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
