conf: handle kernels without or not using SMT

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

/* SPDX-License-Identifier: LGPL-2.1+ */

#include "config.h"

#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <linux/unistd.h>
#include <pwd.h>
#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.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 <termios.h>
#include <unistd.h>

#include "attach.h"

#include "af_unix.h"
#include "attach.h"
#include "caps.h"
#include "cgroups/cgroup.h"
#include "cgroups/cgroup_utils.h"
#include "commands.h"
#include "conf.h"
#include "confile.h"
#include "log.h"
#include "lsm/lsm.h"
#include "lxclock.h"
#include "lxcseccomp.h"
#include "macro.h"
#include "mainloop.h"
#include "memory_utils.h"
#include "mount_utils.h"
#include "namespace.h"
#include "process_utils.h"
#include "sync.h"
#include "syscall_wrappers.h"
#include "terminal.h"
#include "utils.h"

lxc_log_define(attach, lxc);

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

/*
 * The context used to attach to the container.
 * @attach_flags        : the attach flags specified in lxc_attach_options_t
 * @init_pid            : the PID of the container's init process
 * @dfd_init_pid        : file descriptor to /proc/@init_pid
 *                        __Must be closed in attach_context_security_barrier()__!
 * @dfd_self_pid        : file descriptor to /proc/self
 *                        __Must be closed in attach_context_security_barrier()__!
 * @setup_ns_uid        : if CLONE_NEWUSER is specified will contain the uid used
 *                        during attach setup.
 * @setup_ns_gid        : if CLONE_NEWUSER is specified will contain the gid used
 *                        during attach setup.
 * @target_ns_uid       : if CLONE_NEWUSER is specified the uid that the final
 *                        program will be run with.
 * @target_ns_gid       : if CLONE_NEWUSER is specified the gid that the final
 *                        program will be run with.
 * @target_host_uid     : if CLONE_NEWUSER is specified the uid that the final
 *                        program will be run with on the host.
 * @target_host_gid     : if CLONE_NEWUSER is specified the gid that the final
 *                        program will be run with on the host.
 * @lsm_label           : LSM label to be used for the attaching process
 * @container           : the container we're attaching o
 * @personality         : the personality to use for the final program
 * @capability          : the capability mask of the @init_pid
 * @ns_inherited        : flags of namespaces that the final program will inherit
 *                        from @init_pid
 * @ns_fd               : file descriptors to @init_pid's namespaces
 * @core_sched_cookie   : core scheduling cookie
 */
struct attach_context {
        unsigned int ns_clone_flags;
        unsigned int attach_flags;
        int init_pid;
        int init_pidfd;
        int dfd_init_pid;
        int dfd_self_pid;
        uid_t setup_ns_uid;
        gid_t setup_ns_gid;
        uid_t target_ns_uid;
        gid_t target_ns_gid;
        uid_t target_host_uid;
        uid_t target_host_gid;
        char *lsm_label;
        struct lxc_container *container;
        personality_t personality;
        unsigned long long capability_mask;
        int ns_inherited;
        int ns_fd[LXC_NS_MAX];
        struct lsm_ops *lsm_ops;
        __u64 core_sched_cookie;
};

static pid_t pidfd_get_pid(int dfd_init_pid, int pidfd)
{
        __do_free char *line = NULL;
        __do_fclose FILE *f = NULL;
        size_t len = 0;
        char path[STRLITERALLEN("fdinfo/") + INTTYPE_TO_STRLEN(int) + 1 ] = "fdinfo/";
        int ret;

        if (dfd_init_pid < 0 || pidfd < 0)
                return ret_errno(EBADF);

        ret = strnprintf(path + STRLITERALLEN("fdinfo/"), INTTYPE_TO_STRLEN(int), "%d", pidfd);
        if (ret < 0)
                return ret_errno(EIO);

        f = fdopen_at(dfd_init_pid, path, "re", PROTECT_OPEN, PROTECT_LOOKUP_BENEATH);
        if (!f)
                return -errno;

        while (getline(&line, &len, f) != -1) {
                const char *prefix = "Pid:\t";
                const size_t prefix_len = STRLITERALLEN("Pid:\t");
                int pid = -ESRCH;
                char *slider = line;

                if (!strnequal(slider, prefix, prefix_len))
                        continue;

                slider += prefix_len;
                slider = lxc_trim_whitespace_in_place(slider);

                ret = lxc_safe_int(slider, &pid);
                if (ret)
                        return -ret;

                return pid;
        }

        return ret_errno(ENOENT);
}

static inline bool sync_wake_pid(int fd, pid_t pid)
{
        return lxc_write_nointr(fd, &pid, sizeof(pid_t)) == sizeof(pid_t);
}

static inline bool sync_wait_pid(int fd, pid_t *pid)
{
        return lxc_read_nointr(fd, pid, sizeof(pid_t)) == sizeof(pid_t);
}

static inline bool sync_wake_fd(int fd, int fd_send)
{
        return lxc_abstract_unix_send_fds(fd, &fd_send, 1, NULL, 0) > 0;
}

static inline bool sync_wait_fd(int fd, int *fd_recv)
{
        return lxc_abstract_unix_recv_one_fd(fd, fd_recv, NULL, 0) > 0;
}

static inline bool attach_lsm(lxc_attach_options_t *options)
{
        return (options->attach_flags & (LXC_ATTACH_LSM | LXC_ATTACH_LSM_LABEL));
}

static struct attach_context *alloc_attach_context(void)
{
        struct attach_context *ctx;

        ctx = zalloc(sizeof(struct attach_context));
        if (!ctx)
                return ret_set_errno(NULL, ENOMEM);

        ctx->init_pid           = -ESRCH;

        ctx->dfd_self_pid       = -EBADF;
        ctx->dfd_init_pid       = -EBADF;
        ctx->init_pidfd         = -EBADF;

        ctx->setup_ns_uid       = LXC_INVALID_UID;
        ctx->setup_ns_gid       = LXC_INVALID_GID;
        ctx->target_ns_uid      = LXC_INVALID_UID;
        ctx->target_ns_gid      = LXC_INVALID_GID;
        ctx->target_host_uid    = LXC_INVALID_UID;
        ctx->target_host_gid    = LXC_INVALID_GID;

        ctx->core_sched_cookie  = INVALID_SCHED_CORE_COOKIE;

        for (lxc_namespace_t i = 0; i < LXC_NS_MAX; i++)
                ctx->ns_fd[i] = -EBADF;

        return ctx;
}

static int get_personality(const char *name, const char *lxcpath,
                           personality_t *personality)
{
        __do_free char *p = NULL;
        int ret;
        signed long per;

        p = lxc_cmd_get_config_item(name, "lxc.arch", lxcpath);
        if (!p) {
                *personality = LXC_ARCH_UNCHANGED;
                return 0;
        }

        ret = lxc_config_parse_arch(p, &per);
        if (ret < 0)
                return syserror("Failed to parse personality");

