[lttng-ust.git] / liblttng-ust / lttng-ust-comm.c

/*
 * lttng-ust-comm.c
 *
 * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
 * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; only
 * version 2.1 of the License.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#define _LGPL_SOURCE
#define _GNU_SOURCE
#include <stddef.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <dlfcn.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <pthread.h>
#include <semaphore.h>
#include <time.h>
#include <assert.h>
#include <signal.h>
#include <limits.h>
#include <urcu/uatomic.h>
#include <urcu/futex.h>
#include <urcu/compiler.h>

#include <lttng/ust-events.h>
#include <lttng/ust-abi.h>
#include <lttng/ust.h>
#include <lttng/ust-error.h>
#include <lttng/ust-ctl.h>
#include <urcu/tls-compat.h>
#include <ust-comm.h>
#include <ust-fd.h>
#include <usterr-signal-safe.h>
#include <helper.h>
#include "tracepoint-internal.h"
#include "lttng-tracer-core.h"
#include "compat.h"
#include "../libringbuffer/rb-init.h"
#include "lttng-ust-statedump.h"
#include "clock.h"
#include "../libringbuffer/getcpu.h"
#include "getenv.h"

/* Concatenate lttng ust shared library name with its major version number. */
#define LTTNG_UST_LIB_SO_NAME "liblttng-ust.so." __ust_stringify(CONFIG_LTTNG_UST_LIBRARY_VERSION_MAJOR)

/*
 * Has lttng ust comm constructor been called ?
 */
static int initialized;

/*
 * The ust_lock/ust_unlock lock is used as a communication thread mutex.
 * Held when handling a command, also held by fork() to deal with
 * removal of threads, and by exit path.
 *
 * The UST lock is the centralized mutex across UST tracing control and
 * probe registration.
 *
 * ust_exit_mutex must never nest in ust_mutex.
 *
 * ust_fork_mutex must never nest in ust_mutex.
 *
 * ust_mutex_nest is a per-thread nesting counter, allowing the perf
 * counter lazy initialization called by events within the statedump,
 * which traces while the ust_mutex is held.
 *
 * ust_lock nests within the dynamic loader lock (within glibc) because
 * it is taken within the library constructor.
 *
 * The ust fd tracker lock nests within the ust_mutex.
 */
static pthread_mutex_t ust_mutex = PTHREAD_MUTEX_INITIALIZER;

/* Allow nesting the ust_mutex within the same thread. */
static DEFINE_URCU_TLS(int, ust_mutex_nest);

/*
 * ust_exit_mutex protects thread_active variable wrt thread exit. It
 * cannot be done by ust_mutex because pthread_cancel(), which takes an
 * internal libc lock, cannot nest within ust_mutex.
 *
 * It never nests within a ust_mutex.
 */
static pthread_mutex_t ust_exit_mutex = PTHREAD_MUTEX_INITIALIZER;

/*
 * ust_fork_mutex protects base address statedump tracing against forks. It
 * prevents the dynamic loader lock to be taken (by base address statedump
 * tracing) while a fork is happening, thus preventing deadlock issues with
 * the dynamic loader lock.
 */
static pthread_mutex_t ust_fork_mutex = PTHREAD_MUTEX_INITIALIZER;

/* Should the ust comm thread quit ? */
static int lttng_ust_comm_should_quit;

/*
 * This variable can be tested by applications to check whether
 * lttng-ust is loaded. They simply have to define their own
 * "lttng_ust_loaded" weak symbol, and test it. It is set to 1 by the
 * library constructor.
 */
int lttng_ust_loaded __attribute__((weak));

/*
 * Return 0 on success, -1 if should quit.
 * The lock is taken in both cases.
 * Signal-safe.
 */
int ust_lock(void)
{
        sigset_t sig_all_blocked, orig_mask;
        int ret, oldstate;

        ret = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &oldstate);
        if (ret) {
                ERR("pthread_setcancelstate: %s", strerror(ret));
        }
        if (oldstate != PTHREAD_CANCEL_ENABLE) {
                ERR("pthread_setcancelstate: unexpected oldstate");
        }
        sigfillset(&sig_all_blocked);
        ret = pthread_sigmask(SIG_SETMASK, &sig_all_blocked, &orig_mask);
        if (ret) {
                ERR("pthread_sigmask: %s", strerror(ret));
        }
        if (!URCU_TLS(ust_mutex_nest)++)
                pthread_mutex_lock(&ust_mutex);
        ret = pthread_sigmask(SIG_SETMASK, &orig_mask, NULL);
        if (ret) {
                ERR("pthread_sigmask: %s", strerror(ret));
        }
        if (lttng_ust_comm_should_quit) {
                return -1;
        } else {
                return 0;
        }
}

/*
 * ust_lock_nocheck() can be used in constructors/destructors, because
 * they are already nested within the dynamic loader lock, and therefore
 * have exclusive access against execution of liblttng-ust destructor.
 * Signal-safe.
 */
void ust_lock_nocheck(void)
{
        sigset_t sig_all_blocked, orig_mask;
        int ret, oldstate;

        ret = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &oldstate);
        if (ret) {
                ERR("pthread_setcancelstate: %s", strerror(ret));
        }
        if (oldstate != PTHREAD_CANCEL_ENABLE) {
                ERR("pthread_setcancelstate: unexpected oldstate");
        }
        sigfillset(&sig_all_blocked);
        ret = pthread_sigmask(SIG_SETMASK, &sig_all_blocked, &orig_mask);
        if (ret) {
                ERR("pthread_sigmask: %s", strerror(ret));
        }
        if (!URCU_TLS(ust_mutex_nest)++)
                pthread_mutex_lock(&ust_mutex);
        ret = pthread_sigmask(SIG_SETMASK, &orig_mask, NULL);
        if (ret) {
                ERR("pthread_sigmask: %s", strerror(ret));
        }
}

/*
 * Signal-safe.
 */
void ust_unlock(void)
{
        sigset_t sig_all_blocked, orig_mask;
        int ret, oldstate;

        sigfillset(&sig_all_blocked);
        ret = pthread_sigmask(SIG_SETMASK, &sig_all_blocked, &orig_mask);
        if (ret) {
                ERR("pthread_sigmask: %s", strerror(ret));
        }
        if (!--URCU_TLS(ust_mutex_nest))
                pthread_mutex_unlock(&ust_mutex);
        ret = pthread_sigmask(SIG_SETMASK, &orig_mask, NULL);
        if (ret) {
                ERR("pthread_sigmask: %s", strerror(ret));
        }
        ret = pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, &oldstate);
        if (ret) {
                ERR("pthread_setcancelstate: %s", strerror(ret));
        }
        if (oldstate != PTHREAD_CANCEL_DISABLE) {
                ERR("pthread_setcancelstate: unexpected oldstate");
        }
}

/*
 * Wait for either of these before continuing to the main
 * program:
 * - the register_done message from sessiond daemon
 *   (will let the sessiond daemon enable sessions before main
 *   starts.)
 * - sessiond daemon is not reachable.
 * - timeout (ensuring applications are resilient to session
 *   daemon problems).
 */
static sem_t constructor_wait;
/*
 * Doing this for both the global and local sessiond.
 */
enum {
        sem_count_initial_value = 4,
};

static int sem_count = sem_count_initial_value;

/*
 * Counting nesting within lttng-ust. Used to ensure that calling fork()
 * from liblttng-ust does not execute the pre/post fork handlers.
 */
static DEFINE_URCU_TLS(int, lttng_ust_nest_count);

/*
 * Info about socket and associated listener thread.
 */
struct sock_info {
        const char *name;
        pthread_t ust_listener; /* listener thread */
        int root_handle;
        int registration_done;
        int allowed;
        int global;
        int thread_active;

        char sock_path[PATH_MAX];
        int socket;
        int notify_socket;

        char wait_shm_path[PATH_MAX];
        char *wait_shm_mmap;
        /* Keep track of lazy state dump not performed yet. */
        int statedump_pending;
        int initial_statedump_done;
        /* Keep procname for statedump */
        char procname[LTTNG_UST_PROCNAME_LEN];
};

/* Socket from app (connect) to session daemon (listen) for communication */
struct sock_info global_apps = {
        .name = "global",
        .global = 1,

        .root_handle = -1,
        .registration_done = 0,
        .allowed = 0,
        .thread_active = 0,

