attach: simplify significantly

[mirror_lxc.git] / src / lxc / attach.c

/*
 * lxc: linux Container library
 *
 * (C) Copyright IBM Corp. 2007, 2008
 *
 * Authors:
 * Daniel Lezcano <daniel.lezcano at free.fr>
 *
 * 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; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * 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 _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <pwd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <linux/unistd.h>
#include <sys/mount.h>
#include <sys/param.h>
#include <sys/prctl.h>
#include <sys/socket.h>
#include <sys/syscall.h>
#include <sys/wait.h>

#include <lxc/lxccontainer.h>

#ifndef HAVE_DECL_PR_CAPBSET_DROP
#define PR_CAPBSET_DROP 24
#endif

#ifndef HAVE_DECL_PR_SET_NO_NEW_PRIVS
#define PR_SET_NO_NEW_PRIVS 38
#endif

#ifndef HAVE_DECL_PR_GET_NO_NEW_PRIVS
#define PR_GET_NO_NEW_PRIVS 39
#endif

#include "af_unix.h"
#include "attach.h"
#include "caps.h"
#include "cgroup.h"
#include "commands.h"
#include "conf.h"
#include "config.h"
#include "confile.h"
#include "log.h"
#include "lsm/lsm.h"
#include "lxclock.h"
#include "lxcseccomp.h"
#include "namespace.h"
#include "utils.h"

#if HAVE_SYS_PERSONALITY_H
#include <sys/personality.h>
#endif

#ifndef SOCK_CLOEXEC
#define SOCK_CLOEXEC 02000000
#endif

#ifndef MS_REC
#define MS_REC 16384
#endif

#ifndef MS_SLAVE
#define MS_SLAVE (1 << 19)
#endif

lxc_log_define(lxc_attach, lxc);

/* /proc/pid-to-str/current\0 = (5 + 21 + 7 + 1) */
#define __LSMATTRLEN (5 + (LXC_NUMSTRLEN64) + 7 + 1)
static int lsm_open(pid_t pid, int on_exec)
{
        const char *name;
        char path[__LSMATTRLEN];
        int ret = -1;
        int labelfd = -1;

        name = lsm_name();

        if (strcmp(name, "nop") == 0)
                return 0;

        if (strcmp(name, "none") == 0)
                return 0;

        /* We don't support on-exec with AppArmor */
        if (strcmp(name, "AppArmor") == 0)
                on_exec = 0;

        if (on_exec)
                ret = snprintf(path, __LSMATTRLEN, "/proc/%d/attr/exec", pid);
        else
                ret = snprintf(path, __LSMATTRLEN, "/proc/%d/attr/current", pid);
        if (ret < 0 || ret >= __LSMATTRLEN)
                return -1;

        labelfd = open(path, O_RDWR);
        if (labelfd < 0) {
                SYSERROR("%s - Unable to open file descriptor to set LSM label",
                         strerror(errno));
                return -1;
        }

        return labelfd;
}

static int lsm_set_label_at(int lsm_labelfd, int on_exec, char *lsm_label)
{
        int fret = -1;
        const char *name;
        char *command = NULL;

        name = lsm_name();

        if (strcmp(name, "nop") == 0)
                return 0;

        if (strcmp(name, "none") == 0)
                return 0;

        /* We don't support on-exec with AppArmor */
        if (strcmp(name, "AppArmor") == 0)
                on_exec = 0;

        if (strcmp(name, "AppArmor") == 0) {
                int size;

                command =
                    malloc(strlen(lsm_label) + strlen("changeprofile ") + 1);
                if (!command) {
                        SYSERROR("Failed to write apparmor profile.");
                        goto out;
                }

                size = sprintf(command, "changeprofile %s", lsm_label);
                if (size < 0) {
                        SYSERROR("Failed to write apparmor profile.");
                        goto out;
                }

                if (write(lsm_labelfd, command, size + 1) < 0) {
                        SYSERROR("Unable to set LSM label: %s.", command);
                        goto out;
                }
                INFO("Set LSM label to: %s.", command);
        } else if (strcmp(name, "SELinux") == 0) {
                if (write(lsm_labelfd, lsm_label, strlen(lsm_label) + 1) < 0) {
                        SYSERROR("Unable to set LSM label: %s.", lsm_label);
                        goto out;
                }
                INFO("Set LSM label to: %s.", lsm_label);
        } else {
                ERROR("Unable to restore label for unknown LSM: %s.", name);
                goto out;
        }
        fret = 0;

out:
        free(command);

        if (lsm_labelfd != -1)
                close(lsm_labelfd);

        return fret;
}

/* /proc/pid-to-str/status\0 = (5 + 21 + 7 + 1) */
#define __PROC_STATUS_LEN (5 + (LXC_NUMSTRLEN64) + 7 + 1)
static struct lxc_proc_context_info *lxc_proc_get_context_info(pid_t pid)
{
        int ret;
        bool found;
        FILE *proc_file;
        char proc_fn[__PROC_STATUS_LEN];
        size_t line_bufsz = 0;
        char *line = NULL;
        struct lxc_proc_context_info *info = NULL;