        *personality = per;
        return 0;
}

static int userns_setup_ids(struct attach_context *ctx,
                            lxc_attach_options_t *options)
{
        __do_free char *line = NULL;
        __do_fclose FILE *f_gidmap = NULL, *f_uidmap = NULL;
        size_t len = 0;
        uid_t init_ns_uid = LXC_INVALID_UID;
        gid_t init_ns_gid = LXC_INVALID_GID;
        uid_t nsuid, hostuid, range_uid;
        gid_t nsgid, hostgid, range_gid;

        if (!(options->namespaces & CLONE_NEWUSER))
                return 0;

        f_uidmap = fdopen_at(ctx->dfd_init_pid, "uid_map", "re", PROTECT_OPEN, PROTECT_LOOKUP_BENEATH);
        if (!f_uidmap)
                return syserror("Failed to open uid_map");

        while (getline(&line, &len, f_uidmap) != -1) {
                if (sscanf(line, "%u %u %u", &nsuid, &hostuid, &range_uid) != 3)
                        continue;

                if (0 >= nsuid && 0 < nsuid + range_uid) {
                        ctx->setup_ns_uid = 0;
                        TRACE("Container has mapping for uid 0");
                        break;
                }

                if (ctx->target_host_uid >= hostuid && ctx->target_host_uid < hostuid + range_uid) {
                        init_ns_uid = (ctx->target_host_uid - hostuid) + nsuid;
                        TRACE("Container runs with uid %d", init_ns_uid);
                }
        }

        f_gidmap = fdopen_at(ctx->dfd_init_pid, "gid_map", "re", PROTECT_OPEN, PROTECT_LOOKUP_BENEATH);
        if (!f_gidmap)
                return syserror("Failed to open gid_map");

        while (getline(&line, &len, f_gidmap) != -1) {
                if (sscanf(line, "%u %u %u", &nsgid, &hostgid, &range_gid) != 3)
                        continue;

                if (0 >= nsgid && 0 < nsgid + range_gid) {
                        ctx->setup_ns_gid = 0;
                        TRACE("Container has mapping for gid 0");
                        break;
                }

                if (ctx->target_host_gid >= hostgid && ctx->target_host_gid < hostgid + range_gid) {
                        init_ns_gid = (ctx->target_host_gid - hostgid) + nsgid;
                        TRACE("Container runs with gid %d", init_ns_gid);
                }
        }

        if (ctx->setup_ns_uid == LXC_INVALID_UID)
                ctx->setup_ns_uid = init_ns_uid;

        if (ctx->setup_ns_gid == LXC_INVALID_UID)
                ctx->setup_ns_gid = init_ns_gid;

        return 0;
}

static void userns_target_ids(struct attach_context *ctx, lxc_attach_options_t *options)
{
        if (options->uid != LXC_INVALID_UID)
                ctx->target_ns_uid = options->uid;
        else if (options->namespaces & CLONE_NEWUSER)
                ctx->target_ns_uid = ctx->setup_ns_uid;
        else
                ctx->target_ns_uid = 0;

        if (ctx->target_ns_uid == LXC_INVALID_UID)
                WARN("Invalid uid specified");

        if (options->gid != LXC_INVALID_GID)
                ctx->target_ns_gid = options->gid;
        else if (options->namespaces & CLONE_NEWUSER)
                ctx->target_ns_gid = ctx->setup_ns_gid;
        else
                ctx->target_ns_gid = 0;

        if (ctx->target_ns_gid == LXC_INVALID_GID)
                WARN("Invalid gid specified");
}

static int parse_init_status(struct attach_context *ctx, lxc_attach_options_t *options)
{
        __do_free char *line = NULL;
        __do_fclose FILE *f = NULL;
        size_t len = 0;
        bool caps_found = false;
        int ret;

        f = fdopen_at(ctx->dfd_init_pid, "status", "re", PROTECT_OPEN, PROTECT_LOOKUP_BENEATH);
        if (!f)
                return syserror("Failed to open status file");

        while (getline(&line, &len, f) != -1) {
                signed long value = -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) {
                        ctx->target_host_uid = (uid_t)value;
                        TRACE("Container's init process runs with hostuid %d", ctx->target_host_uid);
                        goto next;
                }

                ret = sscanf(line, "Gid: %ld", &value);
                if (ret != EOF && ret == 1) {
                        ctx->target_host_gid = (gid_t)value;
                        TRACE("Container's init process runs with hostgid %d", ctx->target_host_gid);
                        goto next;
                }

                ret = sscanf(line, "CapBnd: %llx", &ctx->capability_mask);
                if (ret != EOF && ret == 1) {
                        caps_found = true;
                        goto next;
                }

        next:
                if (ctx->target_host_uid != LXC_INVALID_UID &&
                    ctx->target_host_gid != LXC_INVALID_GID &&
                    caps_found)
                        break;

        }

        ret = userns_setup_ids(ctx, options);
        if (ret)
                return syserror_ret(ret, "Failed to get setup ids");
        userns_target_ids(ctx, options);

        return 0;
}

static bool pidfd_setns_supported(struct attach_context *ctx)
{
        int ret;

        /*
         * The ability to attach to time namespaces came after the introduction
         * of of using pidfds for attaching to namespaces. To avoid having to
         * special-case both CLONE_NEWUSER and CLONE_NEWTIME handling, let's
         * use CLONE_NEWTIME as gatekeeper.
         */
        if (ctx->init_pidfd >= 0)
                ret = setns(ctx->init_pidfd, CLONE_NEWTIME);
        else
                ret = -EOPNOTSUPP;
        TRACE("Attaching to namespaces via pidfds %s",
              ret ? "unsupported" : "supported");
        return ret == 0;
}

static int get_attach_context(struct attach_context *ctx,
                              struct lxc_container *container,
                              lxc_attach_options_t *options)
{
        __do_free char *lsm_label = NULL;
        int ret;
        char path[LXC_PROC_PID_LEN];

        ctx->container = container;
        ctx->attach_flags = options->attach_flags;

        ctx->dfd_self_pid = open_at(-EBADF, "/proc/self",
                                    PROTECT_OPATH_FILE & ~O_NOFOLLOW,
                                    (PROTECT_LOOKUP_ABSOLUTE_WITH_SYMLINKS & ~RESOLVE_NO_XDEV), 0);
        if (ctx->dfd_self_pid < 0)
                return syserror("Failed to open /proc/self");