        .sock_path = LTTNG_DEFAULT_RUNDIR "/" LTTNG_UST_SOCK_FILENAME,
        .socket = -1,
        .notify_socket = -1,

        .wait_shm_path = "/" LTTNG_UST_WAIT_FILENAME,

        .statedump_pending = 0,
        .initial_statedump_done = 0,
        .procname[0] = '\0'
};

/* TODO: allow global_apps_sock_path override */

struct sock_info local_apps = {
        .name = "local",
        .global = 0,
        .root_handle = -1,
        .registration_done = 0,
        .allowed = 0,   /* Check setuid bit first */
        .thread_active = 0,

        .socket = -1,
        .notify_socket = -1,

        .statedump_pending = 0,
        .initial_statedump_done = 0,
        .procname[0] = '\0'
};

static int wait_poll_fallback;

static const char *cmd_name_mapping[] = {
        [ LTTNG_UST_RELEASE ] = "Release",
        [ LTTNG_UST_SESSION ] = "Create Session",
        [ LTTNG_UST_TRACER_VERSION ] = "Get Tracer Version",

        [ LTTNG_UST_TRACEPOINT_LIST ] = "Create Tracepoint List",
        [ LTTNG_UST_WAIT_QUIESCENT ] = "Wait for Quiescent State",
        [ LTTNG_UST_REGISTER_DONE ] = "Registration Done",
        [ LTTNG_UST_TRACEPOINT_FIELD_LIST ] = "Create Tracepoint Field List",

        /* Session FD commands */
        [ LTTNG_UST_CHANNEL ] = "Create Channel",
        [ LTTNG_UST_SESSION_START ] = "Start Session",
        [ LTTNG_UST_SESSION_STOP ] = "Stop Session",

        /* Channel FD commands */
        [ LTTNG_UST_STREAM ] = "Create Stream",
        [ LTTNG_UST_EVENT ] = "Create Event",

        /* Event and Channel FD commands */
        [ LTTNG_UST_CONTEXT ] = "Create Context",
        [ LTTNG_UST_FLUSH_BUFFER ] = "Flush Buffer",

        /* Event, Channel and Session commands */
        [ LTTNG_UST_ENABLE ] = "Enable",
        [ LTTNG_UST_DISABLE ] = "Disable",

        /* Tracepoint list commands */
        [ LTTNG_UST_TRACEPOINT_LIST_GET ] = "List Next Tracepoint",
        [ LTTNG_UST_TRACEPOINT_FIELD_LIST_GET ] = "List Next Tracepoint Field",

        /* Event FD commands */
        [ LTTNG_UST_FILTER ] = "Create Filter",
        [ LTTNG_UST_EXCLUSION ] = "Add exclusions to event",
};

static const char *str_timeout;
static int got_timeout_env;

extern void lttng_ring_buffer_client_overwrite_init(void);
extern void lttng_ring_buffer_client_overwrite_rt_init(void);
extern void lttng_ring_buffer_client_discard_init(void);
extern void lttng_ring_buffer_client_discard_rt_init(void);
extern void lttng_ring_buffer_metadata_client_init(void);
extern void lttng_ring_buffer_client_overwrite_exit(void);
extern void lttng_ring_buffer_client_overwrite_rt_exit(void);
extern void lttng_ring_buffer_client_discard_exit(void);
extern void lttng_ring_buffer_client_discard_rt_exit(void);
extern void lttng_ring_buffer_metadata_client_exit(void);

static char *get_map_shm(struct sock_info *sock_info);

ssize_t lttng_ust_read(int fd, void *buf, size_t len)
{
        ssize_t ret;
        size_t copied = 0, to_copy = len;

        do {
                ret = read(fd, buf + copied, to_copy);
                if (ret > 0) {
                        copied += ret;
                        to_copy -= ret;
                }
        } while ((ret > 0 && to_copy > 0)
                || (ret < 0 && errno == EINTR));
        if (ret > 0) {
                ret = copied;
        }
        return ret;
}
/*
 * Returns the HOME directory path. Caller MUST NOT free(3) the returned
 * pointer.
 */
static
const char *get_lttng_home_dir(void)
{
       const char *val;

       val = (const char *) lttng_getenv("LTTNG_HOME");
       if (val != NULL) {
               return val;
       }
       return (const char *) lttng_getenv("HOME");
}

/*
 * Force a read (imply TLS fixup for dlopen) of TLS variables.
 */
static
void lttng_fixup_nest_count_tls(void)
{
        asm volatile ("" : : "m" (URCU_TLS(lttng_ust_nest_count)));
}

static
void lttng_fixup_ust_mutex_nest_tls(void)
{
        asm volatile ("" : : "m" (URCU_TLS(ust_mutex_nest)));
}

/*
 * Fixup urcu bp TLS.
 */
static
void lttng_fixup_urcu_bp_tls(void)
{
        rcu_read_lock();
        rcu_read_unlock();
}

void lttng_ust_fixup_tls(void)
{
        lttng_fixup_urcu_bp_tls();
        lttng_fixup_ringbuffer_tls();
        lttng_fixup_vtid_tls();
        lttng_fixup_nest_count_tls();
        lttng_fixup_procname_tls();
        lttng_fixup_ust_mutex_nest_tls();
        lttng_ust_fixup_perf_counter_tls();
        lttng_ust_fixup_fd_tracker_tls();
        lttng_fixup_cgroup_ns_tls();
        lttng_fixup_ipc_ns_tls();
        lttng_fixup_net_ns_tls();
        lttng_fixup_uts_ns_tls();
}

int lttng_get_notify_socket(void *owner)
{
        struct sock_info *info = owner;

        return info->notify_socket;
}


LTTNG_HIDDEN
char* lttng_ust_sockinfo_get_procname(void *owner)
{
        struct sock_info *info = owner;

        return info->procname;
}

static
void print_cmd(int cmd, int handle)
{
        const char *cmd_name = "Unknown";

        if (cmd >= 0 && cmd < LTTNG_ARRAY_SIZE(cmd_name_mapping)
                        && cmd_name_mapping[cmd]) {
                cmd_name = cmd_name_mapping[cmd];
        }
        DBG("Message Received \"%s\" (%d), Handle \"%s\" (%d)",
                cmd_name, cmd,
                lttng_ust_obj_get_name(handle), handle);
}

static
int setup_global_apps(void)
{
        int ret = 0;
        assert(!global_apps.wait_shm_mmap);

        global_apps.wait_shm_mmap = get_map_shm(&global_apps);
        if (!global_apps.wait_shm_mmap) {
                WARN("Unable to get map shm for global apps. Disabling LTTng-UST global tracing.");
                global_apps.allowed = 0;
                ret = -EIO;
                goto error;
        }

        global_apps.allowed = 1;
        lttng_ust_getprocname(global_apps.procname);
error:
        return ret;
}
static
int setup_local_apps(void)
{
        int ret = 0;
        const char *home_dir;
        uid_t uid;

        assert(!local_apps.wait_shm_mmap);

        uid = getuid();
        /*
         * Disallow per-user tracing for setuid binaries.
         */
        if (uid != geteuid()) {
                assert(local_apps.allowed == 0);
                ret = 0;
                goto end;
        }
        home_dir = get_lttng_home_dir();
        if (!home_dir) {
                WARN("HOME environment variable not set. Disabling LTTng-UST per-user tracing.");
                assert(local_apps.allowed == 0);
                ret = -ENOENT;
                goto end;
        }
        local_apps.allowed = 1;
        snprintf(local_apps.sock_path, PATH_MAX, "%s/%s/%s",
                home_dir,
                LTTNG_DEFAULT_HOME_RUNDIR,
                LTTNG_UST_SOCK_FILENAME);
        snprintf(local_apps.wait_shm_path, PATH_MAX, "/%s-%u",
                LTTNG_UST_WAIT_FILENAME,
                uid);

        local_apps.wait_shm_mmap = get_map_shm(&local_apps);
        if (!local_apps.wait_shm_mmap) {
                WARN("Unable to get map shm for local apps. Disabling LTTng-UST per-user tracing.");
                local_apps.allowed = 0;
                ret = -EIO;
                goto end;
        }

        lttng_ust_getprocname(local_apps.procname);
end:
        return ret;
}

/*
 * Get socket timeout, in ms.
 * -1: wait forever. 0: don't wait. >0: timeout, in ms.
 */
static
long get_timeout(void)
{
        long constructor_delay_ms = LTTNG_UST_DEFAULT_CONSTRUCTOR_TIMEOUT_MS;

        if (!got_timeout_env) {
                str_timeout = lttng_getenv("LTTNG_UST_REGISTER_TIMEOUT");
                got_timeout_env = 1;
        }
        if (str_timeout)
                constructor_delay_ms = strtol(str_timeout, NULL, 10);
        /* All negative values are considered as "-1". */
        if (constructor_delay_ms < -1)
                constructor_delay_ms = -1;
        return constructor_delay_ms;
}

