当用户通过 SUID 位提升为 root 时，smbd 不会启动或停止

在 Ubuntu 12.04 下，我编写了以下 C 程序来帮助我在运行自动备份时关闭服务器的 apache2 和 samba 服务。请注意，在 Makefile 中我设置了 SUID 位，这样程序在由低级用户运行时将具有 root 权限tmv。

服务.c：

#include <stdio.h>
#include <stdlib.h>

void usage(char * arg0) {
    printf("Usage: %s start|stop\n", arg0);
    exit(1);
}

int main(int argc, char ** argv) {
    fprintf(stderr, "Running as: ");
    system("whoami");
    if (argc != 2) usage(argv[0]);
    if (!strcmp(argv[1], "stop")) {
        printf("Before running rsync, we need to shut down apache2 and smbd.\n");
        system("service apache2 stop");
        system("service smbd stop");
    } else if (!strcmp(argv[1], "start")) {
        printf("After running rsync, we need to start apache2 and smbd.\n");
        system("service apache2 start");
        system("service smbd start");
    } else {
        usage(argv[0]);
    }
    return 0;
}

生成文件：

all: services.c
    gcc -o services services.c
    chown root:tmv services
    chmod u+s services      # allow elevation to root
    chmod o-rx services     # only user tmv should execute

这是我得到的：

tmv@patience:~$ ./services start
Running as: root
After running rsync, we need to start apache2 and smbd.
 * Starting web server apache2                                           [ OK ] 
start: Unable to connect to system bus: Failed to connect to socket /var/run/dbus/system_bus_socket: No such file or directory

以 root 身份运行工作正常：

# ./services start
Running as: root
After running rsync, we need to start apache2 and smbd.
 * Starting web server apache2                                           [ OK ] 
smbd start/running, process 8515

任何想法为什么我./services在以用户身份运行时无法按预期工作tmv？是不是也需要配置一些环境变量？