        /* Read capabilities. */
        ret = snprintf(proc_fn, __PROC_STATUS_LEN, "/proc/%d/status", pid);
        if (ret < 0 || ret >= __PROC_STATUS_LEN)
                goto on_error;

        proc_file = fopen(proc_fn, "r");
        if (!proc_file) {
                SYSERROR("Could not open %s.", proc_fn);
                goto on_error;
        }

        info = calloc(1, sizeof(*info));
        if (!info) {
                SYSERROR("Could not allocate memory.");
                fclose(proc_file);
                return NULL;
        }

        found = false;
        while (getline(&line, &line_bufsz, proc_file) != -1) {
                ret = sscanf(line, "CapBnd: %llx", &info->capability_mask);
                if (ret != EOF && ret == 1) {
                        found = true;
                        break;
                }
        }

        free(line);
        fclose(proc_file);

        if (!found) {
                SYSERROR("Could not read capability bounding set from %s.",
                         proc_fn);
                errno = ENOENT;
                goto on_error;
        }

        info->lsm_label = lsm_process_label_get(pid);
        info->ns_inherited = 0;
        memset(info->ns_fd, -1, sizeof(int) * LXC_NS_MAX);

        return info;

on_error:
        free(info);
        return NULL;
}

static inline void lxc_proc_close_ns_fd(struct lxc_proc_context_info *ctx)
{
        int i;

        for (i = 0; i < LXC_NS_MAX; i++) {
                if (ctx->ns_fd[i] < 0)
                        continue;
                close(ctx->ns_fd[i]);
                ctx->ns_fd[i] = -EBADF;
        }
}

static void lxc_proc_put_context_info(struct lxc_proc_context_info *ctx)
{
        free(ctx->lsm_label);
        if (ctx->container)
                lxc_container_put(ctx->container);
        lxc_proc_close_ns_fd(ctx);
        free(ctx);
}

/**
 * in_same_namespace - Check whether two processes are in the same namespace.
 * @pid1 - PID of the first process.
 * @pid2 - PID of the second process.
 * @ns   - Name of the namespace to check. Must correspond to one of the names
 *         for the namespaces as shown in /proc/<pid/ns/
 *
 * If the two processes are not in the same namespace returns an fd to the
 * namespace of the second process identified by @pid2. If the two processes are
 * in the same namespace returns -EINVAL, -1 if an error occurred.
 */
static int in_same_namespace(pid_t pid1, pid_t pid2, const char *ns)
{
        int ns_fd1 = -1, ns_fd2 = -1, ret = -1;
        struct stat ns_st1, ns_st2;

        ns_fd1 = lxc_preserve_ns(pid1, ns);
        if (ns_fd1 < 0) {
                /* The kernel does not support this namespace. This is not an
                 * error.
                 */
                if (errno == ENOENT)
                        return -EINVAL;

                goto out;
        }

        ns_fd2 = lxc_preserve_ns(pid2, ns);
        if (ns_fd2 < 0)
                goto out;

        ret = fstat(ns_fd1, &ns_st1);
        if (ret < 0)
                goto out;

        ret = fstat(ns_fd2, &ns_st2);
        if (ret < 0)
                goto out;

        /* processes are in the same namespace */
        ret = -EINVAL;
        if ((ns_st1.st_dev == ns_st2.st_dev ) && (ns_st1.st_ino == ns_st2.st_ino))
                goto out;

        /* processes are in different namespaces */
        ret = ns_fd2;
        ns_fd2 = -1;

out:

        if (ns_fd1 >= 0)
                close(ns_fd1);
        if (ns_fd2 >= 0)
                close(ns_fd2);

        return ret;
}

static int lxc_attach_to_ns(pid_t pid, struct lxc_proc_context_info *ctx)
{
        int i, ret;

        for (i = 0; i < LXC_NS_MAX; i++) {
                if (ctx->ns_fd[i] < 0)
                        continue;

                ret = setns(ctx->ns_fd[i], ns_info[i].clone_flag);
                if (ret < 0) {
                        SYSERROR("Failed to attach to %s namespace of %d",
                                 ns_info[i].proc_name, pid);
                        return -1;
                }

                DEBUG("Attached to %s namespace of %d", ns_info[i].proc_name, pid);
        }

        return 0;
}

static int lxc_attach_remount_sys_proc(void)
{
        int ret;

        ret = unshare(CLONE_NEWNS);
        if (ret < 0) {
                SYSERROR("Failed to unshare mount namespace.");
                return -1;
        }

        if (detect_shared_rootfs()) {
                if (mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL)) {
                        SYSERROR("Failed to make / rslave.");
                        ERROR("Continuing...");
                }
        }

        /* Assume /proc is always mounted, so remount it. */
        ret = umount2("/proc", MNT_DETACH);
        if (ret < 0) {
                SYSERROR("Failed to unmount /proc.");
                return -1;
        }

        ret = mount("none", "/proc", "proc", 0, NULL);
        if (ret < 0) {
                SYSERROR("Failed to remount /proc.");
                return -1;
        }

        /* Try to umount /sys. If it's not a mount point, we'll get EINVAL, then
         * we ignore it because it may not have been mounted in the first place.
         */
        ret = umount2("/sys", MNT_DETACH);
        if (ret < 0 && errno != EINVAL) {
                SYSERROR("Failed to unmount /sys.");
                return -1;
        } else if (ret == 0) {
                /* Remount it. */
                ret = mount("none", "/sys", "sysfs", 0, NULL);
                if (ret < 0) {
                        SYSERROR("Failed to remount /sys.");
                        return -1;
                }
        }

        return 0;
}

static int lxc_attach_drop_privs(struct lxc_proc_context_info *ctx)
{
        int cap, last_cap;

        last_cap = lxc_caps_last_cap();
        for (cap = 0; cap <= last_cap; cap++) {
                if (ctx->capability_mask & (1LL << cap))
                        continue;

                if (prctl(PR_CAPBSET_DROP, cap, 0, 0, 0)) {
                        SYSERROR("Failed to drop capability %d", cap);
                        return -1;
                }
                TRACE("Dropped capability %d", cap);
        }

        return 0;
}

static int lxc_attach_set_environment(enum lxc_attach_env_policy_t policy,
                                      char **extra_env, char **extra_keep)
{
        if (policy == LXC_ATTACH_CLEAR_ENV) {
                int path_kept = 0;
                char **extra_keep_store = NULL;

                if (extra_keep) {
                        size_t count, i;