/* Timeout for notify socket send and recv. */
static
long get_notify_sock_timeout(void)
{
        return get_timeout();
}

/* Timeout for connecting to cmd and notify sockets. */
static
long get_connect_sock_timeout(void)
{
        return get_timeout();
}

/*
 * Return values: -1: wait forever. 0: don't wait. 1: timeout wait.
 */
static
int get_constructor_timeout(struct timespec *constructor_timeout)
{
        long constructor_delay_ms;
        int ret;

        constructor_delay_ms = get_timeout();

        switch (constructor_delay_ms) {
        case -1:/* fall-through */
        case 0:
                return constructor_delay_ms;
        default:
                break;
        }

        /*
         * If we are unable to find the current time, don't wait.
         */
        ret = clock_gettime(CLOCK_REALTIME, constructor_timeout);
        if (ret) {
                /* Don't wait. */
                return 0;
        }
        constructor_timeout->tv_sec += constructor_delay_ms / 1000UL;
        constructor_timeout->tv_nsec +=
                (constructor_delay_ms % 1000UL) * 1000000UL;
        if (constructor_timeout->tv_nsec >= 1000000000UL) {
                constructor_timeout->tv_sec++;
                constructor_timeout->tv_nsec -= 1000000000UL;
        }
        /* Timeout wait (constructor_delay_ms). */
        return 1;
}

static
void get_allow_blocking(void)
{
        const char *str_allow_blocking =
                lttng_getenv("LTTNG_UST_ALLOW_BLOCKING");

        if (str_allow_blocking) {
                DBG("%s environment variable is set",
                        "LTTNG_UST_ALLOW_BLOCKING");
                lttng_ust_ringbuffer_set_allow_blocking();
        }
}

static
int register_to_sessiond(int socket, enum ustctl_socket_type type)
{
        return ustcomm_send_reg_msg(socket,
                type,
                CAA_BITS_PER_LONG,
                lttng_alignof(uint8_t) * CHAR_BIT,
                lttng_alignof(uint16_t) * CHAR_BIT,
                lttng_alignof(uint32_t) * CHAR_BIT,
                lttng_alignof(uint64_t) * CHAR_BIT,
                lttng_alignof(unsigned long) * CHAR_BIT);
}

static
int send_reply(int sock, struct ustcomm_ust_reply *lur)
{
        ssize_t len;

        len = ustcomm_send_unix_sock(sock, lur, sizeof(*lur));
        switch (len) {
        case sizeof(*lur):
                DBG("message successfully sent");
                return 0;
        default:
                if (len == -ECONNRESET) {
                        DBG("remote end closed connection");
                        return 0;
                }
                if (len < 0)
                        return len;
                DBG("incorrect message size: %zd", len);
                return -EINVAL;
        }
}

static
void decrement_sem_count(unsigned int count)
{
        int ret;

        assert(uatomic_read(&sem_count) >= count);

        if (uatomic_read(&sem_count) <= 0) {
                return;
        }

        ret = uatomic_add_return(&sem_count, -count);
        if (ret == 0) {
                ret = sem_post(&constructor_wait);
                assert(!ret);
        }
}

static
int handle_register_done(struct sock_info *sock_info)
{
        if (sock_info->registration_done)
                return 0;
        sock_info->registration_done = 1;

        decrement_sem_count(1);
        if (!sock_info->statedump_pending) {
                sock_info->initial_statedump_done = 1;
                decrement_sem_count(1);
        }

        return 0;
}

static
int handle_register_failed(struct sock_info *sock_info)
{
        if (sock_info->registration_done)
                return 0;
        sock_info->registration_done = 1;
        sock_info->initial_statedump_done = 1;

        decrement_sem_count(2);

        return 0;
}

/*
 * Only execute pending statedump after the constructor semaphore has
 * been posted by the current listener thread. This means statedump will
 * only be performed after the "registration done" command is received
 * from this thread's session daemon.
 *
 * This ensures we don't run into deadlock issues with the dynamic
 * loader mutex, which is held while the constructor is called and
 * waiting on the constructor semaphore. All operations requiring this
 * dynamic loader lock need to be postponed using this mechanism.
 *
 * In a scenario with two session daemons connected to the application,
 * it is possible that the first listener thread which receives the
 * registration done command issues its statedump while the dynamic
 * loader lock is still held by the application constructor waiting on
 * the semaphore. It will however be allowed to proceed when the
 * second session daemon sends the registration done command to the
 * second listener thread. This situation therefore does not produce
 * a deadlock.
 */
static
void handle_pending_statedump(struct sock_info *sock_info)
{
        if (sock_info->registration_done && sock_info->statedump_pending) {
                sock_info->statedump_pending = 0;
                pthread_mutex_lock(&ust_fork_mutex);
                lttng_handle_pending_statedump(sock_info);
                pthread_mutex_unlock(&ust_fork_mutex);

                if (!sock_info->initial_statedump_done) {
                        sock_info->initial_statedump_done = 1;
                        decrement_sem_count(1);
                }
        }
}

static
int handle_message(struct sock_info *sock_info,
                int sock, struct ustcomm_ust_msg *lum)
{
        int ret = 0;
        const struct lttng_ust_objd_ops *ops;
        struct ustcomm_ust_reply lur;
        union ust_args args;
        char ctxstr[LTTNG_UST_SYM_NAME_LEN];    /* App context string. */
        ssize_t len;

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

        if (ust_lock()) {
                ret = -LTTNG_UST_ERR_EXITING;
                goto error;
        }

        ops = objd_ops(lum->handle);
        if (!ops) {
                ret = -ENOENT;
                goto error;
        }

        switch (lum->cmd) {
        case LTTNG_UST_REGISTER_DONE:
                if (lum->handle == LTTNG_UST_ROOT_HANDLE)
                        ret = handle_register_done(sock_info);
                else
                        ret = -EINVAL;
                break;
        case LTTNG_UST_RELEASE:
                if (lum->handle == LTTNG_UST_ROOT_HANDLE)
                        ret = -EPERM;
                else
                        ret = lttng_ust_objd_unref(lum->handle, 1);
                break;
        case LTTNG_UST_FILTER:
        {
                /* Receive filter data */
                struct lttng_ust_filter_bytecode_node *bytecode;

                if (lum->u.filter.data_size > FILTER_BYTECODE_MAX_LEN) {
                        ERR("Filter data size is too large: %u bytes",
                                lum->u.filter.data_size);
                        ret = -EINVAL;
                        goto error;
                }

                if (lum->u.filter.reloc_offset > lum->u.filter.data_size) {
                        ERR("Filter reloc offset %u is not within data",
                                lum->u.filter.reloc_offset);
                        ret = -EINVAL;
                        goto error;
                }

                bytecode = zmalloc(sizeof(*bytecode) + lum->u.filter.data_size);
                if (!bytecode) {
                        ret = -ENOMEM;
                        goto error;
                }
                len = ustcomm_recv_unix_sock(sock, bytecode->bc.data,
                                lum->u.filter.data_size);
                switch (len) {
                case 0: /* orderly shutdown */
                        ret = 0;
                        free(bytecode);
                        goto error;
                default:
                        if (len == lum->u.filter.data_size) {
                                DBG("filter data received");
                                break;
                        } else if (len < 0) {
                                DBG("Receive failed from lttng-sessiond with errno %d", (int) -len);
                                if (len == -ECONNRESET) {
                                        ERR("%s remote end closed connection", sock_info->name);
                                        ret = len;
                                        free(bytecode);
                                        goto error;
                                }
                                ret = len;
                                free(bytecode);
                                goto error;
                        } else {
                                DBG("incorrect filter data message size: %zd", len);
                                ret = -EINVAL;
                                free(bytecode);
                                goto error;
                        }
                }
                bytecode->bc.len = lum->u.filter.data_size;
                bytecode->bc.reloc_offset = lum->u.filter.reloc_offset;
                bytecode->bc.seqnum = lum->u.filter.seqnum;
                if (ops->cmd) {
                        ret = ops->cmd(lum->handle, lum->cmd,
                                        (unsigned long) bytecode,
                                        &args, sock_info);
                        if (ret) {
                                free(bytecode);
                        }
                        /* don't free bytecode if everything went fine. */
                } else {
                        ret = -ENOSYS;
                        free(bytecode);
                }
                break;
        }
        case LTTNG_UST_EXCLUSION:
        {
                /* Receive exclusion names */
                struct lttng_ust_excluder_node *node;
                unsigned int count;

