/* vi: set sw=4 ts=4: */
/*
 * Mini nsenter implementation for busybox.
 *
 * Copyright (C) 2016 by Bartosz Golaszewski <bartekgola@gmail.com>
 *
 * Licensed under GPLv2 or later, see file LICENSE in this source tree.
 */
//config:config NSENTER
//config:       bool "nsenter (6.5 kb)"
//config:       default y
//config:       help
//config:       Run program with namespaces of other processes.

//applet:IF_NSENTER(APPLET(nsenter, BB_DIR_USR_BIN, BB_SUID_DROP))

//kbuild:lib-$(CONFIG_NSENTER) += nsenter.o

//usage:#define nsenter_trivial_usage
//usage:       "[OPTIONS] [PROG ARGS]"
//usage:#define nsenter_full_usage "\n"
//usage:     "\n        -t PID          Target process to get namespaces from"
//usage:     "\n        -m[FILE]        Enter mount namespace"
//usage:     "\n        -u[FILE]        Enter UTS namespace (hostname etc)"
//usage:     "\n        -i[FILE]        Enter System V IPC namespace"
//usage:     "\n        -n[FILE]        Enter network namespace"
//usage:     "\n        -p[FILE]        Enter pid namespace"
//usage:     "\n        -U[FILE]        Enter user namespace"
//usage:     "\n        -S UID          Set uid in entered namespace"
//usage:     "\n        -G GID          Set gid in entered namespace"
//usage:        IF_LONG_OPTS(
//usage:     "\n        --preserve-credentials  Don't touch uids or gids"
//usage:        )
//usage:     "\n        -r[DIR]         Set root directory"
//usage:     "\n        -w[DIR]         Set working directory"
//usage:     "\n        -F              Don't fork before exec'ing PROG"

#include <sched.h>
#ifndef CLONE_NEWUTS
# define CLONE_NEWUTS  0x04000000
#endif
#ifndef CLONE_NEWIPC
# define CLONE_NEWIPC  0x08000000
#endif
#ifndef CLONE_NEWUSER
# define CLONE_NEWUSER 0x10000000
#endif
#ifndef CLONE_NEWPID
# define CLONE_NEWPID  0x20000000
#endif
#ifndef CLONE_NEWNET
# define CLONE_NEWNET  0x40000000
#endif

#include "libbb.h"

struct namespace_descr {
        int flag;               /* value passed to setns() */
        char ns_nsfile8[8];     /* "ns/" + namespace file in process' procfs entry */
};

struct namespace_ctx {
        char *path;             /* optional path to a custom ns file */
        int fd;                 /* opened namespace file descriptor */
};

enum {
        OPT_user        = 1 << 0,
        OPT_ipc         = 1 << 1,
        OPT_uts         = 1 << 2,
        OPT_network     = 1 << 3,
        OPT_pid         = 1 << 4,
        OPT_mount       = 1 << 5,
        OPT_target      = 1 << 6,
        OPT_setuid      = 1 << 7,
        OPT_setgid      = 1 << 8,
        OPT_root        = 1 << 9,
        OPT_wd          = 1 << 10,
        OPT_nofork      = 1 << 11,
        OPT_prescred    = (1 << 12) * ENABLE_LONG_OPTS,
};
enum {
        NS_USR_POS = 0,
        NS_IPC_POS,
        NS_UTS_POS,
        NS_NET_POS,
        NS_PID_POS,
        NS_MNT_POS,
        NS_COUNT,
};
/*
 * The order is significant in nsenter.
 * The user namespace comes first, so that it is entered first.
 * This gives an unprivileged user the potential to enter other namespaces.
 */
static const struct namespace_descr ns_list[] = {
        { CLONE_NEWUSER, "ns/user", },
        { CLONE_NEWIPC,  "ns/ipc",  },
        { CLONE_NEWUTS,  "ns/uts",  },
        { CLONE_NEWNET,  "ns/net",  },
        { CLONE_NEWPID,  "ns/pid",  },
        { CLONE_NEWNS,   "ns/mnt",  },
};
/*
 * Upstream nsenter doesn't support the short option for --preserve-credentials
 * "+": stop on first non-option
 */
static const char opt_str[] ALIGN1 = "+""U::i::u::n::p::m::""t:+S:+G:+r::w::F";

#if ENABLE_LONG_OPTS
static const char nsenter_longopts[] ALIGN1 =
        "user\0"                        Optional_argument       "U"
        "ipc\0"                         Optional_argument       "i"
        "uts\0"                         Optional_argument       "u"
        "net\0"                         Optional_argument       "n"
        "pid\0"                         Optional_argument       "p"
        "mount\0"                       Optional_argument       "m"
        "target\0"                      Required_argument       "t"
        "setuid\0"                      Required_argument       "S"
        "setgid\0"                      Required_argument       "G"
        "root\0"                        Optional_argument       "r"
        "wd\0"                          Optional_argument       "w"
        "no-fork\0"                     No_argument             "F"
        "preserve-credentials\0"        No_argument             "\xff"
        ;
#endif