                        for (count = 0; extra_keep[count]; count++);

                        extra_keep_store = calloc(count, sizeof(char *));
                        if (!extra_keep_store) {
                                SYSERROR("Failed to allocate memory for storing current "
                                         "environment variable values that will be kept.");
                                return -1;
                        }
                        for (i = 0; i < count; i++) {
                                char *v = getenv(extra_keep[i]);
                                if (v) {
                                        extra_keep_store[i] = strdup(v);
                                        if (!extra_keep_store[i]) {
                                                SYSERROR("Failed to allocate memory for storing current "
                                                         "environment variable values that will be kept.");
                                                while (i > 0)
                                                        free(extra_keep_store[--i]);
                                                free(extra_keep_store);
                                                return -1;
                                        }
                                        if (strcmp(extra_keep[i], "PATH") == 0)
                                                path_kept = 1;
                                }
                                /* Calloc sets entire array to zero, so we don't
                                 * need an else.
                                 */
                        }
                }

                if (clearenv()) {
                        char **p;

                        SYSERROR("Failed to clear environment.");
                        if (extra_keep_store) {
                                for (p = extra_keep_store; *p; p++)
                                        free(*p);
                                free(extra_keep_store);
                        }
                        return -1;
                }

                if (extra_keep_store) {
                        size_t i;

                        for (i = 0; extra_keep[i]; i++) {
                                if (extra_keep_store[i]) {
                                        if (setenv(extra_keep[i], extra_keep_store[i], 1) < 0)
                                                SYSERROR("Unable to set environment variable.");
                                }
                                free(extra_keep_store[i]);
                        }
                        free(extra_keep_store);
                }

                /* Always set a default path; shells and execlp tend to be fine
                 * without it, but there is a disturbing number of C programs
                 * out there that just assume that getenv("PATH") is never NULL
                 * and then die a painful segfault death.
                 */
                if (!path_kept)
                        setenv("PATH", "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", 1);
        }

        if (putenv("container=lxc")) {
                SYSERROR("Failed to set environment variable.");
                return -1;
        }

        /* Set extra environment variables. */
        if (extra_env) {
                for (; *extra_env; extra_env++) {
                        /* Duplicate the string, just to be on the safe side,
                         * because putenv does not do it for us.
                         */
                        char *p = strdup(*extra_env);
                        /* We just assume the user knows what they are doing, so
                         * we don't do any checks.
                         */
                        if (!p) {
                                SYSERROR("Failed to allocate memory for additional environment "
                                         "variables.");
                                return -1;
                        }
                        putenv(p);
                }
        }

        return 0;
}

static char *lxc_attach_getpwshell(uid_t uid)
{
        int fd, ret;
        pid_t pid;
        int pipes[2];
        char *result = NULL;

        /* We need to fork off a process that runs the getent program, and we
         * need to capture its output, so we use a pipe for that purpose.
         */
        ret = pipe(pipes);
        if (ret < 0)
                return NULL;

        pid = fork();
        if (pid < 0) {
                close(pipes[0]);
                close(pipes[1]);
                return NULL;
        }

        if (pid) {
                int status;
                FILE *pipe_f;
                int found = 0;
                size_t line_bufsz = 0;
                char *line = NULL;

                close(pipes[1]);

                pipe_f = fdopen(pipes[0], "r");
                while (getline(&line, &line_bufsz, pipe_f) != -1) {
                        int i;
                        long value;
                        char *token;
                        char *endptr = NULL, *saveptr = NULL;

                        /* If we already found something, just continue to read
                         * until the pipe doesn't deliver any more data, but
                         * don't modify the existing data structure.
                         */
                        if (found)
                                continue;

                        /* Trim line on the right hand side. */
                        for (i = strlen(line); i > 0 && (line[i - 1] == '\n' || line[i - 1] == '\r'); --i)
                                line[i - 1] = '\0';

                        /* Split into tokens: first: user name. */
                        token = strtok_r(line, ":", &saveptr);
                        if (!token)
                                continue;
                        /* next: dummy password field */
                        token = strtok_r(NULL, ":", &saveptr);
                        if (!token)
                                continue;
                        /* next: user id */
                        token = strtok_r(NULL, ":", &saveptr);
                        value = token ? strtol(token, &endptr, 10) : 0;
                        if (!token || !endptr || *endptr || value == LONG_MIN || value == LONG_MAX)
                                continue;
                        /* dummy sanity check: user id matches */
                        if ((uid_t) value != uid)
                                continue;
                        /* skip fields: gid, gecos, dir, go to next field 'shell' */
                        for (i = 0; i < 4; i++) {
                                token = strtok_r(NULL, ":", &saveptr);
                                if (!token)
                                        break;
                        }
                        if (!token)
                                continue;
                        free(result);
                        result = strdup(token);

                        /* Sanity check that there are no fields after that. */
                        token = strtok_r(NULL, ":", &saveptr);
                        if (token)
                                continue;

                        found = 1;
                }

                free(line);
                fclose(pipe_f);
        again:
                if (waitpid(pid, &status, 0) < 0) {
                        if (errno == EINTR)
                                goto again;
                        free(result);
                        return NULL;
                }

