上一次我们讲过，如何用setrlimit限制进程的资源，并以时间和内存为例。这次我们来试试看更多的资源限制，以防止恶意代码的攻击，增加系统的安全性。

超时

超时上次不是讲过了吗？嗯，上次讲的是“忙”的超时，而这次是“sleep”的超时。通过实验发现，如果程序一直咋sleep，父进程是无法检测到子进程超时的，我想这应该是因为sleep的时间本身不算在进程的CPU时间内。那啥办呢？好办，加定时器（alarm）。例如：

signal(SIGALRM, my_alarm_handler);
alarm(timeLimit * 2);

这样，当进程流逝的时间超过alarm的限制时，父进程也会受到相应的信号。

例如对于这样的代码：

#include <unistd.h>

int main ()
{
    sleep(15);

    return 0;
}

运行结果如下：

quxiao@quxiao-laptop:~/judge$ ./linux_judge_svn/linux/judge.o /home/quxiao/judge/a_and_b.in /home/quxiao/judge/out.txt 1 64 /home/quxiao/judge/just_sleep.o input file: /home/quxiao/judge/a_and_b.in output file: /home/quxiao/judge/out.txt time limit: 1 memory limit: 67108864 cmd: /home/quxiao/judge/just_sleep.o ================parent process===================== child exit status: 14 exit abnormally. exit by signal: 14 | 14 parent process is finished. child process time: 0 ms maximum resident set size:    292

生成/打开文件

有时我们并不希望子进程有文件操作（当然，除了输入输出设备这些特殊文件），可以通过修改RLIMIT_NOFILE的值来达到此效果。

如果程序会生成一些文件，代码如下：

for (i = 0; i < 10; i ++)
{
    char template[] = "template-XXXXXX";
    fd = mkstemp(template);
    if ( fd == -1 )
    {
        printf("can't create file!\n");
    }
}

那么，修改过限制之后的代码运行结果就是：

quxiao@quxiao-laptop:~/judge$ ./linux_judge_svn/linux/judge.o /home/quxiao/judge/a_and_b.in /home/quxiao/judge/out.txt 1 64 /home/quxiao/judge/too_many_file.o input file: /home/quxiao/judge/a_and_b.in output file: /home/quxiao/judge/out.txt time limit: 1 memory limit: 67108864 cmd: /home/quxiao/judge/too_many_file.o ================parent process===================== child exit status: 0 exit normally. parent process is finished. child process time: 0 ms

maximum resident set size:    364

quxiao@quxiao-laptop:~/judge$ cat out.txt can’t create file! can’t create file! can’t create file! can’t create file! can’t create file! can’t create file! can’t create file! can’t create file! can’t create file!

不过，从例子中可以看出，表面上父进程并没有检测到子进程的任何异常，但我们可以从输出文件中看到子进程的生成文件是受到限制的。另外，还有一点值得考虑，那就是子进程还是生成了一个文件的，RLIMIT_NOFILE到底应该设置成多少，还要再研究研究。

生成子进程

同样，让进程可以任意地生成子进程也是很危险的。RLIMIT_NPROC可以限制生成子进程的数目。对于如下代码：

    for (i = 0; i < 3; i ++)
    {
        pid_t pid;
        if ( (pid=fork()) == -1 )
        {
            printf("error!\n");
        }
        else if ( pid == 0 )
        {
            printf("this is child process\n");
        }
        else
        {
            printf("this is parent process\n");
        }
    }

运行结果如下：

quxiao@quxiao-laptop:~/judge$ ./linux_judge_svn/linux/judge.o /home/quxiao/judge/a_and_b.in /home/quxiao/judge/out.txt 1 64 /home/quxiao/judge/too_many_child.o input file: /home/quxiao/judge/a_and_b.in output file: /home/quxiao/judge/out.txt time limit: 1 memory limit: 67108864 cmd: /home/quxiao/judge/too_many_child.o ================parent process===================== child exit status: 0 exit normally. parent process is finished. child process time: 0 ms

maximum resident set size:    340

quxiao@quxiao-laptop:~/judge$ cat out.txt error! error! error!

runtime error

当程序出现一些运行时错误的时候，父进程也可以通过信号检测到。这里其实跟资源限制没太大关系，但检测这方面的信息也是很重要的。举2个例子，1个是除0，1个是数组越界。

除0程序：

int main ()
{
    int a, b;
    a = 1;
    b = 0;
    a = a / b;

    return 0;
}

quxiao@quxiao-laptop:~/judge$ ./linux_judge_svn/linux/judge.o /home/quxiao/judge/a_and_b.in /home/quxiao/judge/out.txt 1 64 /home/quxiao/judge/divide0.o input file: /home/quxiao/judge/a_and_b.in output file: /home/quxiao/judge/out.txt time limit: 1 memory limit: 67108864 cmd: /home/quxiao/judge/divide0.o ================parent process===================== child exit status: 8 exit abnormally. exit by signal: 8 | 8 parent process is finished. child process time: 0 ms

maximum resident set size:    292

数组越界程序：

const int MAX = 100;

int main ()
{
    int array[MAX];
    int i;
    i = 0;
    while ( ++i )   /* so dangerous! */
    {
        array[i] = 0;
    }

    return 0;
}

quxiao@quxiao-laptop:~/judge$ ./linux_judge_svn/linux/judge.o /home/quxiao/judge/a_and_b.in /home/quxiao/judge/out.txt 1 64 /home/quxiao/judge/re.o input file: /home/quxiao/judge/a_and_b.in output file: /home/quxiao/judge/out.txt time limit: 1 memory limit: 67108864 cmd: /home/quxiao/judge/re.o ================parent process===================== child exit status: 11 exit abnormally. exit by signal: 11 | 11 parent process is finished. child process time: 0 ms

maximum resident set size:    288