                count = lum->u.exclusion.count;
                if (count == 0) {
                        /* There are no names to read */
                        ret = 0;
                        goto error;
                }
                node = zmalloc(sizeof(*node) +
                                count * LTTNG_UST_SYM_NAME_LEN);
                if (!node) {
                        ret = -ENOMEM;
                        goto error;
                }
                node->excluder.count = count;
                len = ustcomm_recv_unix_sock(sock, node->excluder.names,
                                count * LTTNG_UST_SYM_NAME_LEN);
                switch (len) {
                case 0: /* orderly shutdown */
                        ret = 0;
                        free(node);
                        goto error;
                default:
                        if (len == count * LTTNG_UST_SYM_NAME_LEN) {
                                DBG("Exclusion data received");
                                break;
                        } else if (len < 0) {
                                DBG("Receive failed from lttng-sessiond with errno %d", (int) -len);
                                if (len == -ECONNRESET) {
                                        ERR("%s remote end closed connection", sock_info->name);
                                        ret = len;
                                        free(node);
                                        goto error;
                                }
                                ret = len;
                                free(node);
                                goto error;
                        } else {
                                DBG("Incorrect exclusion data message size: %zd", len);
                                ret = -EINVAL;
                                free(node);
                                goto error;
                        }
                }
                if (ops->cmd) {
                        ret = ops->cmd(lum->handle, lum->cmd,
                                        (unsigned long) node,
                                        &args, sock_info);
                        if (ret) {
                                free(node);
                        }
                        /* Don't free exclusion data if everything went fine. */
                } else {
                        ret = -ENOSYS;
                        free(node);
                }
                break;
        }
        case LTTNG_UST_CHANNEL:
        {
                void *chan_data;
                int wakeup_fd;

                len = ustcomm_recv_channel_from_sessiond(sock,
                                &chan_data, lum->u.channel.len,
                                &wakeup_fd);
                switch (len) {
                case 0: /* orderly shutdown */
                        ret = 0;
                        goto error;
                default:
                        if (len == lum->u.channel.len) {
                                DBG("channel data received");
                                break;
                        } else if (len < 0) {
                                DBG("Receive failed from lttng-sessiond with errno %d", (int) -len);
                                if (len == -ECONNRESET) {
                                        ERR("%s remote end closed connection", sock_info->name);
                                        ret = len;
                                        goto error;
                                }
                                ret = len;
                                goto error;
                        } else {
                                DBG("incorrect channel data message size: %zd", len);
                                ret = -EINVAL;
                                goto error;
                        }
                }
                args.channel.chan_data = chan_data;
                args.channel.wakeup_fd = wakeup_fd;
                if (ops->cmd)
                        ret = ops->cmd(lum->handle, lum->cmd,
                                        (unsigned long) &lum->u,
                                        &args, sock_info);
                else
                        ret = -ENOSYS;
                break;
        }
        case LTTNG_UST_STREAM:
        {
                /* Receive shm_fd, wakeup_fd */
                ret = ustcomm_recv_stream_from_sessiond(sock,
                        NULL,
                        &args.stream.shm_fd,
                        &args.stream.wakeup_fd);
                if (ret) {
                        goto error;
                }

                if (ops->cmd)
                        ret = ops->cmd(lum->handle, lum->cmd,
                                        (unsigned long) &lum->u,
                                        &args, sock_info);
                else
                        ret = -ENOSYS;
                break;
        }
        case LTTNG_UST_CONTEXT:
                switch (lum->u.context.ctx) {
                case LTTNG_UST_CONTEXT_APP_CONTEXT:
                {
                        char *p;
                        size_t ctxlen, recvlen;

                        ctxlen = strlen("$app.") + lum->u.context.u.app_ctx.provider_name_len - 1
                                        + strlen(":") + lum->u.context.u.app_ctx.ctx_name_len;
                        if (ctxlen >= LTTNG_UST_SYM_NAME_LEN) {
                                ERR("Application context string length size is too large: %zu bytes",
                                        ctxlen);
                                ret = -EINVAL;
                                goto error;
                        }
                        strcpy(ctxstr, "$app.");
                        p = &ctxstr[strlen("$app.")];
                        recvlen = ctxlen - strlen("$app.");
                        len = ustcomm_recv_unix_sock(sock, p, recvlen);
                        switch (len) {
                        case 0: /* orderly shutdown */
                                ret = 0;
                                goto error;
                        default:
                                if (len == recvlen) {
                                        DBG("app context data received");
                                        break;
                                } else if (len < 0) {
                                        DBG("Receive failed from lttng-sessiond with errno %d", (int) -len);
                                        if (len == -ECONNRESET) {
                                                ERR("%s remote end closed connection", sock_info->name);
                                                ret = len;
                                                goto error;
                                        }
                                        ret = len;
                                        goto error;
                                } else {
                                        DBG("incorrect app context data message size: %zd", len);
                                        ret = -EINVAL;
                                        goto error;
                                }
                        }
                        /* Put : between provider and ctxname. */
                        p[lum->u.context.u.app_ctx.provider_name_len - 1] = ':';
                        args.app_context.ctxname = ctxstr;
                        break;
                }
                default:
                        break;
                }
                if (ops->cmd) {
                        ret = ops->cmd(lum->handle, lum->cmd,
                                        (unsigned long) &lum->u,
                                        &args, sock_info);
                } else {
                        ret = -ENOSYS;
                }
                break;
        default:
                if (ops->cmd)
                        ret = ops->cmd(lum->handle, lum->cmd,
                                        (unsigned long) &lum->u,
                                        &args, sock_info);
                else
                        ret = -ENOSYS;
                break;
        }

        lur.handle = lum->handle;
        lur.cmd = lum->cmd;
        lur.ret_val = ret;
        if (ret >= 0) {
                lur.ret_code = LTTNG_UST_OK;
        } else {
                /*
                 * Use -LTTNG_UST_ERR as wildcard for UST internal
                 * error that are not caused by the transport, except if
                 * we already have a more precise error message to
                 * report.
                 */
                if (ret > -LTTNG_UST_ERR) {
                        /* Translate code to UST error. */
                        switch (ret) {
                        case -EEXIST:
                                lur.ret_code = -LTTNG_UST_ERR_EXIST;
                                break;
                        case -EINVAL:
                                lur.ret_code = -LTTNG_UST_ERR_INVAL;
                                break;
                        case -ENOENT:
                                lur.ret_code = -LTTNG_UST_ERR_NOENT;
                                break;
                        case -EPERM:
                                lur.ret_code = -LTTNG_UST_ERR_PERM;
                                break;
                        case -ENOSYS:
                                lur.ret_code = -LTTNG_UST_ERR_NOSYS;
                                break;
                        default:
                                lur.ret_code = -LTTNG_UST_ERR;
                                break;
                        }
                } else {
                        lur.ret_code = ret;
                }
        }
        if (ret >= 0) {
                switch (lum->cmd) {
                case LTTNG_UST_TRACER_VERSION:
                        lur.u.version = lum->u.version;
                        break;
                case LTTNG_UST_TRACEPOINT_LIST_GET:
                        memcpy(&lur.u.tracepoint, &lum->u.tracepoint, sizeof(lur.u.tracepoint));
                        break;
                }
        }
        DBG("Return value: %d", lur.ret_val);

        ust_unlock();

        /*
         * Performed delayed statedump operations outside of the UST
         * lock. We need to take the dynamic loader lock before we take
         * the UST lock internally within handle_pending_statedump().
          */
        handle_pending_statedump(sock_info);

        if (ust_lock()) {
                ret = -LTTNG_UST_ERR_EXITING;
                goto error;
        }

        ret = send_reply(sock, &lur);
        if (ret < 0) {
                DBG("error sending reply");
                goto error;
        }