        ctx->init_pidfd = lxc_cmd_get_init_pidfd(container->name, container->config_path);
        if (ctx->init_pidfd >= 0)
                ctx->init_pid = pidfd_get_pid(ctx->dfd_self_pid, ctx->init_pidfd);
        else
                ctx->init_pid = lxc_cmd_get_init_pid(container->name, container->config_path);
        if (ctx->init_pid < 0)
                return syserror_ret(-1, "Failed to get init pid");

        ret = lxc_cmd_get_clone_flags(container->name, container->config_path);
        if (ret < 0)
                SYSERROR("Failed to retrieve namespace flags");
        ctx->ns_clone_flags = ret;

        ret = core_scheduling_cookie_get(ctx->init_pid, &ctx->core_sched_cookie);
        if (ret || !core_scheduling_cookie_valid(ctx->core_sched_cookie))
                INFO("Container does not run in a separate core scheduling domain");
        else
                INFO("Container runs in separate core scheduling domain %llu",
                     (llu)ctx->core_sched_cookie);

        ret = strnprintf(path, sizeof(path), "/proc/%d", ctx->init_pid);
        if (ret < 0)
                return ret_errno(EIO);

        ctx->dfd_init_pid = open_at(-EBADF, path,
                                    PROTECT_OPATH_DIRECTORY,
                                    (PROTECT_LOOKUP_ABSOLUTE & ~RESOLVE_NO_XDEV), 0);
        if (ctx->dfd_init_pid < 0)
                return syserror("Failed to open /proc/%d", ctx->init_pid);

        if (ctx->init_pidfd >= 0) {
                ret = lxc_raw_pidfd_send_signal(ctx->init_pidfd, 0, NULL, 0);
                if (ret)
                        return syserror("Container process exited or PID has been recycled");
                else
                        TRACE("Container process still running and PID was not recycled");

                if (!pidfd_setns_supported(ctx)) {
                        /* We can't risk leaking file descriptors during attach. */
                        if (close(ctx->init_pidfd))
                                return syserror("Failed to close pidfd");

                        ctx->init_pidfd = -EBADF;
                        TRACE("Attaching to namespaces via pidfds not supported");
                }
        }

        /* Determine which namespaces the container was created with. */
        if (options->namespaces == -1) {
                options->namespaces = ctx->ns_clone_flags;
                if (options->namespaces == -1)
                        return syserror_set(-EINVAL, "Failed to automatically determine the namespaces which the container uses");

                for (lxc_namespace_t 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;

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

        ret = parse_init_status(ctx, options);
        if (ret)
                return syserror("Failed to open parse file");

        ctx->lsm_ops = lsm_init_static();

        if (attach_lsm(options)) {
                if (ctx->attach_flags & LXC_ATTACH_LSM_LABEL)
                        lsm_label = options->lsm_label;
                else
                        lsm_label = ctx->lsm_ops->process_label_get_at(ctx->lsm_ops, ctx->dfd_init_pid);
                if (!lsm_label)
                        WARN("No security context received");
                else
                        INFO("Retrieved security context %s", lsm_label);
        }

        ret = get_personality(container->name, container->config_path, &ctx->personality);
        if (ret)
                return syserror_ret(ret, "Failed to get personality of the container");

        if (!ctx->container->lxc_conf) {
                ctx->container->lxc_conf = lxc_conf_init();
                if (!ctx->container->lxc_conf)
                        return syserror_set(-ENOMEM, "Failed to allocate new lxc config");
        }

        ctx->lsm_label = move_ptr(lsm_label);
        return 0;
}

static int same_nsfd(int dfd_pid1, int dfd_pid2, const char *ns_path)
{
        int ret;
        struct stat ns_st1, ns_st2;

        ret = fstatat(dfd_pid1, ns_path, &ns_st1, 0);
        if (ret)
                return -errno;

        ret = fstatat(dfd_pid2, ns_path, &ns_st2, 0);
        if (ret)
                return -errno;

        /* processes are in the same namespace */
        if ((ns_st1.st_dev == ns_st2.st_dev) &&
            (ns_st1.st_ino == ns_st2.st_ino))
                return 1;

        return 0;
}

static int same_ns(int dfd_pid1, int dfd_pid2, const char *ns_path)
{
        __do_close int ns_fd2 = -EBADF;
        int ret = -1;

        ns_fd2 = open_at(dfd_pid2, ns_path, PROTECT_OPEN_WITH_TRAILING_SYMLINKS,
                         (PROTECT_LOOKUP_BENEATH_WITH_MAGICLINKS &
                          ~(RESOLVE_NO_XDEV | RESOLVE_BENEATH)), 0);
        if (ns_fd2 < 0) {
                if (errno == ENOENT)
                        return -ENOENT;
                return syserror("Failed to open %d(%s)", dfd_pid2, ns_path);
        }

        ret = same_nsfd(dfd_pid1, dfd_pid2, ns_path);
        switch (ret) {
        case -ENOENT:
                __fallthrough;
        case 1:
                return ret_errno(ENOENT);
        case 0:
                /* processes are in different namespaces */
                return move_fd(ns_fd2);
        }

        return ret;
}

static int __prepare_namespaces_pidfd(struct attach_context *ctx)
{
        for (lxc_namespace_t i = 0; i < LXC_NS_MAX; i++) {
                int ret;

                ret = same_nsfd(ctx->dfd_self_pid,
                                ctx->dfd_init_pid,
                                ns_info[i].proc_path);
                switch (ret) {
                case -ENOENT:
                        __fallthrough;
                case 1:
                        ctx->ns_inherited &= ~ns_info[i].clone_flag;
                        TRACE("Shared %s namespace doesn't need attach", ns_info[i].proc_name);
                        continue;
                case 0:
                        TRACE("Different %s namespace needs attach", ns_info[i].proc_name);
                        continue;
                }

                return syserror("Failed to determine whether %s namespace is shared",
                                ns_info[i].proc_name);
        }

        return 0;
}

static int __prepare_namespaces_nsfd(struct attach_context *ctx,
                                     lxc_attach_options_t *options)
{
        for (lxc_namespace_t i = 0; i < LXC_NS_MAX; i++) {
                lxc_namespace_t j;

                if (options->namespaces & ns_info[i].clone_flag)
                        ctx->ns_fd[i] = open_at(ctx->dfd_init_pid,
                                                ns_info[i].proc_path,
                                                PROTECT_OPEN_WITH_TRAILING_SYMLINKS,
                                                (PROTECT_LOOKUP_BENEATH_WITH_MAGICLINKS &
                                                 ~(RESOLVE_NO_XDEV | RESOLVE_BENEATH)),
                                                0);
                else if (ctx->ns_inherited & ns_info[i].clone_flag)
                        ctx->ns_fd[i] = same_ns(ctx->dfd_self_pid,
                                                ctx->dfd_init_pid,
                                                ns_info[i].proc_path);
                else
                        continue;

                if (ctx->ns_fd[i] >= 0)
                        continue;

                if (ctx->ns_fd[i] == -ENOENT) {
                        ctx->ns_inherited &= ~ns_info[i].clone_flag;
                        continue;
                }