                /* Some sanity checks. If anything even hinted at going wrong,
                 * we can't be sure we have a valid result, so we assume we
                 * don't.
                 */

                if (!WIFEXITED(status)) {
                        free(result);
                        return NULL;
                }

                if (WEXITSTATUS(status) != 0) {
                        free(result);
                        return NULL;
                }

                if (!found) {
                        free(result);
                        return NULL;
                }

                return result;
        } else {
                char uid_buf[32];
                char *arguments[] = {
                        "getent",
                        "passwd",
                        uid_buf,
                        NULL
                };

                close(pipes[0]);

                /* We want to capture stdout. */
                dup2(pipes[1], 1);
                close(pipes[1]);

                /* Get rid of stdin/stderr, so we try to associate it with
                 * /dev/null.
                 */
                fd = open("/dev/null", O_RDWR);
                if (fd < 0) {
                        close(0);
                        close(2);
                } else {
                        dup2(fd, 0);
                        dup2(fd, 2);
                        close(fd);
                }

                /* Finish argument list. */
                ret = snprintf(uid_buf, sizeof(uid_buf), "%ld", (long) uid);
                if (ret <= 0)
                        exit(-1);

                /* Try to run getent program. */
                (void) execvp("getent", arguments);
                exit(-1);
        }
}

static void lxc_attach_get_init_uidgid(uid_t *init_uid, gid_t *init_gid)
{
        FILE *proc_file;
        char proc_fn[__PROC_STATUS_LEN];
        int ret;
        char *line = NULL;
        size_t line_bufsz = 0;
        long value = -1;
        uid_t uid = (uid_t)-1;
        gid_t gid = (gid_t)-1;

        /* Read capabilities. */
        snprintf(proc_fn, __PROC_STATUS_LEN, "/proc/%d/status", 1);

        proc_file = fopen(proc_fn, "r");
        if (!proc_file)
                return;

        while (getline(&line, &line_bufsz, proc_file) != -1) {
                /* Format is: real, effective, saved set user, fs we only care
                 * about real uid.
                 */
                ret = sscanf(line, "Uid: %ld", &value);
                if (ret != EOF && ret == 1) {
                        uid = (uid_t)value;
                } else {
                        ret = sscanf(line, "Gid: %ld", &value);
                        if (ret != EOF && ret == 1)
                                gid = (gid_t)value;
                }
                if (uid != (uid_t)-1 && gid != (gid_t)-1)
                        break;
        }

        fclose(proc_file);
        free(line);

        /* Only override arguments if we found something. */
        if (uid != (uid_t)-1)
                *init_uid = uid;
        if (gid != (gid_t)-1)
                *init_gid = gid;

        /* TODO: we should also parse supplementary groups and use
         * setgroups() to set them.
         */
}

struct attach_clone_payload {
        int ipc_socket;
        lxc_attach_options_t *options;
        struct lxc_proc_context_info *init_ctx;
        lxc_attach_exec_t exec_function;
        void *exec_payload;
};

static int attach_child_main(void *data);

/* Help the optimizer along if it doesn't know that exit always exits. */
#define rexit(c)                                                               \
        do {                                                                   \
                int __c = (c);                                                 \
                _exit(__c);                                                    \
                return __c;                                                    \
        } while (0)

/* Define default options if no options are supplied by the user. */
static lxc_attach_options_t attach_static_default_options = LXC_ATTACH_OPTIONS_DEFAULT;

static bool fetch_seccomp(struct lxc_container *c,
                          lxc_attach_options_t *options)
{
        char *path;

        if (!(options->namespaces & CLONE_NEWNS) ||
            !(options->attach_flags & LXC_ATTACH_LSM)) {
                free(c->lxc_conf->seccomp);
                c->lxc_conf->seccomp = NULL;
                return true;
        }

        /* Remove current setting. */
        if (!c->set_config_item(c, "lxc.seccomp", "") &&
            !c->set_config_item(c, "lxc.seccomp.profile", "")) {
                return false;
        }

        /* Fetch the current profile path over the cmd interface. */
        path = c->get_running_config_item(c, "lxc.seccomp.profile");
        if (!path) {
                INFO("Failed to get running config item for lxc.seccomp.profile");
                path = c->get_running_config_item(c, "lxc.seccomp");
        }
        if (!path) {
                INFO("Failed to get running config item for lxc.seccomp");
                return true;
        }

        /* Copy the value into the new lxc_conf. */
        if (!c->set_config_item(c, "lxc.seccomp.profile", path)) {
                free(path);
                return false;
        }
        free(path);

        /* Attempt to parse the resulting config. */
        if (lxc_read_seccomp_config(c->lxc_conf) < 0) {
                ERROR("Error reading seccomp policy.");
                return false;
        }

        INFO("Retrieved seccomp policy.");
        return true;
}

static bool no_new_privs(struct lxc_container *c, lxc_attach_options_t *options)
{
        char *val;

        /* Remove current setting. */
        if (!c->set_config_item(c, "lxc.no_new_privs", ""))
                return false;

        /* Retrieve currently active setting. */
        val = c->get_running_config_item(c, "lxc.no_new_privs");
        if (!val) {
                INFO("Failed to get running config item for lxc.no_new_privs.");
                return false;
        }