        /*
         * LTTNG_UST_TRACEPOINT_FIELD_LIST_GET needs to send the field
         * after the reply.
         */
        if (lur.ret_code == LTTNG_UST_OK) {
                switch (lum->cmd) {
                case LTTNG_UST_TRACEPOINT_FIELD_LIST_GET:
                        len = ustcomm_send_unix_sock(sock,
                                &args.field_list.entry,
                                sizeof(args.field_list.entry));
                        if (len < 0) {
                                ret = len;
                                goto error;
                        }
                        if (len != sizeof(args.field_list.entry)) {
                                ret = -EINVAL;
                                goto error;
                        }
                }
        }

error:
        ust_unlock();

        return ret;
}

static
void cleanup_sock_info(struct sock_info *sock_info, int exiting)
{
        int ret;

        if (sock_info->root_handle != -1) {
                ret = lttng_ust_objd_unref(sock_info->root_handle, 1);
                if (ret) {
                        ERR("Error unref root handle");
                }
                sock_info->root_handle = -1;
        }
        sock_info->registration_done = 0;
        sock_info->initial_statedump_done = 0;

        /*
         * wait_shm_mmap, socket and notify socket are used by listener
         * threads outside of the ust lock, so we cannot tear them down
         * ourselves, because we cannot join on these threads. Leave
         * responsibility of cleaning up these resources to the OS
         * process exit.
         */
        if (exiting)
                return;

        if (sock_info->socket != -1) {
                ret = ustcomm_close_unix_sock(sock_info->socket);
                if (ret) {
                        ERR("Error closing ust cmd socket");
                }
                sock_info->socket = -1;
        }
        if (sock_info->notify_socket != -1) {
                ret = ustcomm_close_unix_sock(sock_info->notify_socket);
                if (ret) {
                        ERR("Error closing ust notify socket");
                }
                sock_info->notify_socket = -1;
        }
        if (sock_info->wait_shm_mmap) {
                long page_size;

                page_size = sysconf(_SC_PAGE_SIZE);
                if (page_size <= 0) {
                        if (!page_size) {
                                errno = EINVAL;
                        }
                        PERROR("Error in sysconf(_SC_PAGE_SIZE)");
                } else {
                        ret = munmap(sock_info->wait_shm_mmap, page_size);
                        if (ret) {
                                ERR("Error unmapping wait shm");
                        }
                }
                sock_info->wait_shm_mmap = NULL;
        }
}

/*
 * Using fork to set umask in the child process (not multi-thread safe).
 * We deal with the shm_open vs ftruncate race (happening when the
 * sessiond owns the shm and does not let everybody modify it, to ensure
 * safety against shm_unlink) by simply letting the mmap fail and
 * retrying after a few seconds.
 * For global shm, everybody has rw access to it until the sessiond
 * starts.
 */
static
int get_wait_shm(struct sock_info *sock_info, size_t mmap_size)
{
        int wait_shm_fd, ret;
        pid_t pid;

        /*
         * Try to open read-only.
         */
        wait_shm_fd = shm_open(sock_info->wait_shm_path, O_RDONLY, 0);
        if (wait_shm_fd >= 0) {
                int32_t tmp_read;
                ssize_t len;
                size_t bytes_read = 0;

                /*
                 * Try to read the fd. If unable to do so, try opening
                 * it in write mode.
                 */
                do {
                        len = read(wait_shm_fd,
                                &((char *) &tmp_read)[bytes_read],
                                sizeof(tmp_read) - bytes_read);
                        if (len > 0) {
                                bytes_read += len;
                        }
                } while ((len < 0 && errno == EINTR)
                        || (len > 0 && bytes_read < sizeof(tmp_read)));
                if (bytes_read != sizeof(tmp_read)) {
                        ret = close(wait_shm_fd);
                        if (ret) {
                                ERR("close wait_shm_fd");
                        }
                        goto open_write;
                }
                goto end;
        } else if (wait_shm_fd < 0 && errno != ENOENT) {
                /*
                 * Real-only open did not work, and it's not because the
                 * entry was not present. It's a failure that prohibits
                 * using shm.
                 */
                ERR("Error opening shm %s", sock_info->wait_shm_path);
                goto end;
        }

open_write:
        /*
         * If the open failed because the file did not exist, or because
         * the file was not truncated yet, try creating it ourself.
         */
        URCU_TLS(lttng_ust_nest_count)++;
        pid = fork();
        URCU_TLS(lttng_ust_nest_count)--;
        if (pid > 0) {
                int status;

                /*
                 * Parent: wait for child to return, in which case the
                 * shared memory map will have been created.
                 */
                pid = wait(&status);
                if (pid < 0 || !WIFEXITED(status) || WEXITSTATUS(status) != 0) {
                        wait_shm_fd = -1;
                        goto end;
                }
                /*
                 * Try to open read-only again after creation.
                 */
                wait_shm_fd = shm_open(sock_info->wait_shm_path, O_RDONLY, 0);
                if (wait_shm_fd < 0) {
                        /*
                         * Real-only open did not work. It's a failure
                         * that prohibits using shm.
                         */
                        ERR("Error opening shm %s", sock_info->wait_shm_path);
                        goto end;
                }
                goto end;
        } else if (pid == 0) {
                int create_mode;

                /* Child */
                create_mode = S_IRUSR | S_IWUSR | S_IRGRP;
                if (sock_info->global)
                        create_mode |= S_IROTH | S_IWGRP | S_IWOTH;
                /*
                 * We're alone in a child process, so we can modify the
                 * process-wide umask.
                 */
                umask(~create_mode);
                /*
                 * Try creating shm (or get rw access).
                 * We don't do an exclusive open, because we allow other
                 * processes to create+ftruncate it concurrently.
                 */
                wait_shm_fd = shm_open(sock_info->wait_shm_path,
                                O_RDWR | O_CREAT, create_mode);
                if (wait_shm_fd >= 0) {
                        ret = ftruncate(wait_shm_fd, mmap_size);
                        if (ret) {
                                PERROR("ftruncate");
                                _exit(EXIT_FAILURE);
                        }
                        _exit(EXIT_SUCCESS);
                }
                /*
                 * For local shm, we need to have rw access to accept
                 * opening it: this means the local sessiond will be
                 * able to wake us up. For global shm, we open it even
                 * if rw access is not granted, because the root.root
                 * sessiond will be able to override all rights and wake
                 * us up.
                 */
                if (!sock_info->global && errno != EACCES) {
                        ERR("Error opening shm %s", sock_info->wait_shm_path);
                        _exit(EXIT_FAILURE);
                }
                /*
                 * The shm exists, but we cannot open it RW. Report
                 * success.
                 */
                _exit(EXIT_SUCCESS);
        } else {
                return -1;
        }
end:
        if (wait_shm_fd >= 0 && !sock_info->global) {
                struct stat statbuf;

                /*
                 * Ensure that our user is the owner of the shm file for
                 * local shm. If we do not own the file, it means our
                 * sessiond will not have access to wake us up (there is
                 * probably a rogue process trying to fake our
                 * sessiond). Fallback to polling method in this case.
                 */
                ret = fstat(wait_shm_fd, &statbuf);
                if (ret) {
                        PERROR("fstat");
                        goto error_close;
                }
                if (statbuf.st_uid != getuid())
                        goto error_close;
        }
        return wait_shm_fd;

error_close:
        ret = close(wait_shm_fd);
        if (ret) {
                PERROR("Error closing fd");
        }
        return -1;
}

static
char *get_map_shm(struct sock_info *sock_info)
{
        long page_size;
        int wait_shm_fd, ret;
        char *wait_shm_mmap;

        page_size = sysconf(_SC_PAGE_SIZE);
        if (page_size <= 0) {
                if (!page_size) {
                        errno = EINVAL;
                }
                PERROR("Error in sysconf(_SC_PAGE_SIZE)");
                goto error;
        }

        lttng_ust_lock_fd_tracker();
        wait_shm_fd = get_wait_shm(sock_info, page_size);
        if (wait_shm_fd < 0) {
                lttng_ust_unlock_fd_tracker();
                goto error;
        }

        ret = lttng_ust_add_fd_to_tracker(wait_shm_fd);
        if (ret < 0) {
                ret = close(wait_shm_fd);
                if (!ret) {
                        PERROR("Error closing fd");
                }
                lttng_ust_unlock_fd_tracker();
                goto error;
        }

        wait_shm_fd = ret;
        lttng_ust_unlock_fd_tracker();

        wait_shm_mmap = mmap(NULL, page_size, PROT_READ,
                  MAP_SHARED, wait_shm_fd, 0);