                /* We failed to preserve the namespace. */
                SYSERROR("Failed to preserve %s namespace of %d",
                         ns_info[i].proc_name, ctx->init_pid);

                /* Close all already opened file descriptors before we return an
                 * error, so we don't leak them.
                 */
                for (j = 0; j < i; j++)
                        close_prot_errno_disarm(ctx->ns_fd[j]);

                return ret_errno(EINVAL);
        }

        return 0;
}

static int prepare_namespaces(struct attach_context *ctx,
                              lxc_attach_options_t *options)
{
        if (ctx->init_pidfd < 0)
                return __prepare_namespaces_nsfd(ctx, options);

        return __prepare_namespaces_pidfd(ctx);
}

static inline void put_namespaces(struct attach_context *ctx)
{
        if (ctx->init_pidfd < 0) {
                for (int i = 0; i < LXC_NS_MAX; i++)
                        close_prot_errno_disarm(ctx->ns_fd[i]);
        }
}

static int __attach_namespaces_pidfd(struct attach_context *ctx,
                                     lxc_attach_options_t *options)
{
        unsigned int ns_flags = options->namespaces | ctx->ns_inherited;
        int ret;

        /* The common case is to attach to all namespaces. */
        ret = setns(ctx->init_pidfd, ns_flags);
        if (ret)
                return syserror("Failed to attach to namespaces via pidfd");

        /* We can't risk leaking file descriptors into the container. */
        if (close(ctx->init_pidfd))
                return syserror("Failed to close pidfd");
        ctx->init_pidfd = -EBADF;

        return log_trace(0, "Attached to container namespaces via pidfd");
}

static int __attach_namespaces_nsfd(struct attach_context *ctx,
                                    lxc_attach_options_t *options)
{
        int fret = 0;

        for (lxc_namespace_t i = 0; i < LXC_NS_MAX; i++) {
                int ret;

                if (ctx->ns_fd[i] < 0)
                        continue;

                ret = setns(ctx->ns_fd[i], ns_info[i].clone_flag);
                if (ret)
                        return syserror("Failed to attach to %s namespace of %d",
                                        ns_info[i].proc_name, ctx->init_pid);

                if (close(ctx->ns_fd[i])) {
                        fret = -errno;
                        SYSERROR("Failed to close file descriptor for %s namespace",
                                 ns_info[i].proc_name);
                }
                ctx->ns_fd[i] = -EBADF;
        }

        return fret;
}

static int attach_namespaces(struct attach_context *ctx,
                             lxc_attach_options_t *options)
{
        if (lxc_log_trace()) {
                for (lxc_namespace_t i = 0; i < LXC_NS_MAX; i++) {
                        if (ns_info[i].clone_flag & options->namespaces) {
                                TRACE("Attaching to %s namespace", ns_info[i].proc_name);
                                continue;
                        }
                        if (ns_info[i].clone_flag & ctx->ns_inherited) {
                                TRACE("Sharing %s namespace", ns_info[i].proc_name);
                                continue;
                        }
                        TRACE("Inheriting %s namespace", ns_info[i].proc_name);
                }
        }

        if (ctx->init_pidfd < 0)
                return __attach_namespaces_nsfd(ctx, options);

        return __attach_namespaces_pidfd(ctx, options);
}

static void put_attach_context(struct attach_context *ctx)
{
        if (ctx) {
                if (!(ctx->attach_flags & LXC_ATTACH_LSM_LABEL))
                        free_disarm(ctx->lsm_label);
                close_prot_errno_disarm(ctx->dfd_init_pid);

                if (ctx->container) {
                        lxc_container_put(ctx->container);
                        ctx->container = NULL;
                }

                put_namespaces(ctx);
                free(ctx);
        }
}

/*
 * Place anything in here that needs to be get rid of before we move into the
 * container's context and fail hard if we can't.
 */
static bool attach_context_security_barrier(struct attach_context *ctx)
{
        if (ctx) {
                if (close(ctx->dfd_self_pid))
                        return false;
                ctx->dfd_self_pid = -EBADF;

                if (close(ctx->dfd_init_pid))
                        return false;
                ctx->dfd_init_pid = -EBADF;
        }

        return true;
}

int lxc_attach_remount_sys_proc(void)
{
        int ret;

        ret = unshare(CLONE_NEWNS);
        if (ret < 0)
                return syserror("Failed to unshare mount namespace");

        if (detect_shared_rootfs() && mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL))
                SYSERROR("Failed to recursively turn root mount tree into dependent mount. Continuing...");

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

        ret = mount("none", "/proc", "proc", 0, NULL);
        if (ret < 0)
                return syserror("Failed to remount /proc");

        /*
         * 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)
                return syserror("Failed to unmount /sys");

        /* Remount it. */
        if (ret == 0 && mount("none", "/sys", "sysfs", 0, NULL))
                return syserror("Failed to remount /sys");

        return 0;
}

static int drop_capabilities(struct attach_context *ctx)
{
        int ret;
        __u32 last_cap;

        ret = lxc_caps_last_cap(&last_cap);
        if (ret)
                return syserror_ret(ret, "%d - Failed to drop capabilities", ret);

        for (__u32 cap = 0; cap <= last_cap; cap++) {
                if (ctx->capability_mask & (1LL << cap))
                        continue;

                if (prctl(PR_CAPBSET_DROP, prctl_arg(cap), prctl_arg(0),
                          prctl_arg(0), prctl_arg(0)))
                        return syserror("Failed to drop capability %d", cap);

                TRACE("Dropped capability %d", cap);
        }

        return 0;
}

static int lxc_attach_set_environment(struct attach_context *ctx,
                                      enum lxc_attach_env_policy_t policy,
                                      char **extra_env, char **extra_keep)
{
        int ret;

        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 = zalloc(count * sizeof(char *));
                        if (!extra_keep_store)
                                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]) {
                                                while (i > 0)
                                                        free(extra_keep_store[--i]);

                                                free(extra_keep_store);
                                                return -1;
                                        }

                                        if (strequal(extra_keep[i], "PATH"))
                                                path_kept = 1;
                                }
                        }
                }