        /* Set currently active setting. */
        if (!c->set_config_item(c, "lxc.no_new_privs", val)) {
                free(val);
                return false;
        }
        free(val);

        return true;
}

static signed long get_personality(const char *name, const char *lxcpath)
{
        char *p;
        signed long ret;

        p = lxc_cmd_get_config_item(name, "lxc.arch", lxcpath);
        if (!p)
                return -1;

        ret = lxc_config_parse_arch(p);
        free(p);

        return ret;
}

int lxc_attach(const char *name, const char *lxcpath,
               lxc_attach_exec_t exec_function, void *exec_payload,
               lxc_attach_options_t *options, pid_t *attached_process)
{
        int i, ret, status;
        int ipc_sockets[2];
        char *cwd, *new_cwd;
        signed long personality;
        pid_t attached_pid, expected, init_pid, pid;
        struct lxc_proc_context_info *init_ctx;

        ret = access("/proc/self/ns", X_OK);
        if (ret) {
                ERROR("Does this kernel version support namespaces?");
                return -1;
        }

        if (!options)
                options = &attach_static_default_options;

        init_pid = lxc_cmd_get_init_pid(name, lxcpath);
        if (init_pid < 0) {
                ERROR("Failed to get init pid.");
                return -1;
        }

        init_ctx = lxc_proc_get_context_info(init_pid);
        if (!init_ctx) {
                ERROR("Failed to get context of init process: %ld", (long)init_pid);
                return -1;
        }

        personality = get_personality(name, lxcpath);
        if (init_ctx->personality < 0) {
                ERROR("Failed to get personality of the container");
                lxc_proc_put_context_info(init_ctx);
                return -1;
        }
        init_ctx->personality = personality;

        init_ctx->container = lxc_container_new(name, lxcpath);
        if (!init_ctx->container) {
                lxc_proc_put_context_info(init_ctx);
                return -1;
        }

        if (!init_ctx->container->lxc_conf) {
                init_ctx->container->lxc_conf = lxc_conf_init();
                if (!init_ctx->container->lxc_conf) {
                        lxc_proc_put_context_info(init_ctx);
                        return -ENOMEM;
                }
        }

        if (!fetch_seccomp(init_ctx->container, options))
                WARN("Failed to get seccomp policy.");

        if (!no_new_privs(init_ctx->container, options))
                WARN("Could not determine whether PR_SET_NO_NEW_PRIVS is set.");

        cwd = getcwd(NULL, 0);

        /* Determine which namespaces the container was created with
         * by asking lxc-start, if necessary.
         */
        if (options->namespaces == -1) {
                options->namespaces = lxc_cmd_get_clone_flags(name, lxcpath);
                /* call failed */
                if (options->namespaces == -1) {
                        ERROR("Failed to automatically determine the "
                              "namespaces which the container uses");
                        free(cwd);
                        lxc_proc_put_context_info(init_ctx);
                        return -1;
                }

                for (i = 0; i < LXC_NS_MAX; i++) {
                        if (ns_info[i].clone_flag & CLONE_NEWCGROUP)
                                if (!(options->attach_flags & LXC_ATTACH_MOVE_TO_CGROUP) ||
                                    !cgns_supported())
                                        continue;

                        if (ns_info[i].clone_flag & options->namespaces)
                                continue;

                        init_ctx->ns_inherited |= ns_info[i].clone_flag;
                }
        }

        pid = getpid();
        for (i = 0; i < LXC_NS_MAX; i++) {
                int j, saved_errno;

                if (options->namespaces & ns_info[i].clone_flag)
                        init_ctx->ns_fd[i] = lxc_preserve_ns(init_pid, ns_info[i].proc_name);
                else if (init_ctx->ns_inherited & ns_info[i].clone_flag)
                        init_ctx->ns_fd[i] = in_same_namespace(pid, init_pid, ns_info[i].proc_name);
                else
                        continue;
                if (init_ctx->ns_fd[i] >= 0)
                        continue;

                if (init_ctx->ns_fd[i] == -EINVAL) {
                        DEBUG("Inheriting %s namespace from %d",
                              ns_info[i].proc_name, pid);
                        init_ctx->ns_inherited &= ~ns_info[i].clone_flag;
                        continue;
                }

                /* We failed to preserve the namespace. */
                saved_errno = errno;
                /* Close all already opened file descriptors before we return an
                 * error, so we don't leak them.
                 */
                for (j = 0; j < i; j++)
                        close(init_ctx->ns_fd[j]);

                errno = saved_errno;
                SYSERROR("Failed to attach to %s namespace of %d",
                         ns_info[i].proc_name, pid);
                free(cwd);
                lxc_proc_put_context_info(init_ctx);
                return -1;
        }

        /* Create a socket pair for IPC communication; set SOCK_CLOEXEC in order
         * to make sure we don't irritate other threads that want to fork+exec
         * away
         *
         * IMPORTANT: if the initial process is multithreaded and another call
         * just fork()s away without exec'ing directly after, the socket fd will
         * exist in the forked process from the other thread and any close() in
         * our own child process will not really cause the socket to close
         * properly, potentiall causing the parent to hang.
         *
         * For this reason, while IPC is still active, we have to use shutdown()
         * if the child exits prematurely in order to signal that the socket is
         * closed and cannot assume that the child exiting will automatically do
         * that.
         *
         * IPC mechanism: (X is receiver)
         *   initial process        intermediate          attached
         *        X           <---  send pid of
         *                          attached proc,
         *                          then exit
         *    send 0 ------------------------------------>    X
         *                                              [do initialization]
         *        X  <------------------------------------  send 1
         *   [add to cgroup, ...]
         *    send 2 ------------------------------------>    X
         *                                              [set LXC_ATTACH_NO_NEW_PRIVS]
         *        X  <------------------------------------  send 3
         *   [open LSM label fd]
         *    send 4 ------------------------------------>    X
         *                                              [set LSM label]
         *   close socket                                 close socket
         *                                                run program
         */
        ret = socketpair(PF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets);
        if (ret < 0) {
                SYSERROR("Could not set up required IPC mechanism for attaching.");
                free(cwd);
                lxc_proc_put_context_info(init_ctx);
                return -1;
        }