        /* close shm fd immediately after taking the mmap reference */
        lttng_ust_lock_fd_tracker();
        ret = close(wait_shm_fd);
        if (!ret) {
                lttng_ust_delete_fd_from_tracker(wait_shm_fd);
        } else {
                PERROR("Error closing fd");
        }
        lttng_ust_unlock_fd_tracker();

        if (wait_shm_mmap == MAP_FAILED) {
                DBG("mmap error (can be caused by race with sessiond). Fallback to poll mode.");
                goto error;
        }
        return wait_shm_mmap;

error:
        return NULL;
}

static
void wait_for_sessiond(struct sock_info *sock_info)
{
        /* Use ust_lock to check if we should quit. */
        if (ust_lock()) {
                goto quit;
        }
        if (wait_poll_fallback) {
                goto error;
        }
        ust_unlock();

        assert(sock_info->wait_shm_mmap);

        DBG("Waiting for %s apps sessiond", sock_info->name);
        /* Wait for futex wakeup */
        if (uatomic_read((int32_t *) sock_info->wait_shm_mmap))
                goto end_wait;

        while (futex_async((int32_t *) sock_info->wait_shm_mmap,
                        FUTEX_WAIT, 0, NULL, NULL, 0)) {
                switch (errno) {
                case EWOULDBLOCK:
                        /* Value already changed. */
                        goto end_wait;
                case EINTR:
                        /* Retry if interrupted by signal. */
                        break;  /* Get out of switch. */
                case EFAULT:
                        wait_poll_fallback = 1;
                        DBG(
"Linux kernels 2.6.33 to 3.0 (with the exception of stable versions) "
"do not support FUTEX_WAKE on read-only memory mappings correctly. "
"Please upgrade your kernel "
"(fix is commit 9ea71503a8ed9184d2d0b8ccc4d269d05f7940ae in Linux kernel "
"mainline). LTTng-UST will use polling mode fallback.");
                        if (ust_debug())
                                PERROR("futex");
                        goto end_wait;
                }
        }
end_wait:
        return;

quit:
        ust_unlock();
        return;

error:
        ust_unlock();
        return;
}

/*
 * This thread does not allocate any resource, except within
 * handle_message, within mutex protection. This mutex protects against
 * fork and exit.
 * The other moment it allocates resources is at socket connection, which
 * is also protected by the mutex.
 */
static
void *ust_listener_thread(void *arg)
{
        struct sock_info *sock_info = arg;
        int sock, ret, prev_connect_failed = 0, has_waited = 0, fd;
        long timeout;

        lttng_ust_fixup_tls();
        /*
         * If available, add '-ust' to the end of this thread's
         * process name
         */
        ret = lttng_ust_setustprocname();
        if (ret) {
                ERR("Unable to set UST process name");
        }

        /* Restart trying to connect to the session daemon */
restart:
        if (prev_connect_failed) {
                /* Wait for sessiond availability with pipe */
                wait_for_sessiond(sock_info);
                if (has_waited) {
                        has_waited = 0;
                        /*
                         * Sleep for 5 seconds before retrying after a
                         * sequence of failure / wait / failure. This
                         * deals with a killed or broken session daemon.
                         */
                        sleep(5);
                } else {
                        has_waited = 1;
                }
                prev_connect_failed = 0;
        }

        if (ust_lock()) {
                goto quit;
        }

        if (sock_info->socket != -1) {
                /* FD tracker is updated by ustcomm_close_unix_sock() */
                ret = ustcomm_close_unix_sock(sock_info->socket);
                if (ret) {
                        ERR("Error closing %s ust cmd socket",
                                sock_info->name);
                }
                sock_info->socket = -1;
        }
        if (sock_info->notify_socket != -1) {
                /* FD tracker is updated by ustcomm_close_unix_sock() */
                ret = ustcomm_close_unix_sock(sock_info->notify_socket);
                if (ret) {
                        ERR("Error closing %s ust notify socket",
                                sock_info->name);
                }
                sock_info->notify_socket = -1;
        }


        /*
         * Register. We need to perform both connect and sending
         * registration message before doing the next connect otherwise
         * we may reach unix socket connect queue max limits and block
         * on the 2nd connect while the session daemon is awaiting the
         * first connect registration message.
         */
        /* Connect cmd socket */
        lttng_ust_lock_fd_tracker();
        ret = ustcomm_connect_unix_sock(sock_info->sock_path,
                get_connect_sock_timeout());
        if (ret < 0) {
                lttng_ust_unlock_fd_tracker();
                DBG("Info: sessiond not accepting connections to %s apps socket", sock_info->name);
                prev_connect_failed = 1;

                /*
                 * If we cannot find the sessiond daemon, don't delay
                 * constructor execution.
                 */
                ret = handle_register_failed(sock_info);
                assert(!ret);
                ust_unlock();
                goto restart;
        }
        fd = ret;
        ret = lttng_ust_add_fd_to_tracker(fd);
        if (ret < 0) {
                ret = close(fd);
                if (ret) {
                        PERROR("close on sock_info->socket");
                }
                ret = -1;
                lttng_ust_unlock_fd_tracker();
                ust_unlock();
                goto quit;
        }

        sock_info->socket = ret;
        lttng_ust_unlock_fd_tracker();

        ust_unlock();
        /*
         * Unlock/relock ust lock because connect is blocking (with
         * timeout). Don't delay constructors on the ust lock for too
         * long.
         */
        if (ust_lock()) {
                goto quit;
        }

        /*
         * Create only one root handle per listener thread for the whole
         * process lifetime, so we ensure we get ID which is statically
         * assigned to the root handle.
         */
        if (sock_info->root_handle == -1) {
                ret = lttng_abi_create_root_handle();
                if (ret < 0) {
                        ERR("Error creating root handle");
                        goto quit;
                }
                sock_info->root_handle = ret;
        }

        ret = register_to_sessiond(sock_info->socket, USTCTL_SOCKET_CMD);
        if (ret < 0) {
                ERR("Error registering to %s ust cmd socket",
                        sock_info->name);
                prev_connect_failed = 1;
                /*
                 * If we cannot register to the sessiond daemon, don't
                 * delay constructor execution.
                 */
                ret = handle_register_failed(sock_info);
                assert(!ret);
                ust_unlock();
                goto restart;
        }

        ust_unlock();
        /*
         * Unlock/relock ust lock because connect is blocking (with
         * timeout). Don't delay constructors on the ust lock for too
         * long.
         */
        if (ust_lock()) {
                goto quit;
        }

        /* Connect notify socket */
        lttng_ust_lock_fd_tracker();
        ret = ustcomm_connect_unix_sock(sock_info->sock_path,
                get_connect_sock_timeout());
        if (ret < 0) {
                lttng_ust_unlock_fd_tracker();
                DBG("Info: sessiond not accepting connections to %s apps socket", sock_info->name);
                prev_connect_failed = 1;