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

                                for (p = extra_keep_store; *p; p++)
                                        free(*p);

                                free(extra_keep_store);
                        }

                        return syserror("Failed to clear environment");
                }

                if (extra_keep_store) {
                        size_t i;

                        for (i = 0; extra_keep[i]; i++) {
                                if (extra_keep_store[i]) {
                                        ret = setenv(extra_keep[i], extra_keep_store[i], 1);
                                        if (ret < 0)
                                                SYSWARN("Failed 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) {
                        ret = setenv("PATH", "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", 1);
                        if (ret < 0)
                                SYSWARN("Failed to set environment variable");
                }
        }

        ret = putenv("container=lxc");
        if (ret < 0)
                return log_warn(-1, "Failed to set environment variable");

        /* Set container environment variables.*/
        if (ctx->container->lxc_conf) {
                ret = lxc_set_environment(ctx->container->lxc_conf);
                if (ret < 0)
                        return -1;
        }

        /* Set extra environment variables. */
        if (extra_env) {
                for (; *extra_env; extra_env++) {
                        char *p;

                        /* We just assume the user knows what they are doing, so
                         * we don't do any checks.
                         */
                        p = strdup(*extra_env);
                        if (!p)
                                return -1;

                        ret = putenv(p);
                        if (ret < 0)
                                SYSWARN("Failed to set environment variable");
                }
        }

        return 0;
}

static char *lxc_attach_getpwshell(uid_t uid)
{
        __do_free char *line = NULL, *result = NULL;
        __do_fclose FILE *pipe_f = NULL;
        int fd, ret;
        pid_t pid;
        int pipes[2];
        bool found = false;
        size_t line_bufsz = 0;

        /* 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 = pipe2(pipes, O_CLOEXEC);
        if (ret < 0)
                return NULL;

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

        if (!pid) {
                char uid_buf[32];
                char *arguments[] = {
                        "getent",
                        "passwd",
                        uid_buf,
                        NULL
                };

                close(pipes[0]);

                /* We want to capture stdout. */
                ret = dup2(pipes[1], STDOUT_FILENO);
                close(pipes[1]);
                if (ret < 0)
                        _exit(EXIT_FAILURE);

                /* Get rid of stdin/stderr, so we try to associate it with
                 * /dev/null.
                 */
                fd = open_devnull();
                if (fd < 0) {
                        close(STDIN_FILENO);
                        close(STDERR_FILENO);
                } else {
                        (void)dup3(fd, STDIN_FILENO, O_CLOEXEC);
                        (void)dup3(fd, STDERR_FILENO, O_CLOEXEC);
                        close(fd);
                }

                /* Finish argument list. */
                ret = strnprintf(uid_buf, sizeof(uid_buf), "%ld", (long)uid);
                if (ret <= 0)
                        _exit(EXIT_FAILURE);

                /* Try to run getent program. */
                (void)execvp("getent", arguments);
                _exit(EXIT_FAILURE);
        }

        close(pipes[1]);

        pipe_f = fdopen(pipes[0], "re");
        if (!pipe_f) {
                close(pipes[0]);
                goto reap_child;
        }
        /* Transfer ownership of pipes[0] to pipe_f. */
        move_fd(pipes[0]);

        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;

                if (!line)
                        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: placeholder 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;

                /* placeholder conherence 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)
                                continue;
                }

                if (!token)
                        continue;

                free_disarm(result);
                result = strdup(token);

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

                found = true;
        }

reap_child:
        ret = wait_for_pid(pid);
        if (ret < 0)
                return NULL;

        if (!found)
                return NULL;

        return move_ptr(result);
}

static bool fetch_seccomp(struct lxc_container *c, lxc_attach_options_t *options)
{
        __do_free char *path = NULL;
        int ret;
        bool bret;

        if (!attach_lsm(options)) {
                free_disarm(c->lxc_conf->seccomp.seccomp);
                return true;
        }

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

                path = c->get_running_config_item(c, "lxc.seccomp");
                if (!path)
                        return log_info(true, "Failed to retrieve lxc.seccomp");
        }

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

        /* Attempt to parse the resulting config. */
        ret = lxc_read_seccomp_config(c->lxc_conf);
        if (ret < 0)
                return log_error(false, "Failed to retrieve seccomp policy");

        return log_info(true, "Retrieved seccomp policy");
}

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

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

        /* Retrieve currently active setting. */
        val = c->get_running_config_item(c, "lxc.no_new_privs");
        if (!val)
                return log_info(false, "Failed to retrieve lxc.no_new_privs");

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

struct attach_payload {
        int ipc_socket;
        int terminal_pts_fd;
        lxc_attach_options_t *options;
        struct attach_context *ctx;
        lxc_attach_exec_t exec_function;
        void *exec_payload;
};

static void put_attach_payload(struct attach_payload *p)
{
        if (p) {
                close_prot_errno_disarm(p->ipc_socket);
                close_prot_errno_disarm(p->terminal_pts_fd);
                put_attach_context(p->ctx);
                p->ctx = NULL;
        }
}

__noreturn static void do_attach(struct attach_payload *ap)
{
        lxc_attach_exec_t attach_function = move_ptr(ap->exec_function);
        void *attach_function_args = move_ptr(ap->exec_payload);
        int fd_lsm, ret;
        lxc_attach_options_t* options = ap->options;
        struct attach_context *ctx = ap->ctx;
        struct lxc_conf *conf = ctx->container->lxc_conf;

        /*
         * We currently artificially restrict core scheduling to be a pid
         * namespace concept since this makes the code easier. We can revisit
         * this no problem and make this work with shared pid namespaces as
         * well. This check here makes sure that the container was created with
         * a separate pid namespace (ctx->ns_clone_flags) and whether we are
         * actually attaching to this pid namespace (options->namespaces).
         */
        if (core_scheduling_cookie_valid(ctx->core_sched_cookie) &&
            (ctx->ns_clone_flags & CLONE_NEWPID) &&
            (options->namespaces & CLONE_NEWPID)) {
                __u64 core_sched_cookie;

                ret = core_scheduling_cookie_share_with(1);
                if (ret < 0) {
                        SYSERROR("Failed to join core scheduling domain of %d",
                                 ctx->init_pid);
                        goto on_error;
                }

                ret = core_scheduling_cookie_get(getpid(), &core_sched_cookie);
                if (ret || !core_scheduling_cookie_valid(core_sched_cookie) ||
                    (ctx->core_sched_cookie != core_sched_cookie)) {
                        SYSERROR("Invalid core scheduling domain cookie %llu != %llu",
                                 (llu)core_sched_cookie,
                                 (llu)ctx->core_sched_cookie);
                        goto on_error;
                }