        /* Create intermediate subprocess, three reasons:
         *       1. Runs all pthread_atfork handlers and the child will no
         *          longer be threaded (we can't properly setns() in a threaded
         *          process).
         *       2. We can't setns() in the child itself, since we want to make
         *          sure we are properly attached to the pidns.
         *       3. Also, the initial thread has to put the attached process
         *          into the cgroup, which we can only do if we didn't already
         *          setns() (otherwise, user namespaces will hate us).
         */
        pid = fork();
        if (pid < 0) {
                SYSERROR("Failed to create first subprocess.");
                free(cwd);
                lxc_proc_put_context_info(init_ctx);
                return -1;
        }

        if (pid) {
                pid_t to_cleanup_pid = pid;

                /* Initial thread, we close the socket that is for the
                 * subprocesses.
                 */
                close(ipc_sockets[1]);
                free(cwd);

                /* Attach to cgroup, if requested. */
                if (options->attach_flags & LXC_ATTACH_MOVE_TO_CGROUP) {
                        if (!cgroup_attach(name, lxcpath, pid))
                                goto on_error;
                        TRACE("Moved intermediate process %d into container's "
                              "cgroups", pid);
                }

                /* Setup resource limits */
                if (!lxc_list_empty(&init_ctx->container->lxc_conf->limits))
                        if (setup_resource_limits(&init_ctx->container->lxc_conf->limits, pid) < 0)
                                goto on_error;

                /* Let the child process know to go ahead. */
                status = 0;
                ret = lxc_write_nointr(ipc_sockets[0], &status, sizeof(status));
                if (ret != sizeof(status))
                        goto on_error;
                TRACE("Told intermediate process to start initializing");

                /* Get pid of attached process from intermediate process. */
                ret = lxc_read_nointr(ipc_sockets[0], &attached_pid, sizeof(attached_pid));
                if (ret != sizeof(attached_pid))
                        goto on_error;
                TRACE("Received pid %d of attached process in parent pid namespace", attached_pid);

                /* Ignore SIGKILL (CTRL-C) and SIGQUIT (CTRL-\) - issue #313. */
                if (options->stdin_fd == 0) {
                        signal(SIGINT, SIG_IGN);
                        signal(SIGQUIT, SIG_IGN);
                }

                /* Reap intermediate process. */
                ret = wait_for_pid(pid);
                if (ret < 0)
                        goto on_error;
                TRACE("Intermediate process %d exited", pid);

                /* We will always have to reap the attached process now. */
                to_cleanup_pid = attached_pid;

                /* Open LSM fd and send it to child. */
                if ((options->namespaces & CLONE_NEWNS) &&
                    (options->attach_flags & LXC_ATTACH_LSM) &&
                    init_ctx->lsm_label) {
                        int labelfd, on_exec;
                        int ret = -1;

                        on_exec = options->attach_flags & LXC_ATTACH_LSM_EXEC ? 1 : 0;
                        labelfd = lsm_open(attached_pid, on_exec);
                        if (labelfd < 0)
                                goto on_error;
                        TRACE("Opened LSM label file descriptor %d", labelfd);

                        /* Send child fd of the LSM security module to write to. */
                        ret = lxc_abstract_unix_send_fds(ipc_sockets[0], &labelfd, 1, NULL, 0);
                        close(labelfd);
                        if (ret <= 0) {
                                SYSERROR("%d", (int)ret);
                                goto on_error;
                        }
                        TRACE("Sent LSM label file descriptor %d to child", labelfd);
                }

                /* Now shut down communication with child, we're done. */
                shutdown(ipc_sockets[0], SHUT_RDWR);
                close(ipc_sockets[0]);
                lxc_proc_put_context_info(init_ctx);

                /* We're done, the child process should now execute whatever it
                 * is that the user requested. The parent can now track it with
                 * waitpid() or similar.
                 */

                *attached_process = attached_pid;
                return 0;

        on_error:
                /* First shut down the socket, then wait for the pid, otherwise
                 * the pid we're waiting for may never exit.
                 */
                shutdown(ipc_sockets[0], SHUT_RDWR);
                close(ipc_sockets[0]);
                if (to_cleanup_pid)
                        (void)wait_for_pid(to_cleanup_pid);
                lxc_proc_put_context_info(init_ctx);
                return -1;
        }

        /* First subprocess begins here, we close the socket that is for the
         * initial thread.
         */
        close(ipc_sockets[0]);

        /* Wait for the parent to have setup cgroups. */
        expected = 0;
        ret = lxc_read_nointr(ipc_sockets[1], &status, sizeof(status));
        if (ret != sizeof(status) || status != expected) {
                shutdown(ipc_sockets[1], SHUT_RDWR);
                lxc_proc_put_context_info(init_ctx);
                rexit(-1);
        }
        TRACE("Intermediate process starting to initialize");

        /* Attach now, create another subprocess later, since pid namespaces
         * only really affect the children of the current process.
         */
        ret = lxc_attach_to_ns(init_pid, init_ctx);
        if (ret < 0) {
                ERROR("Failed to enter namespaces");
                shutdown(ipc_sockets[1], SHUT_RDWR);
                lxc_proc_put_context_info(init_ctx);
                rexit(-1);
        }
        /* close namespace file descriptors */
        lxc_proc_close_ns_fd(init_ctx);