                /*
                 * If we cannot find the sessiond daemon, don't delay
                 * constructor execution.
                 */
                ret = handle_register_failed(sock_info);
                assert(!ret);
                ust_unlock();
                goto restart;
        }

        fd = ret;
        ret = lttng_ust_add_fd_to_tracker(fd);
        if (ret < 0) {
                ret = close(fd);
                if (ret) {
                        PERROR("close on sock_info->notify_socket");
                }
                ret = -1;
                lttng_ust_unlock_fd_tracker();
                ust_unlock();
                goto quit;
        }

        sock_info->notify_socket = ret;
        lttng_ust_unlock_fd_tracker();

        ust_unlock();
        /*
         * Unlock/relock ust lock because connect is blocking (with
         * timeout). Don't delay constructors on the ust lock for too
         * long.
         */
        if (ust_lock()) {
                goto quit;
        }

        timeout = get_notify_sock_timeout();
        if (timeout >= 0) {
                /*
                 * Give at least 10ms to sessiond to reply to
                 * notifications.
                 */
                if (timeout < 10)
                        timeout = 10;
                ret = ustcomm_setsockopt_rcv_timeout(sock_info->notify_socket,
                                timeout);
                if (ret < 0) {
                        WARN("Error setting socket receive timeout");
                }
                ret = ustcomm_setsockopt_snd_timeout(sock_info->notify_socket,
                                timeout);
                if (ret < 0) {
                        WARN("Error setting socket send timeout");
                }
        } else if (timeout < -1) {
                WARN("Unsupported timeout value %ld", timeout);
        }

        ret = register_to_sessiond(sock_info->notify_socket,
                        USTCTL_SOCKET_NOTIFY);
        if (ret < 0) {
                ERR("Error registering to %s ust notify socket",
                        sock_info->name);
                prev_connect_failed = 1;
                /*
                 * If we cannot register to the sessiond daemon, don't
                 * delay constructor execution.
                 */
                ret = handle_register_failed(sock_info);
                assert(!ret);
                ust_unlock();
                goto restart;
        }
        sock = sock_info->socket;

        ust_unlock();

        for (;;) {
                ssize_t len;
                struct ustcomm_ust_msg lum;

                len = ustcomm_recv_unix_sock(sock, &lum, sizeof(lum));
                switch (len) {
                case 0: /* orderly shutdown */
                        DBG("%s lttng-sessiond has performed an orderly shutdown", sock_info->name);
                        if (ust_lock()) {
                                goto quit;
                        }
                        /*
                         * Either sessiond has shutdown or refused us by closing the socket.
                         * In either case, we don't want to delay construction execution,
                         * and we need to wait before retry.
                         */
                        prev_connect_failed = 1;
                        /*
                         * If we cannot register to the sessiond daemon, don't
                         * delay constructor execution.
                         */
                        ret = handle_register_failed(sock_info);
                        assert(!ret);
                        ust_unlock();
                        goto end;
                case sizeof(lum):
                        print_cmd(lum.cmd, lum.handle);
                        ret = handle_message(sock_info, sock, &lum);
                        if (ret) {
                                ERR("Error handling message for %s socket",
                                        sock_info->name);
                                /*
                                 * Close socket if protocol error is
                                 * detected.
                                 */
                                goto end;
                        }
                        continue;
                default:
                        if (len < 0) {
                                DBG("Receive failed from lttng-sessiond with errno %d", (int) -len);
                        } else {
                                DBG("incorrect message size (%s socket): %zd", sock_info->name, len);
                        }
                        if (len == -ECONNRESET) {
                                DBG("%s remote end closed connection", sock_info->name);
                                goto end;
                        }
                        goto end;
                }

        }
end:
        if (ust_lock()) {
                goto quit;
        }
        /* Cleanup socket handles before trying to reconnect */
        lttng_ust_objd_table_owner_cleanup(sock_info);
        ust_unlock();
        goto restart;   /* try to reconnect */

quit:
        ust_unlock();

        pthread_mutex_lock(&ust_exit_mutex);
        sock_info->thread_active = 0;
        pthread_mutex_unlock(&ust_exit_mutex);
        return NULL;
}

/*
 * Weak symbol to call when the ust malloc wrapper is not loaded.
 */
__attribute__((weak))
void lttng_ust_malloc_wrapper_init(void)
{
}

/*
 * sessiond monitoring thread: monitor presence of global and per-user
 * sessiond by polling the application common named pipe.
 */
void __attribute__((constructor)) lttng_ust_init(void)
{
        struct timespec constructor_timeout;
        sigset_t sig_all_blocked, orig_parent_mask;
        pthread_attr_t thread_attr;
        int timeout_mode;
        int ret;
        void *handle;

        if (uatomic_xchg(&initialized, 1) == 1)
                return;

        /*
         * Fixup interdependency between TLS fixup mutex (which happens
         * to be the dynamic linker mutex) and ust_lock, taken within
         * the ust lock.
         */
        lttng_ust_fixup_tls();

        lttng_ust_loaded = 1;

        /*
         * We need to ensure that the liblttng-ust library is not unloaded to avoid
         * the unloading of code used by the ust_listener_threads as we can not
         * reliably know when they exited. To do that, manually load
         * liblttng-ust.so to increment the dynamic loader's internal refcount for
         * this library so it never becomes zero, thus never gets unloaded from the
         * address space of the process. Since we are already running in the
         * constructor of the LTTNG_UST_LIB_SO_NAME library, calling dlopen will
         * simply increment the refcount and no additionnal work is needed by the
         * dynamic loader as the shared library is already loaded in the address
         * space. As a safe guard, we use the RTLD_NODELETE flag to prevent
         * unloading of the UST library if its refcount becomes zero (which should
         * never happen). Do the return value check but discard the handle at the
         * end of the function as it's not needed.
         */
        handle = dlopen(LTTNG_UST_LIB_SO_NAME, RTLD_LAZY | RTLD_NODELETE);
        if (!handle) {
                ERR("dlopen of liblttng-ust shared library (%s).", LTTNG_UST_LIB_SO_NAME);
        }

        /*
         * We want precise control over the order in which we construct
         * our sub-libraries vs starting to receive commands from
         * sessiond (otherwise leading to errors when trying to create
         * sessiond before the init functions are completed).
         */
        init_usterr();
        lttng_ust_getenv_init();        /* Needs init_usterr() to be completed. */
        init_tracepoint();
        lttng_ust_init_fd_tracker();
        lttng_ust_clock_init();
        lttng_ust_getcpu_init();
        lttng_ust_statedump_init();
        lttng_ring_buffer_metadata_client_init();
        lttng_ring_buffer_client_overwrite_init();
        lttng_ring_buffer_client_overwrite_rt_init();
        lttng_ring_buffer_client_discard_init();
        lttng_ring_buffer_client_discard_rt_init();
        lttng_perf_counter_init();
        /*
         * Invoke ust malloc wrapper init before starting other threads.
         */
        lttng_ust_malloc_wrapper_init();

        timeout_mode = get_constructor_timeout(&constructor_timeout);

        get_allow_blocking();

        ret = sem_init(&constructor_wait, 0, 0);
        if (ret) {
                PERROR("sem_init");
        }

        ret = setup_global_apps();
        if (ret) {
                assert(global_apps.allowed == 0);
                DBG("global apps setup returned %d", ret);
        }

        ret = setup_local_apps();
        if (ret) {
                assert(local_apps.allowed == 0);
                DBG("local apps setup returned %d", ret);
        }

        /* A new thread created by pthread_create inherits the signal mask
         * from the parent. To avoid any signal being received by the
         * listener thread, we block all signals temporarily in the parent,
         * while we create the listener thread.
         */
        sigfillset(&sig_all_blocked);
        ret = pthread_sigmask(SIG_SETMASK, &sig_all_blocked, &orig_parent_mask);
        if (ret) {
                ERR("pthread_sigmask: %s", strerror(ret));
        }

        ret = pthread_attr_init(&thread_attr);
        if (ret) {
                ERR("pthread_attr_init: %s", strerror(ret));
        }
        ret = pthread_attr_setdetachstate(&thread_attr, PTHREAD_CREATE_DETACHED);
        if (ret) {
                ERR("pthread_attr_setdetachstate: %s", strerror(ret));
        }

        if (global_apps.allowed) {
                pthread_mutex_lock(&ust_exit_mutex);
                ret = pthread_create(&global_apps.ust_listener, &thread_attr,
                                ust_listener_thread, &global_apps);
                if (ret) {
                        ERR("pthread_create global: %s", strerror(ret));
                }
                global_apps.thread_active = 1;
                pthread_mutex_unlock(&ust_exit_mutex);
        } else {
                handle_register_done(&global_apps);
        }

        if (local_apps.allowed) {
                pthread_mutex_lock(&ust_exit_mutex);
                ret = pthread_create(&local_apps.ust_listener, &thread_attr,
                                ust_listener_thread, &local_apps);
                if (ret) {
                        ERR("pthread_create local: %s", strerror(ret));
                }
                local_apps.thread_active = 1;
                pthread_mutex_unlock(&ust_exit_mutex);
        } else {
                handle_register_done(&local_apps);
        }
        ret = pthread_attr_destroy(&thread_attr);
        if (ret) {
                ERR("pthread_attr_destroy: %s", strerror(ret));
        }