                INFO("Joined core scheduling domain of %d with cookie %lld",
                     ctx->init_pid, (llu)core_sched_cookie);
        }

        /* 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)
                        goto on_error;

                TRACE("Remounted \"/proc\" and \"/sys\"");
        }

        /* Now perform additional attachments. */
        if (options->attach_flags & LXC_ATTACH_SET_PERSONALITY) {
                long new_personality;

                if (options->personality == LXC_ATTACH_DETECT_PERSONALITY)
                        new_personality = ctx->personality;
                else
                        new_personality = options->personality;

                if (new_personality != LXC_ARCH_UNCHANGED) {
                        ret = lxc_personality(new_personality);
                        if (ret < 0)
                                goto on_error;

                        TRACE("Set new personality");
                }
        }

        if (options->attach_flags & LXC_ATTACH_DROP_CAPABILITIES) {
                ret = drop_capabilities(ctx);
                if (ret < 0)
                        goto on_error;

                TRACE("Dropped capabilities");
        }

        /* 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(ctx,
                                         options->env_policy,
                                         options->extra_env_vars,
                                         options->extra_keep_env);
        if (ret < 0)
                goto on_error;

        TRACE("Set up environment");

        /*
         * This remark only affects fully unprivileged containers:
         * Receive fd for LSM security module before we set{g,u}id(). The reason
         * is that on set{g,u}id() the kernel will a) make us undumpable and b)
         * we will change our effective uid. This means our effective uid will
         * be different from the effective uid of the process that created us
         * which means that this processs no longer has capabilities in our
         * namespace including CAP_SYS_PTRACE. This means we will not be able to
         * read and /proc/<pid> files for the process anymore when /proc is
         * mounted with hidepid={1,2}. So let's get the lsm label fd before the
         * set{g,u}id().
         */
        if (attach_lsm(options) && ctx->lsm_label) {
                if (!sync_wait_fd(ap->ipc_socket, &fd_lsm)) {
                        SYSERROR("Failed to receive lsm label fd");
                        goto on_error;
                }

                TRACE("Received LSM label file descriptor %d from parent", fd_lsm);
        }

        if (options->stdin_fd > 0 && isatty(options->stdin_fd)) {
                ret = lxc_make_controlling_terminal(options->stdin_fd);
                if (ret < 0)
                        goto on_error;
        }

        if ((options->attach_flags & LXC_ATTACH_SETGROUPS) &&
            options->groups.size > 0) {
                if (!lxc_setgroups(options->groups.list, options->groups.size))
                        goto on_error;
        } else {
                if (!lxc_drop_groups() && errno != EPERM)
                        goto on_error;
        }

        if (options->namespaces & CLONE_NEWUSER)
                if (!lxc_switch_uid_gid(ctx->setup_ns_uid, ctx->setup_ns_gid))
                        goto on_error;

        if (attach_lsm(options) && ctx->lsm_label) {
                bool on_exec;

                /* Change into our new LSM profile. */
                on_exec = options->attach_flags & LXC_ATTACH_LSM_EXEC ? true : false;
                ret = ctx->lsm_ops->process_label_set_at(ctx->lsm_ops, fd_lsm, ctx->lsm_label, on_exec);
                close_prot_errno_disarm(fd_lsm);
                if (ret < 0)
                        goto on_error;

                TRACE("Set %s LSM label to \"%s\"", ctx->lsm_ops->name, ctx->lsm_label);
        }

        if (conf->no_new_privs || (options->attach_flags & LXC_ATTACH_NO_NEW_PRIVS)) {
                ret = prctl(PR_SET_NO_NEW_PRIVS, prctl_arg(1), prctl_arg(0),
                            prctl_arg(0), prctl_arg(0));
                if (ret < 0)
                        goto on_error;

                TRACE("Set PR_SET_NO_NEW_PRIVS");
        }

        /* 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 != STDIN_FILENO)
                if (dup2(options->stdin_fd, STDIN_FILENO) < 0)
                        SYSDEBUG("Failed to replace stdin with %d", options->stdin_fd);

        if (options->stdout_fd >= 0 && options->stdout_fd != STDOUT_FILENO)
                if (dup2(options->stdout_fd, STDOUT_FILENO) < 0)
                        SYSDEBUG("Failed to replace stdout with %d", options->stdout_fd);

        if (options->stderr_fd >= 0 && options->stderr_fd != STDERR_FILENO)
                if (dup2(options->stderr_fd, STDERR_FILENO) < 0)
                        SYSDEBUG("Failed to replace stderr with %d", options->stderr_fd);

        /* close the old fds */
        if (options->stdin_fd > STDERR_FILENO)
                close(options->stdin_fd);

        if (options->stdout_fd > STDERR_FILENO)
                close(options->stdout_fd);

        if (options->stderr_fd > STDERR_FILENO)
                close(options->stderr_fd);

        /*
         * Try to remove FD_CLOEXEC flag from stdin/stdout/stderr, but also
         * here, ignore errors.
         */
        for (int fd = STDIN_FILENO; fd <= STDERR_FILENO; fd++) {
                ret = fd_cloexec(fd, false);
                if (ret < 0) {
                        SYSERROR("Failed to clear FD_CLOEXEC from file descriptor %d", fd);
                        goto on_error;
                }
        }

        if (options->attach_flags & LXC_ATTACH_TERMINAL) {
                ret = lxc_terminal_prepare_login(ap->terminal_pts_fd);
                if (ret < 0) {
                        SYSERROR("Failed to prepare terminal file descriptor %d", ap->terminal_pts_fd);
                        goto on_error;
                }

                TRACE("Prepared terminal file descriptor %d", ap->terminal_pts_fd);
        }

        /* Avoid unnecessary syscalls. */
        if (ctx->setup_ns_uid == ctx->target_ns_uid)
                ctx->target_ns_uid = LXC_INVALID_UID;

        if (ctx->setup_ns_gid == ctx->target_ns_gid)
                ctx->target_ns_gid = LXC_INVALID_GID;

        /*
         * Make sure that the processes STDIO is correctly owned by the user
         * that we are switching to.
         */
        ret = fix_stdio_permissions(ctx->target_ns_uid);
        if (ret)
                INFO("Failed to adjust stdio permissions");

        if (conf->seccomp.seccomp) {
                ret = lxc_seccomp_load(conf);
                if (ret < 0)
                        goto on_error;

                TRACE("Loaded seccomp profile");

                ret = lxc_seccomp_send_notifier_fd(&conf->seccomp, ap->ipc_socket);
                if (ret < 0)
                        goto on_error;
                lxc_seccomp_close_notifier_fd(&conf->seccomp);
        }

        if (!lxc_switch_uid_gid(ctx->target_ns_uid, ctx->target_ns_gid))
                goto on_error;

        put_attach_payload(ap);