        /* Attach succeeded, try to cwd. */
        if (options->initial_cwd)
                new_cwd = options->initial_cwd;
        else
                new_cwd = cwd;
        if (new_cwd) {
                ret = chdir(new_cwd);
                if (ret < 0)
                        WARN("Could not change directory to \"%s\"", new_cwd);
        }
        free(cwd);

        /* Now create the real child process. */
        {
                struct attach_clone_payload payload = {
                        .ipc_socket = ipc_sockets[1],
                        .options = options,
                        .init_ctx = init_ctx,
                        .exec_function = exec_function,
                        .exec_payload = exec_payload,
                };
                /* We use clone_parent here to make this subprocess a direct
                 * child of the initial process. Then this intermediate process
                 * can exit and the parent can directly track the attached
                 * process.
                 */
                pid = lxc_clone(attach_child_main, &payload, CLONE_PARENT);
        }

        /* Shouldn't happen, clone() should always return positive pid. */
        if (pid <= 0) {
                SYSERROR("Failed to clone attached process");
                shutdown(ipc_sockets[1], SHUT_RDWR);
                lxc_proc_put_context_info(init_ctx);
                rexit(-1);
        }
        TRACE("Cloned attached process %d", pid);

        /* Tell grandparent the pid of the pid of the newly created child. */
        ret = lxc_write_nointr(ipc_sockets[1], &pid, sizeof(pid));
        if (ret != sizeof(pid)) {
                /* If this really happens here, this is very unfortunate, since
                 * the parent will not know the pid of the attached process and
                 * will not be able to wait for it (and we won't either due to
                 * CLONE_PARENT) so the parent won't be able to reap it and the
                 * attached process will remain a zombie.
                 */
                shutdown(ipc_sockets[1], SHUT_RDWR);
                lxc_proc_put_context_info(init_ctx);
                rexit(-1);
        }
        TRACE("Sending pid %d of attached process", pid);

        /* The rest is in the hands of the initial and the attached process. */
        lxc_proc_put_context_info(init_ctx);
        rexit(0);
}

static int attach_child_main(void* data)
{
        int fd, ret;
        long flags;
#if HAVE_SYS_PERSONALITY_H
        long new_personality;
#endif
        uid_t new_uid;
        gid_t new_gid;
        struct attach_clone_payload* payload = (struct attach_clone_payload*)data;
        int ipc_socket = payload->ipc_socket;
        lxc_attach_options_t* options = payload->options;
        struct lxc_proc_context_info* init_ctx = payload->init_ctx;

        /* A description of the purpose of this functionality is provided in the
         * lxc-attach(1) manual page. We have to remount here and not in the
         * parent process, otherwise /proc may not properly reflect the new pid
         * namespace.
         */
        if (!(options->namespaces & CLONE_NEWNS) &&
            (options->attach_flags & LXC_ATTACH_REMOUNT_PROC_SYS)) {
                ret = lxc_attach_remount_sys_proc();
                if (ret < 0) {
                        shutdown(ipc_socket, SHUT_RDWR);
                        rexit(-1);
                }
        }

        /* Now perform additional attachments. */
#if HAVE_SYS_PERSONALITY_H
        if (options->personality < 0)
                new_personality = init_ctx->personality;
        else
                new_personality = options->personality;

        if (options->attach_flags & LXC_ATTACH_SET_PERSONALITY) {
                ret = personality(new_personality);
                if (ret < 0) {
                        SYSERROR("Could not ensure correct architecture");
                        shutdown(ipc_socket, SHUT_RDWR);
                        rexit(-1);
                }
        }
#endif

        if (options->attach_flags & LXC_ATTACH_DROP_CAPABILITIES) {
                ret = lxc_attach_drop_privs(init_ctx);
                if (ret < 0) {
                        ERROR("Could not drop privileges");
                        shutdown(ipc_socket, SHUT_RDWR);
                        rexit(-1);
                }
        }

        /* Always set the environment (specify (LXC_ATTACH_KEEP_ENV, NULL, NULL)
         * if you want this to be a no-op).
         */
        ret = lxc_attach_set_environment(options->env_policy,
                                         options->extra_env_vars,
                                         options->extra_keep_env);
        if (ret < 0) {
                ERROR("Failed to set initial environment for attached process");
                shutdown(ipc_socket, SHUT_RDWR);
                rexit(-1);
        }

        /* Set {u,g}id. */
        new_uid = 0;
        new_gid = 0;
        /* Ignore errors, we will fall back to root in that case (/proc was not
         * mounted etc.).
         */
        if (options->namespaces & CLONE_NEWUSER)
                lxc_attach_get_init_uidgid(&new_uid, &new_gid);

        if (options->uid != (uid_t)-1)
                new_uid = options->uid;
        if (options->gid != (gid_t)-1)
                new_gid = options->gid;

        /* Setup the controlling tty. */
        if (options->stdin_fd && isatty(options->stdin_fd)) {
                if (setsid() < 0) {
                        SYSERROR("Unable to setsid.");
                        shutdown(ipc_socket, SHUT_RDWR);
                        rexit(-1);
                }

                if (ioctl(options->stdin_fd, TIOCSCTTY, (char *)NULL) < 0) {
                        SYSERROR("Unable to set TIOCSTTY.");
                        shutdown(ipc_socket, SHUT_RDWR);
                        rexit(-1);
                }
        }