        /* Restore original signal mask in parent */
        ret = pthread_sigmask(SIG_SETMASK, &orig_parent_mask, NULL);
        if (ret) {
                ERR("pthread_sigmask: %s", strerror(ret));
        }

        switch (timeout_mode) {
        case 1: /* timeout wait */
                do {
                        ret = sem_timedwait(&constructor_wait,
                                        &constructor_timeout);
                } while (ret < 0 && errno == EINTR);
                if (ret < 0) {
                        switch (errno) {
                        case ETIMEDOUT:
                                ERR("Timed out waiting for lttng-sessiond");
                                break;
                        case EINVAL:
                                PERROR("sem_timedwait");
                                break;
                        default:
                                ERR("Unexpected error \"%s\" returned by sem_timedwait",
                                        strerror(errno));
                        }
                }
                break;
        case -1:/* wait forever */
                do {
                        ret = sem_wait(&constructor_wait);
                } while (ret < 0 && errno == EINTR);
                if (ret < 0) {
                        switch (errno) {
                        case EINVAL:
                                PERROR("sem_wait");
                                break;
                        default:
                                ERR("Unexpected error \"%s\" returned by sem_wait",
                                        strerror(errno));
                        }
                }
                break;
        case 0: /* no timeout */
                break;
        }
}

static
void lttng_ust_cleanup(int exiting)
{
        cleanup_sock_info(&global_apps, exiting);
        cleanup_sock_info(&local_apps, exiting);
        local_apps.allowed = 0;
        global_apps.allowed = 0;
        /*
         * The teardown in this function all affect data structures
         * accessed under the UST lock by the listener thread. This
         * lock, along with the lttng_ust_comm_should_quit flag, ensure
         * that none of these threads are accessing this data at this
         * point.
         */
        lttng_ust_abi_exit();
        lttng_ust_events_exit();
        lttng_perf_counter_exit();
        lttng_ring_buffer_client_discard_rt_exit();
        lttng_ring_buffer_client_discard_exit();
        lttng_ring_buffer_client_overwrite_rt_exit();
        lttng_ring_buffer_client_overwrite_exit();
        lttng_ring_buffer_metadata_client_exit();
        lttng_ust_statedump_destroy();
        exit_tracepoint();
        if (!exiting) {
                /* Reinitialize values for fork */
                sem_count = sem_count_initial_value;
                lttng_ust_comm_should_quit = 0;
                initialized = 0;
        }
}

void __attribute__((destructor)) lttng_ust_exit(void)
{
        int ret;

        /*
         * Using pthread_cancel here because:
         * A) we don't want to hang application teardown.
         * B) the thread is not allocating any resource.
         */

        /*
         * Require the communication thread to quit. Synchronize with
         * mutexes to ensure it is not in a mutex critical section when
         * pthread_cancel is later called.
         */
        ust_lock_nocheck();
        lttng_ust_comm_should_quit = 1;
        ust_unlock();

        pthread_mutex_lock(&ust_exit_mutex);
        /* cancel threads */
        if (global_apps.thread_active) {
                ret = pthread_cancel(global_apps.ust_listener);
                if (ret) {
                        ERR("Error cancelling global ust listener thread: %s",
                                strerror(ret));
                } else {
                        global_apps.thread_active = 0;
                }
        }
        if (local_apps.thread_active) {
                ret = pthread_cancel(local_apps.ust_listener);
                if (ret) {
                        ERR("Error cancelling local ust listener thread: %s",
                                strerror(ret));
                } else {
                        local_apps.thread_active = 0;
                }
        }
        pthread_mutex_unlock(&ust_exit_mutex);

        /*
         * Do NOT join threads: use of sys_futex makes it impossible to
         * join the threads without using async-cancel, but async-cancel
         * is delivered by a signal, which could hit the target thread
         * anywhere in its code path, including while the ust_lock() is
         * held, causing a deadlock for the other thread. Let the OS
         * cleanup the threads if there are stalled in a syscall.
         */
        lttng_ust_cleanup(1);
}

static
void ust_context_ns_reset(void)
{
        lttng_context_pid_ns_reset();
        lttng_context_cgroup_ns_reset();
        lttng_context_ipc_ns_reset();
        lttng_context_mnt_ns_reset();
        lttng_context_net_ns_reset();
        lttng_context_user_ns_reset();
        lttng_context_uts_ns_reset();
}

static
void ust_context_vuids_reset(void)
{
        lttng_context_vuid_reset();
        lttng_context_veuid_reset();
        lttng_context_vsuid_reset();
}

static
void ust_context_vgids_reset(void)
{
        lttng_context_vgid_reset();
        lttng_context_vegid_reset();
        lttng_context_vsgid_reset();
}

/*
 * We exclude the worker threads across fork and clone (except
 * CLONE_VM), because these system calls only keep the forking thread
 * running in the child.  Therefore, we don't want to call fork or clone
 * in the middle of an tracepoint or ust tracing state modification.
 * Holding this mutex protects these structures across fork and clone.
 */
void ust_before_fork(sigset_t *save_sigset)
{
        /*
         * Disable signals. This is to avoid that the child intervenes
         * before it is properly setup for tracing. It is safer to
         * disable all signals, because then we know we are not breaking
         * anything by restoring the original mask.
         */
        sigset_t all_sigs;
        int ret;

        /* Fixup lttng-ust TLS. */
        lttng_ust_fixup_tls();

        if (URCU_TLS(lttng_ust_nest_count))
                return;
        /* Disable signals */
        sigfillset(&all_sigs);
        ret = sigprocmask(SIG_BLOCK, &all_sigs, save_sigset);
        if (ret == -1) {
                PERROR("sigprocmask");
        }

        pthread_mutex_lock(&ust_fork_mutex);

        ust_lock_nocheck();
        urcu_bp_before_fork();
        lttng_ust_lock_fd_tracker();
        lttng_perf_lock();
}

static void ust_after_fork_common(sigset_t *restore_sigset)
{
        int ret;

        DBG("process %d", getpid());
        lttng_perf_unlock();
        lttng_ust_unlock_fd_tracker();
        ust_unlock();

        pthread_mutex_unlock(&ust_fork_mutex);

        /* Restore signals */
        ret = sigprocmask(SIG_SETMASK, restore_sigset, NULL);
        if (ret == -1) {
                PERROR("sigprocmask");
        }
}

void ust_after_fork_parent(sigset_t *restore_sigset)
{
        if (URCU_TLS(lttng_ust_nest_count))
                return;
        DBG("process %d", getpid());
        urcu_bp_after_fork_parent();
        /* Release mutexes and reenable signals */
        ust_after_fork_common(restore_sigset);
}

/*
 * After fork, in the child, we need to cleanup all the leftover state,
 * except the worker thread which already magically disappeared thanks
 * to the weird Linux fork semantics. After tyding up, we call
 * lttng_ust_init() again to start over as a new PID.
 *
 * This is meant for forks() that have tracing in the child between the
 * fork and following exec call (if there is any).
 */
void ust_after_fork_child(sigset_t *restore_sigset)
{
        if (URCU_TLS(lttng_ust_nest_count))
                return;
        lttng_context_vpid_reset();
        lttng_context_vtid_reset();
        lttng_context_procname_reset();
        ust_context_ns_reset();
        ust_context_vuids_reset();
        ust_context_vgids_reset();
        DBG("process %d", getpid());
        /* Release urcu mutexes */
        urcu_bp_after_fork_child();
        lttng_ust_cleanup(0);
        /* Release mutexes and reenable signals */
        ust_after_fork_common(restore_sigset);
        lttng_ust_init();
}

void ust_after_setns(void)
{
        ust_context_ns_reset();
        ust_context_vuids_reset();
        ust_context_vgids_reset();
}

void ust_after_unshare(void)
{
        ust_context_ns_reset();
        ust_context_vuids_reset();
        ust_context_vgids_reset();
}

void ust_after_setuid(void)
{
        ust_context_vuids_reset();
}

void ust_after_seteuid(void)
{
        ust_context_vuids_reset();
}

void ust_after_setreuid(void)
{
        ust_context_vuids_reset();
}

void ust_after_setresuid(void)
{
        ust_context_vuids_reset();
}

void ust_after_setgid(void)
{
        ust_context_vgids_reset();
}

void ust_after_setegid(void)
{
        ust_context_vgids_reset();
}

void ust_after_setregid(void)
{
        ust_context_vgids_reset();
}

void ust_after_setresgid(void)
{
        ust_context_vgids_reset();
}

void lttng_ust_sockinfo_session_enabled(void *owner)
{
        struct sock_info *sock_info = owner;
        sock_info->statedump_pending = 1;
}