        /* We're done, so we can now do whatever the user intended us to do. */
        _exit(attach_function(attach_function_args));

on_error:
        ERROR("Failed to attach to container");
        put_attach_payload(ap);
        _exit(EXIT_FAILURE);
}

static int lxc_attach_terminal(const char *name, const char *lxcpath, struct lxc_conf *conf,
                               struct lxc_terminal *terminal)
{
        int ret;

        lxc_terminal_init(terminal);

        ret = lxc_terminal_create(name, lxcpath, conf, terminal);
        if (ret < 0)
                return syserror("Failed to create terminal");

        return 0;
}

static int lxc_attach_terminal_mainloop_init(struct lxc_terminal *terminal,
                                             struct lxc_async_descr *descr)
{
        int ret;

        ret = lxc_mainloop_open(descr);
        if (ret < 0)
                return syserror("Failed to create mainloop");

        ret = lxc_terminal_mainloop_add(descr, terminal);
        if (ret < 0) {
                lxc_mainloop_close(descr);
                return syserror("Failed to add handlers to mainloop");
        }

        return 0;
}

static inline void lxc_attach_terminal_close_ptx(struct lxc_terminal *terminal)
{
        close_prot_errno_disarm(terminal->ptx);
}

static inline void lxc_attach_terminal_close_pts(struct lxc_terminal *terminal)
{
        close_prot_errno_disarm(terminal->pty);
}

static inline void lxc_attach_terminal_close_peer(struct lxc_terminal *terminal)
{
        close_prot_errno_disarm(terminal->peer);
}

static inline void lxc_attach_terminal_close_log(struct lxc_terminal *terminal)
{
        close_prot_errno_disarm(terminal->log_fd);
}

int lxc_attach(struct lxc_container *container, lxc_attach_exec_t exec_function,
               void *exec_payload, lxc_attach_options_t *options,
               pid_t *attached_process)
{
        int ret_parent = -1;
        struct lxc_async_descr descr = {};
        int ret;
        char *name, *lxcpath;
        int ipc_sockets[2];
        pid_t attached_pid, pid, to_cleanup_pid;
        struct attach_context *ctx;
        struct lxc_terminal terminal;
        struct lxc_conf *conf;

        if (!container)
                return ret_errno(EINVAL);

        if (!lxc_container_get(container))
                return ret_errno(EINVAL);

        name = container->name;
        lxcpath = container->config_path;

        if (!options) {
                options = &attach_static_default_options;
                options->lsm_label = NULL;
        }

        ctx = alloc_attach_context();
        if (!ctx) {
                lxc_container_put(container);
                return syserror_set(-ENOMEM, "Failed to allocate attach context");
        }

        ret = get_attach_context(ctx, container, options);
        if (ret) {
                put_attach_context(ctx);
                return syserror("Failed to get attach context");
        }

        conf = ctx->container->lxc_conf;

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

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

        ret = prepare_namespaces(ctx, options);
        if (ret) {
                put_attach_context(ctx);
                return syserror("Failed to get namespace file descriptors");
        }

        if (options->attach_flags & LXC_ATTACH_TERMINAL) {
                ret = lxc_attach_terminal(name, lxcpath, conf, &terminal);
                if (ret < 0) {
                        put_attach_context(ctx);
                        return syserror("Failed to setup new terminal");
                }

                terminal.log_fd = options->log_fd;
        } else {
                lxc_terminal_init(&terminal);
        }

        /* 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, potentially causing the parent to get stuck.
         *
         * 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        transient process   attached process
         *        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) {
                put_attach_context(ctx);
                return syserror("Could not set up required IPC mechanism for attaching");
        }

        /* Create transient process, two reasons:
         *       1. We can't setns() in the child itself, since we want to make
         *          sure we are properly attached to the pidns.
         *       2. 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) {
                put_attach_context(ctx);
                return syserror("Failed to create first subprocess");
        }

        if (pid == 0) {
                char *cwd, *new_cwd;

                /* close unneeded file descriptors */
                close_prot_errno_disarm(ipc_sockets[0]);

                if (options->attach_flags & LXC_ATTACH_TERMINAL) {
                        lxc_attach_terminal_close_ptx(&terminal);
                        lxc_attach_terminal_close_peer(&terminal);
                        lxc_attach_terminal_close_log(&terminal);
                }

                /* Wait for the parent to have setup cgroups. */
                if (!sync_wait(ipc_sockets[1], ATTACH_SYNC_CGROUP)) {
                        shutdown(ipc_sockets[1], SHUT_RDWR);
                        put_attach_context(ctx);
                        _exit(EXIT_FAILURE);
                }

                if (!attach_context_security_barrier(ctx)) {
                        shutdown(ipc_sockets[1], SHUT_RDWR);
                        put_attach_context(ctx);
                        _exit(EXIT_FAILURE);
                }

                cwd = getcwd(NULL, 0);

                /*
                 * Attach now, create another subprocess later, since pid
                 * namespaces only really affect the children of the current
                 * process.
                 *
                 * Note that this is a crucial barrier. We're no moving into
                 * the container's context so we need to make sure to not leak
                 * anything sensitive. That especially means things such as
                 * open file descriptors!
                 */
                ret = attach_namespaces(ctx, options);
                if (ret < 0) {
                        ERROR("Failed to enter namespaces");
                        shutdown(ipc_sockets[1], SHUT_RDWR);
                        put_attach_context(ctx);
                        _exit(EXIT_FAILURE);
                }

                /* 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_disarm(cwd);

                /* Create attached process. */
                pid = lxc_raw_clone(CLONE_PARENT, NULL);
                if (pid < 0) {
                        SYSERROR("Failed to clone attached process");
                        shutdown(ipc_sockets[1], SHUT_RDWR);
                        put_attach_context(ctx);
                        _exit(EXIT_FAILURE);
                }

                if (pid == 0) {
                        struct attach_payload ap = {
                                .ipc_socket             = ipc_sockets[1],
                                .options                = options,
                                .ctx                    = ctx,
                                .terminal_pts_fd        = terminal.pty,
                                .exec_function          = exec_function,
                                .exec_payload           = exec_payload,
                        };

                        if (options->attach_flags & LXC_ATTACH_TERMINAL) {
                                ret = lxc_terminal_signal_sigmask_safe_blocked(&terminal);
                                if (ret < 0) {
                                        SYSERROR("Failed to reset signal mask");
                                        _exit(EXIT_FAILURE);
                                }
                        }