        /* Try to set the {u,g}id combination. */
        if ((new_gid != 0 || options->namespaces & CLONE_NEWUSER)) {
                if (setgid(new_gid) || setgroups(0, NULL)) {
                        SYSERROR("Switching to container gid.");
                        shutdown(ipc_socket, SHUT_RDWR);
                        rexit(-1);
                }
        }
        if ((new_uid != 0 || options->namespaces & CLONE_NEWUSER) &&
            setuid(new_uid)) {
                SYSERROR("Switching to container uid.");
                shutdown(ipc_socket, SHUT_RDWR);
                rexit(-1);
        }

        if ((init_ctx->container && init_ctx->container->lxc_conf &&
             init_ctx->container->lxc_conf->no_new_privs) ||
            (options->attach_flags & LXC_ATTACH_NO_NEW_PRIVS)) {
                if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
                        SYSERROR("PR_SET_NO_NEW_PRIVS could not be set. "
                                 "Process can use execve() gainable "
                                 "privileges.");
                        shutdown(ipc_socket, SHUT_RDWR);
                        rexit(-1);
                }
                INFO("PR_SET_NO_NEW_PRIVS is set. Process cannot use execve() "
                     "gainable privileges.");
        }

        if ((options->namespaces & CLONE_NEWNS) &&
            (options->attach_flags & LXC_ATTACH_LSM) && init_ctx->lsm_label) {
                int lsm_labelfd, on_exec;

                /* Receive fd for LSM security module. */
                ret = lxc_abstract_unix_recv_fds(ipc_socket, &lsm_labelfd, 1, NULL, 0);
                if (ret <= 0) {
                        shutdown(ipc_socket, SHUT_RDWR);
                        rexit(-1);
                }
                TRACE("Received LSM label file descriptor %d from parent", lsm_labelfd);

                /* Change into our new LSM profile. */
                on_exec = options->attach_flags & LXC_ATTACH_LSM_EXEC ? 1 : 0;
                if (lsm_set_label_at(lsm_labelfd, on_exec, init_ctx->lsm_label) < 0) {
                        SYSERROR("Failed to set LSM label.");
                        shutdown(ipc_socket, SHUT_RDWR);
                        close(lsm_labelfd);
                        rexit(-1);
                }
                close(lsm_labelfd);
        }

        if (init_ctx->container && init_ctx->container->lxc_conf &&
            init_ctx->container->lxc_conf->seccomp &&
            (lxc_seccomp_load(init_ctx->container->lxc_conf) != 0)) {
                ERROR("Failed to load seccomp policy.");
                shutdown(ipc_socket, SHUT_RDWR);
                rexit(-1);
        }

        shutdown(ipc_socket, SHUT_RDWR);
        close(ipc_socket);
        lxc_proc_put_context_info(init_ctx);

        /* The following is done after the communication socket is shut down.
         * That way, all errors that might (though unlikely) occur up until this
         * point will have their messages printed to the original stderr (if
         * logging is so configured) and not the fd the user supplied, if any.
         */

        /* Fd handling for stdin, stdout and stderr; ignore errors here, user
         * may want to make sure the fds are closed, for example.
         */
        if (options->stdin_fd >= 0 && options->stdin_fd != 0)
                dup2(options->stdin_fd, 0);
        if (options->stdout_fd >= 0 && options->stdout_fd != 1)
                dup2(options->stdout_fd, 1);
        if (options->stderr_fd >= 0 && options->stderr_fd != 2)
                dup2(options->stderr_fd, 2);

        /* close the old fds */
        if (options->stdin_fd > 2)
                close(options->stdin_fd);
        if (options->stdout_fd > 2)
                close(options->stdout_fd);
        if (options->stderr_fd > 2)
                close(options->stderr_fd);

        /* Try to remove FD_CLOEXEC flag from stdin/stdout/stderr, but also
         * here, ignore errors.
         */
        for (fd = 0; fd <= 2; fd++) {
                flags = fcntl(fd, F_GETFL);
                if (flags < 0)
                        continue;
                if (flags & FD_CLOEXEC)
                        if (fcntl(fd, F_SETFL, flags & ~FD_CLOEXEC) < 0)
                                SYSERROR("Unable to clear FD_CLOEXEC from file descriptor.");
        }

        /* We're done, so we can now do whatever the user intended us to do. */
        rexit(payload->exec_function(payload->exec_payload));
}

int lxc_attach_run_command(void* payload)
{
        lxc_attach_command_t* cmd = (lxc_attach_command_t*)payload;

        execvp(cmd->program, cmd->argv);
        SYSERROR("Failed to exec \"%s\".", cmd->program);
        return -1;
}

int lxc_attach_run_shell(void* payload)
{
        uid_t uid;
        struct passwd *passwd;
        char *user_shell;

        /* Ignore payload parameter. */
        (void)payload;

        uid = getuid();
        passwd = getpwuid(uid);

        /* This probably happens because of incompatible nss implementations in
         * host and container (remember, this code is still using the host's
         * glibc but our mount namespace is in the container) we may try to get
         * the information by spawning a [getent passwd uid] process and parsing
         * the result.
         */
        if (!passwd)
                user_shell = lxc_attach_getpwshell(uid);
        else
                user_shell = passwd->pw_shell;
        if (user_shell)
                execlp(user_shell, user_shell, (char *)NULL);

        /* Executed if either no passwd entry or execvp fails, we will fall back
         * on /bin/sh as a default shell.
         */
        execlp("/bin/sh", "/bin/sh", (char *)NULL);
        SYSERROR("Failed to execute shell");
        if (!passwd)
                free(user_shell);
        return -1;
}