netns-exec.c

/*
 * Copyright (C) 2016 Pekka Ekman
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version
 * 2 of the License, or (at your option) any later version.
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <errno.h>
#include <string.h>
#include <stdbool.h>

#include "namespace.h"


#define EXIT_INVALID_ARGS 2
#define EXIT_NOT_FOUND 127
#define EXIT_CANNOT_EXEC 126
#define EXIT_OTHER_ERROR 125


static void usage(const char *argv0, bool is_error)
{
    fprintf(
        stderr,
        "\n"
        "usage:\n"
        "\n"
        "run command in netns:\n"
        "    %1$s [-h | --help] [--] <netns> <command> [<args> ...]\n"
        "\n"
        "run shell in netns:\n"
        "    %1$s [-h | --help] [--] <netns>\n"
        "\n",
        argv0);

    exit(is_error ? EXIT_INVALID_ARGS : 0);
}


/*
 * Check if namespace name is valid.
 *
 * The iproute2 code doesn't seem to do any validation. That is
 * probably because it's normally run as root, so the lack of
 * validation doesn't allow the user to do anything they couldn't do
 * already. This is not true for a suid root program, so we have to do
 * the validation.
 *
 * Because the namespace is implemented as a file, the character '/'
 * and the names ".", ".." and "" are forbidden. (Character '\0' is
 * also forbidden, but cannot be passed in a command line argument.)
 */
static bool is_valid_ns(const char *ns)
{
    return ns[0] != '\0'
        && strcmp(ns, ".") != 0 && strcmp(ns, "..") != 0
        && strchr(ns, '/') == NULL;
}


static void drop_root_privileges(void)
{
    if (setuid(getuid()) < 0) {
        perror("error setting user id");
        exit(EXIT_OTHER_ERROR);
    }
    if (setgid(getgid()) < 0) {
        perror("error setting group id");
        exit(EXIT_OTHER_ERROR);
    }
}


int main(int argc, char *argv[])
{
    // parse arguments
    char *argv0 = argv[0];
    if (argc < 2 || (argc == 2 && strcmp(argv[1], "--") == 0))
        // no netns given
        usage(argv0, true);
    if (strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-h") == 0)
        usage(argv0, false);

    if (strcmp(argv[1], "--") == 0) {
        argv++;
        argc--;
    }
    else if (argv[1][0] == '-') {
        // invalid command line option
        fprintf(stderr, "Unknown option: %s\n", argv[1]);
        usage(argv0, true);
    }

    if (! is_valid_ns(argv[1])) {
        fputs("Invalid namespace\n", stderr);
        usage(argv0, true);
    }


    // switch network namespace using iproute2's function
    if (netns_switch(argv[1]) < 0)
        return EXIT_OTHER_ERROR;


    drop_root_privileges();


    char **command_argv;

    // if no command given, start shell
    if (argc == 2) {
        static char *shell_argv[] = { "/bin/sh", NULL };
        char *shell = getenv("SHELL");
        if (shell != NULL)
            shell_argv[0] = shell;
        command_argv = shell_argv;
    }
    else {
        command_argv = &argv[2];
    }


    // execute the command
    execvp(command_argv[0], command_argv);

    fprintf(stderr, "error executing \"%s\": %s",
            command_argv[0], strerror(errno));
    if (errno == ENOENT)
        return EXIT_NOT_FOUND;
    else
        return EXIT_CANNOT_EXEC;
}