                        /* Does not return. */
                        do_attach(&ap);
                }
                TRACE("Attached process %d started initializing", pid);

                if (options->attach_flags & LXC_ATTACH_TERMINAL)
                        lxc_attach_terminal_close_pts(&terminal);

                /* Tell grandparent the pid of the pid of the newly created child. */
                if (!sync_wake_pid(ipc_sockets[1], 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);
                        put_attach_context(ctx);
                        _exit(EXIT_FAILURE);
                }

                /* The rest is in the hands of the initial and the attached process. */
                put_attach_context(ctx);
                _exit(EXIT_SUCCESS);
        }
        TRACE("Transient process %d started initializing", pid);

        to_cleanup_pid = pid;

        /* close unneeded file descriptors */
        close_prot_errno_disarm(ipc_sockets[1]);
        put_namespaces(ctx);
        if (options->attach_flags & LXC_ATTACH_TERMINAL)
                lxc_attach_terminal_close_pts(&terminal);

        /* Attach to cgroup, if requested. */
        if (options->attach_flags & LXC_ATTACH_MOVE_TO_CGROUP) {
                /*
                 * If this is the unified hierarchy cgroup_attach() is
                 * enough.
                 */
                ret = cgroup_attach(conf, name, lxcpath, pid);
                if (ret) {
                        call_cleaner(cgroup_exit) struct cgroup_ops *cgroup_ops = NULL;
                        if (!ERRNO_IS_NOT_SUPPORTED(ret)) {
                                SYSERROR("Failed to attach cgroup");
                                goto on_error;
                        }

                        cgroup_ops = cgroup_init(conf);
                        if (!cgroup_ops)
                                goto on_error;

                        if (!cgroup_ops->attach(cgroup_ops, conf, name, lxcpath, pid))
                                goto on_error;
                }

                TRACE("Moved transient process %d into container cgroup", pid);
        }

        /*
         * Close sensitive file descriptors we don't need anymore. Even if
         * we're the parent.
         */
        if (!attach_context_security_barrier(ctx))
                goto on_error;

        /* Setup /proc limits */
        ret = setup_proc_filesystem(conf, pid);
        if (ret < 0)
                goto on_error;

        /* Setup resource limits */
        ret = setup_resource_limits(conf, pid);
        if (ret < 0)
                goto on_error;

        if (options->attach_flags & LXC_ATTACH_TERMINAL) {
                ret = lxc_attach_terminal_mainloop_init(&terminal, &descr);
                if (ret < 0)
                        goto on_error;

                TRACE("Initialized terminal mainloop");
        }

        /* Let the child process know to go ahead. */
        if (!sync_wake(ipc_sockets[0], ATTACH_SYNC_CGROUP))
                goto close_mainloop;

        TRACE("Told transient process to start initializing");

        /* Get pid of attached process from transient process. */
        if (!sync_wait_pid(ipc_sockets[0], &attached_pid))
                goto close_mainloop;

        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 == STDIN_FILENO) {
                signal(SIGINT, SIG_IGN);
                signal(SIGQUIT, SIG_IGN);
        }

        /* Reap transient process. */
        ret = wait_for_pid(pid);
        if (ret < 0)
                goto close_mainloop;

        TRACE("Transient 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 (attach_lsm(options) && ctx->lsm_label) {
                __do_close int fd_lsm = -EBADF;
                bool on_exec;

                on_exec = options->attach_flags & LXC_ATTACH_LSM_EXEC ? true : false;
                fd_lsm = ctx->lsm_ops->process_label_fd_get(ctx->lsm_ops, attached_pid, on_exec);
                if (fd_lsm < 0)
                        goto close_mainloop;

                TRACE("Opened LSM label file descriptor %d", fd_lsm);

                /* Send child fd of the LSM security module to write to. */
                if (!sync_wake_fd(ipc_sockets[0], fd_lsm)) {
                        SYSERROR("Failed to send lsm label fd");
                        goto close_mainloop;
                }

                TRACE("Sent LSM label file descriptor %d to child", fd_lsm);
        }

        if (conf->seccomp.seccomp) {
                ret = lxc_seccomp_recv_notifier_fd(&conf->seccomp, ipc_sockets[0]);
                if (ret < 0)
                        goto close_mainloop;

                ret = lxc_seccomp_add_notifier(name, lxcpath, &conf->seccomp);
                if (ret < 0)
                        goto close_mainloop;
        }

        /* 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;

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

        ret_parent = 0;
        to_cleanup_pid = -1;

        if (options->attach_flags & LXC_ATTACH_TERMINAL) {
                ret = lxc_mainloop(&descr, -1);
                if (ret < 0) {
                        ret_parent = -1;
                        to_cleanup_pid = attached_pid;
                }
        }

close_mainloop:
        if (options->attach_flags & LXC_ATTACH_TERMINAL)
                lxc_mainloop_close(&descr);

on_error:
        if (ipc_sockets[0] >= 0) {
                shutdown(ipc_sockets[0], SHUT_RDWR);
                close_prot_errno_disarm(ipc_sockets[0]);
        }

        if (to_cleanup_pid > 0)
                (void)wait_for_pid(to_cleanup_pid);

        if (options->attach_flags & LXC_ATTACH_TERMINAL) {
                lxc_terminal_delete(&terminal);
                lxc_terminal_conf_free(&terminal);
        }

        put_attach_context(ctx);
        return ret_parent;
}

int lxc_attach_run_command(void *payload)
{
        int ret = -1;
        lxc_attach_command_t *cmd = payload;

        ret = execvp(cmd->program, cmd->argv);
        if (ret < 0) {
                switch (errno) {
                case ENOEXEC:
                        ret = 126;
                        break;
                case ENOENT:
                        ret = 127;
                        break;
                }
        }

        return syserror_ret(ret, "Failed to exec \"%s\"", cmd->program);
}

int lxc_attach_run_shell(void* payload)
{
        __do_free char *buf = NULL;
        uid_t uid;
        struct passwd pwent;
        struct passwd *pwentp = NULL;
        char *user_shell;
        ssize_t bufsize;
        int ret;

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

        uid = getuid();

        bufsize = sysconf(_SC_GETPW_R_SIZE_MAX);
        if (bufsize < 0)
                bufsize = 1024;

        buf = malloc(bufsize);
        if (buf) {
                ret = getpwuid_r(uid, &pwent, buf, bufsize, &pwentp);
                if (!pwentp) {
                        if (ret == 0)
                                WARN("Could not find matched password record");

                        WARN("Failed to get password record - %u", 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 (!pwentp)
                user_shell = lxc_attach_getpwshell(uid);
        else
                user_shell = pwent.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 (!pwentp)
                free(user_shell);

        return -1;
}