as we're serializing globally via
                 * tfork_acquire_sighandling() and tfork_release_sighandling().
                 */
                TFORK_ANNOTATE_BENIGN_RACE(&state->waiter_pid);

                state->waiter_pid = pid;

                close(status_sp_waiter_fd);
                close(event_pipe_waiter_fd);
                close(ready_pipe_worker_fd);

                set_blocking(event_pipe_caller_fd, false);

                /*
                 * wait for the waiter to get ready.
                 */
                nread = sys_read(status_sp_caller_fd, &c, sizeof(char));
                if (nread != sizeof(char)) {
                        return -1;
                }

                /*
                 * Notify the worker to start.
                 */
                nwritten = sys_write(ready_pipe_caller_fd,
                                     &(char){0}, sizeof(char));
                if (nwritten != sizeof(char)) {
                        close(ready_pipe_caller_fd);
                        return -1;
                }
                close(ready_pipe_caller_fd);

                *_event_fd = event_pipe_caller_fd;
                *_status_fd = status_sp_caller_fd;

                return pid;
        }

#ifndef HAVE_PTHREAD
        /* cleanup sigchld_handler */
        tfork_atfork_child();
#endif

        /*
         * The "waiter" child.
         */
        setproctitle("tfork waiter process");
        CatchSignal(SIGCHLD, SIG_DFL);

        close(status_sp_caller_fd);
        close(event_pipe_caller_fd);
        close(ready_pipe_caller_fd);

        pid = fork();
        if (pid == -1) {
                state->waiter_errno = errno;
                _exit(0);
        }
        if (pid == 0) {
                /*
                 * The worker child.
                 */

                close(status_sp_waiter_fd);
                close(event_pipe_waiter_fd);

                /*
                 * Wait for the caller to give us a go!
                 */
                nread = sys_read(ready_pipe_worker_fd, &c, sizeof(char));
                if (nread != sizeof(char)) {
                        _exit(1);
                }
                close(ready_pipe_worker_fd);

                return 0;
        }
        state->worker_pid = pid;

        close(ready_pipe_worker_fd);

        /*
         * We're going to stay around until child2 exits, so lets close all fds
         * other then the pipe fd we may have inherited from the caller.
         *
         * Dup event_sp_waiter_fd and status_sp_waiter_fd onto fds 0 and 1 so we
         * can then call closefrom(2).
         */
        if (event_pipe_waiter_fd > 0) {
                int dup_fd = 0;

                if (status_sp_waiter_fd == 0) {
                        dup_fd = 1;
                }

                do {
                        fd = dup2(event_pipe_waiter_fd, dup_fd);
                } while ((fd == -1) && (errno == EINTR));
                if (fd == -1) {
                        state->waiter_errno = errno;
                        kill(state->worker_pid, SIGKILL);
                        state->worker_pid = -1;
                        _exit(1);
                }
                event_pipe_waiter_fd = fd;
        }

        if (status_sp_waiter_fd > 1) {
                do {
                        fd = dup2(status_sp_waiter_fd, 1);
                } while ((fd == -1) && (errno == EINTR));
                if (fd == -1) {
                        state->waiter_errno = errno;
                        kill(state->worker_pid, SIGKILL);
                        state->worker_pid = -1;
                        _exit(1);
                }
                status_sp_waiter_fd = fd;
        }

        closefrom(2);

        /* Tell the caller we're ready */
        nwritten = sys_write(status_sp_waiter_fd, &(char){0}, sizeof(char));
        if (nwritten != sizeof(char)) {
                _exit(1);
        }

        tfork_global_free();
        state = NULL;

        do {
                ret = waitpid(pid, &status, 0);
        } while ((ret == -1) && (errno == EINTR));
        if (ret == -1) {
                status = errno;
                kill(pid, SIGKILL);
        }

        /*
         * This writes the worker child exit status via our internal socketpair
         * so the tfork_status() implementation can read it from its end.
         */
        nwritten = sys_write(status_sp_waiter_fd, &status, sizeof(status));
        if (nwritten == -1) {
                if (errno != EPIPE && errno != ECONNRESET) {
                        _exit(errno);
                }
                /*
                 * The caller exitted and didn't call tfork_status().
                 */
                _exit(0);
        }
        if (nwritten != sizeof(status)) {
                _exit(1);
        }

        /*
         * This write to the event_fd returned by tfork_event_fd() and notifies
         * the caller that the worker child is done and he may now call
         * tfork_status().
         */
        nwritten = sys_write(event_pipe_waiter_fd, &(char){0}, sizeof(char));
        if (nwritten != sizeof(char)) {
                _exit(1);
        }

        /*
         * Wait for our parent (the process that called tfork_create()) to
         * close() the socketpair fd in tfork_status().
         *
         * Again, the caller might have exitted without calling tfork_status().
         */
        nread = sys_read(status_sp_waiter_fd, &c, 1);
        if (nread == -1) {
                if (errno == EPIPE || errno == ECONNRESET) {
                        _exit(0);
                }
                _exit(errno);
        }
        if (nread != 1) {
                _exit(255);
        }