/*
 * Open a file and return the new descriptor. If a full path is provided in
 * fs_path, then the file to which it points is opened. Otherwise (fd_path is
 * NULL) the routine builds a path to a procfs file using the following
 * template: '/proc/<target_pid>/<target_file>'.
 */
static int open_by_path_or_target(const char *path,
                                  pid_t target_pid, const char *target_file)
{
        char proc_path_buf[sizeof("/proc/%u/1234567890") + sizeof(int)*3];

        if (!path) {
                if (target_pid == 0) {
                        /* Example:
                         * "nsenter -p PROG" - neither -pFILE nor -tPID given.
                         */
                        bb_show_usage();
                }
                snprintf(proc_path_buf, sizeof(proc_path_buf),
                         "/proc/%u/%s", (unsigned)target_pid, target_file);
                path = proc_path_buf;
        }

        return xopen(path, O_RDONLY);
}

int nsenter_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int nsenter_main(int argc UNUSED_PARAM, char **argv)
{
        int i;
        unsigned int opts;
        const char *root_dir_str = NULL;
        const char *wd_str = NULL;
        struct namespace_ctx ns_ctx_list[NS_COUNT];
        int setgroups_failed;
        int root_fd, wd_fd;
        int target_pid = 0;
        int uid = 0;
        int gid = 0;

        memset(ns_ctx_list, 0, sizeof(ns_ctx_list));

        opts = getopt32long(argv, opt_str, nsenter_longopts,
                        &ns_ctx_list[NS_USR_POS].path,
                        &ns_ctx_list[NS_IPC_POS].path,
                        &ns_ctx_list[NS_UTS_POS].path,
                        &ns_ctx_list[NS_NET_POS].path,
                        &ns_ctx_list[NS_PID_POS].path,
                        &ns_ctx_list[NS_MNT_POS].path,
                        &target_pid, &uid, &gid,
                        &root_dir_str, &wd_str
        );
        argv += optind;

        root_fd = wd_fd = -1;
        if (opts & OPT_root)
                root_fd = open_by_path_or_target(root_dir_str,
                                                 target_pid, "root");
        if (opts & OPT_wd)
                wd_fd = open_by_path_or_target(wd_str, target_pid, "cwd");

        for (i = 0; i < NS_COUNT; i++) {
                const struct namespace_descr *ns = &ns_list[i];
                struct namespace_ctx *ns_ctx = &ns_ctx_list[i];

                ns_ctx->fd = -1;
                if (opts & (1 << i))
                        ns_ctx->fd = open_by_path_or_target(ns_ctx->path,
                                        target_pid, ns->ns_nsfile8);
        }

        /*
         * Entering the user namespace without --preserve-credentials implies
         * --setuid & --setgid and clearing root's groups.
         */
        setgroups_failed = 0;
        if ((opts & OPT_user) && !(opts & OPT_prescred)) {
                opts |= (OPT_setuid | OPT_setgid);
                /*
                 * We call setgroups() before and after setns() and only
                 * bail-out if it fails twice.
                 */
                setgroups_failed = (setgroups(0, NULL) < 0);
        }

        for (i = 0; i < NS_COUNT; i++) {
                const struct namespace_descr *ns = &ns_list[i];
                struct namespace_ctx *ns_ctx = &ns_ctx_list[i];

                if (ns_ctx->fd < 0)
                        continue;
                if (setns(ns_ctx->fd, ns->flag)) {
                        bb_perror_msg_and_die(
                                "setns(): can't reassociate to namespace '%s'",
                                ns->ns_nsfile8 + 3 /* skip over "ns/" */
                        );
                }
                close(ns_ctx->fd); /* should close fds, to not confuse exec'ed PROG */
                /*ns_ctx->fd = -1;*/
        }

        if (root_fd >= 0) {
                if (wd_fd < 0) {
                        /*
                         * Save the current working directory if we're not
                         * changing it.
                         */
                        wd_fd = xopen(".", O_RDONLY);
                }
                xfchdir(root_fd);
                xchroot(".");
                close(root_fd);
                /*root_fd = -1;*/
        }

        if (wd_fd >= 0) {
                xfchdir(wd_fd);
                close(wd_fd);
                /*wd_fd = -1;*/
        }

        /*
         * Entering the pid namespace implies forking unless it's been
         * explicitly requested by the user not to.
         */
        if (!(opts & OPT_nofork) && (opts & OPT_pid)) {
                xvfork_parent_waits_and_exits();
                /* Child continues */
        }

        if (opts & OPT_setgid) {
                if (setgroups(0, NULL) < 0 && setgroups_failed)
                        bb_simple_perror_msg_and_die("setgroups");
                xsetgid(gid);
        }
        if (opts & OPT_setuid)
                xsetuid(uid);

        exec_prog_or_SHELL(argv);
}