使用有趣的 Linux 病毒程序解释基本 Linux 系统调用

如果你对编写 Linux 系统编程感兴趣，你应该学习所有基本的库/系统调用。本文有一个示例 C 程序，其中包含一组系统调用，可帮助您了解这些基本库调用的用法。

下面给出的示例 C 代码执行以下操作：

• 自动打开一些终端
• 显示会话以 root 或非 root 身份运行的消息
• 在所有打开的终端上显示上述消息

以下是以下示例代码中涵盖的 13 个重要的库或系统调用。

1. memset() ：此函数用常量字节 c 填充 s 指向的内存区域的前 n 个字节。
2. fopen() ：此函数打开文件，其名称是其第一个参数指向的字符串，并将流与它相关联。
3. getcwd() ：此函数返回一个以 null 结尾的字符串，其中包含一个绝对路径名，该路径名是调用进程的当前工作目录
4. getuid() ：该函数返回调用进程的真实用户ID
5. snprintf() ：此函数根据格式生成输出并将输出写入缓冲区。
6. fwrite() ：此函数用于将数据写入流
7. fflush() ：此函数强制将所有用户空间缓冲数据写入特定流
8. fclose() ：此函数刷新关联的流并关闭底层文件描述符。
9. system() : 这个函数执行一个命令
10. sleep() ：此函数使调用进程休眠，直到经过指定的秒数或没有忽略的信号到达。
11. opendir() : 这个函数打开一个目录流
12. readdir() ：此函数读取作为流打开的目录
13. atoi() ：此函数将 ascii 参数转换为整数。

以下是显示如何使用上述所有 13 个系统调用的 C 代码。

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<dirent.h>
#include<sys/types.h>
#include<pwd.h>

// A buffer to hold current working directory
char cwd[512];

void inform(char *path, char *binary_name)
{
    // Declare variables for file operations
    FILE *fp = NULL;

    // A counter to be used in loop
    unsigned int counter = 0;

    // A buffer to hold the information message
    char msg[1024];
    // memset function initializes the bytes
    // in the buffer 'msg' with NULL characters
    memset(msg, '\0', sizeof(msg));

    memset(cwd, '\0', sizeof(cwd));

    // Check for the path to be non NULL
    if(NULL== path)
    {
         printf("\n NULL path detected\n");
         return;
    }

    // fopen will open the file represented
    // by 'path' in read write mode.
    fp = fopen(path,"r+");

    if(!fp)
    {
        printf("\n Failed to open %s\n",path);
        return;
    }
    else
    {
        printf("\n Successfully opened %s\n",path);
    }

    // getcwd() gives us the current working directory
    // of the environemt from which this binary was
    // executed
    if(NULL == getcwd(cwd,sizeof(cwd)))
    {
        printf("\n Failed to get current directory\n");
        return;
    }

    // getuid() returns the real user ID of the calling
    // process.
    // getuid() returns 0 for root and non zero for
    // any other user.
    if( 0 != getuid())
    {
        // This functions fills the buffer 'msg' with the formatted string by replacing %s in the harcoded string with the appropriate values
        snprintf(msg,sizeof(msg),"\n\n\nYOU ARE NOT ROOT!!!!!");
    }
    else
    {
       snprintf(msg, sizeof(msg),"\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nYOU ARE ROOT!!!!!!!!!!!!!!");
    }

   // Make sure the information8 is printed 25 times on each
   // open terminal
   for(counter=0;counter<25;counter++)
   {
       printf("\n fwrite()\n");
       // Write the information message on to the terminal
       fwrite(msg, strlen(msg), 1, fp);
       // Flush the message to the stdout of the terminal
       fflush(fp);
       // Wait for one second.
       sleep(1);
   }
   // close the file representing the terminal
   fclose(fp);

}

int main(int argc, char *argv[])
{
    // Since we will do some directory operations
    // So declare some variables for it.
    DIR *dp = NULL;
    struct dirent *ptr = NULL;

    // This variable will contain the path to
    // terminal
    char *path = NULL;

    // Used as a counter in loops
    int i =0;

    // Step1 :
    // Open 5 terminals each after 2 seconds
    // of delay.
    for(;i<5;i++)
    {
        // The system API executes a shell command
        // We try to execute two commands here
        // Both of these commands will open up
        // a terminal. We have used two commands
        // just in case one of them fails.
        system("gnome-terminal");
        system("/usr/bin/xterm");

        // This call is used to cause a delay in
        // program execution. The argument to this
        // function is the number of seconds for
        // which the delay is required
        sleep(2);
    }

    // Give user some 60 seconds before issuing
    // a information message.
    sleep(60);

    // Now, open the directory /dev/pts which
    // corresponds to the open command terminals.
    dp = opendir("/dev/pts");
    if(NULL == dp)
    {
        printf("\n Failed to open /dev/pts\n");
        return 0;
    }

    // Now iterate over each element in the
    // directory untill all the elements are
    // iterated upon.
    while ( NULL != (ptr = readdir(dp)) )
    {
        // ptr->d_name gives the current device
        // name or the terminal name as a device.
        // All the numeric names correspond to
        // open terminals.

        // To check the numeric values we use
        // atoi().
        // Function atoi() converts the ascii
        // value into integer

        switch(atoi(ptr->d_name))
        {
            // Initialize 'path' accordingly

            case 0:path = "/dev/pts/0";
                   break;
            case 1:
                   path = "/dev/pts/1";
                   break;
            case 2:
                   path = "/dev/pts/2";
                   break;
            case 3:
                   path = "/dev/pts/3";
                   break;
            case 4:
                   path = "/dev/pts/4";
                   break;
            case 5:
                   path = "/dev/pts/5";
                   break;
            case 6:
                   path = "/dev/pts/6";
                   break;
            case 7:
                   path = "/dev/pts/8";
                   break;
            case 9:
                   path = "/dev/pts/9";
                   break;
            default:
                   break;
         }
         if(path)
         {
             // Call this function to throw some information.
             // Pass the path to terminal where the information
             // is to be sent and the binary name of this
             // program
             inform(path, argv[0]);
             // Before next iteration, make path point to
             // NULL
             path = NULL;
         }

    }

    sleep(60);

    return 0;
}

上面的代码本身是不言自明的，因为它包含足够的注释来解释这些系统调用的作用。如果您是 Linux 系统编程的新手，这段代码足以让您充分了解所有这些重要功能的用法。有关更多详细信息和高级用法，请仔细阅读他们的手册页。

这段代码是一个有趣的基本病毒程序的模拟。编译并执行上述 c 程序后，它将执行以下操作。此代码在 Linux mint 上进行了测试。但是，它应该适用于所有 ubuntu 衍生产品。

• 用户将在 1 秒后看到 5 个终端一个接一个地打开。
• 虽然用户会想知道刚刚发生了什么，但他所有打开的终端都会慢慢开始重复获取有关登录是 root 还是非 root 的信息。
• 请注意，为了您的学习目的，代码中启用了调试日志记录，如果您想玩得开心，请注释掉调试 printf，然后执行它。