        _exit(0);
}

static int tfork_create_reap_waiter(pid_t waiter_pid)
{
        pid_t pid;
        int waiter_status;

        if (waiter_pid == -1) {
                return 0;
        }

        kill(waiter_pid, SIGKILL);

        do {
                pid = waitpid(waiter_pid, &waiter_status, 0);
        } while ((pid == -1) && (errno == EINTR));
        assert(pid == waiter_pid);

        return 0;
}

struct tfork *tfork_create(void)
{
        struct tfork_state *state = NULL;
        struct tfork *t = NULL;
        pid_t pid;
        int saved_errno;
        int ret = 0;

#ifdef HAVE_PTHREAD
        ret = pthread_once(&tfork_global_is_initialized,
                           tfork_global_initialize);
        if (ret != 0) {
                return NULL;
        }
#else
        tfork_global_initialize();
#endif

        state = tfork_global_get();
        if (state == NULL) {
                return NULL;
        }
        *state = (struct tfork_state) {
                .waiter_pid = -1,
                .waiter_errno = ECANCELED,
                .worker_pid = -1,
        };

        t = malloc(sizeof(struct tfork));
        if (t == NULL) {
                ret = -1;
                goto cleanup;
        }

        *t = (struct tfork) {
                .event_fd = -1,
                .status_fd = -1,
                .waiter_pid = -1,
                .worker_pid = -1,
        };

        ret = tfork_install_sigchld_handler(&state->waiter_pid);
        if (ret != 0) {
                goto cleanup;
        }

        pid = tfork_start_waiter_and_worker(state,
                                            &t->event_fd,
                                            &t->status_fd);
        if (pid == -1) {
                ret = -1;
                goto cleanup;
        }
        if (pid == 0) {
                /* In the worker */
                tfork_global_free();
                t->worker_pid = 0;
                return t;
        }

        /*
         * In a threaded process there's no data race on t->waiter_pid as
         * we're serializing globally via tfork_acquire_sighandling() and
         * tfork_release_sighandling().
         */
        TFORK_ANNOTATE_BENIGN_RACE(&t->waiter_pid);

        t->waiter_pid = pid;
        t->worker_pid = state->worker_pid;

cleanup:
        if (ret == -1) {
                saved_errno = errno;

                if (t != NULL) {
                        if (t->status_fd != -1) {
                                close(t->status_fd);
                        }
                        if (t->event_fd != -1) {
                                close(t->event_fd);
                        }

                        ret = tfork_create_reap_waiter(state->waiter_pid);
                        assert(ret == 0);

                        free(t);
                        t = NULL;
                }
        }

        ret = tfork_uninstall_sigchld_handler();
        assert(ret == 0);

        tfork_global_free();

        if (ret == -1) {
                errno = saved_errno;
        }
        return t;
}

pid_t tfork_child_pid(const struct tfork *t)
{
        return t->worker_pid;
}

int tfork_event_fd(struct tfork *t)
{
        int fd = t->event_fd;

        assert(t->event_fd != -1);
        t->event_fd = -1;

        return fd;
}

int tfork_status(struct tfork **_t, bool wait)
{
        struct tfork *t = *_t;
        int status;
        ssize_t nread;
        int waiter_status;
        pid_t pid;
        int ret;

        if (t == NULL) {
                return -1;
        }

        if (wait) {
                set_blocking(t->status_fd, true);

                nread = sys_read(t->status_fd, &status, sizeof(int));
        } else {
                set_blocking(t->status_fd, false);

                nread = read(t->status_fd, &status, sizeof(int));
                if ((nread == -1) &&
                    ((errno == EAGAIN) || (errno == EWOULDBLOCK) || errno == EINTR)) {
                        errno = EAGAIN;
                        return -1;
                }
        }
        if (nread != sizeof(int)) {
                return -1;
        }

        ret = tfork_install_sigchld_handler(&t->waiter_pid);
        if (ret != 0) {
                return -1;
        }

        /*
         * This triggers process exit in the waiter.
         * We write to the fd as well as closing it, as any tforked sibling
         * processes will also have the writable end of this socket open.
         *
         */
        {
                size_t nwritten;
                nwritten = sys_write(t->status_fd, &(char){0}, sizeof(char));
                if (nwritten != sizeof(char)) {
                        close(t->status_fd);
                        return -1;
                }
        }
        close(t->status_fd);

        do {
                pid = waitpid(t->waiter_pid, &waiter_status, 0);
        } while ((pid == -1) && (errno == EINTR));
        assert(pid == t->waiter_pid);

        if (t->event_fd != -1) {
                close(t->event_fd);
                t->event_fd = -1;
        }

        free(t);
        t = NULL;
        *_t = NULL;

        ret = tfork_uninstall_sigchld_handler();
        assert(ret == 0);

        return status;
}

int tfork_destroy(struct tfork **_t)
{
        struct tfork *t = *_t;
        int ret;

        if (t == NULL) {
                errno = EINVAL;
                return -1;
        }

        kill(t->worker_pid, SIGKILL);

        ret = tfork_status(_t, true);
        if (ret == -1) {
                return -1;
        }

        return 0;
}