如何测试 SOCK_CLOEXEC/ O_CLOEXEC 的功能（执行时关闭）

Question

我有一个已接受的文件描述符来设置 SOCK_CLOEXEC。谁能告诉我如何测试已设置 O_CLOEXEC/SOCK_CLOEXEC 的 FD（接受的套接字不是任何 fd）的功能

accepted_fd =   accept4(lsn_fd, (struct sockaddr *)& tcp_remote, &size,  SOCK_CLOEXEC);

请通过任何方法，任何测试它的代码提供给我，除非通过测试 FD_CLOEXE 是否被返回

fdf = fcntl(accept_fd, F_GETFD);

分叉代码：

  pid_t childpid;
    childpid = fork();
    if (childpid >= 0) /* fork succeeded */
      {
        if (childpid == 0) /* fork() returns 0 to the child process */
          {
            rc = recv_gd_flow2(0, accepted_fd, count+1); /* wrapper function calls recv() */
            CHECK_VALUE("recv_gd_flow, with 0", rc, 0, goto cleanup);

            execl("/bin/ls", "ls", NULL); //EXEC ls

            //CHILD
            if(accepted_fd != INVALID_SOCKET)
              printf("I am Child and sock is closed\n");
            else
              printf("I am Child and sock is opend\n");
          }
        else{

          if(accepted_fd != INVALID_SOCKET)
            printf("I am Parent and sock is closed\n");
          else
            printf("I am Parent and sock is opend\n");
          //

        }
      }
     // The purpose of this code to see if child has a privileged to get a leaked fd, i think he shouldn't  

Answer 1

更新

我注意到您在代码中添加了对execl() 的调用，并在执行后 测试套接字描述符。这是行不通的。来自execl man-page：

RETURN VALUE
       The  exec() functions return only if an error has occurred.  The return
       value is -1, and errno is set to indicate the error.

成功执行后的代码将永远不会执行。必须执行测试的是执行过程。

/更新

它被称为close-on-exec。在fork() 之后，您需要在子进程中执行某些操作。仅分叉不足以测试此标志。

这是一个示例（错误处理最少，并且有一些快捷方式）：

accept4.c:

/*
  gcc -g -Wall -Wextra -std=c99 -D _GNU_SOURCE accept4.c
*/
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/wait.h>

int main (void)
{
    int ld = -1;
    int ad = -1;
    int ret = -1;
    struct sockaddr_in serv;
    int reuse = 1;
    pid_t pid = 0;
    pid_t w = 0;
    int status = 0;

    memset(&serv, 0, sizeof(serv));
    serv.sin_family = AF_INET;
    serv.sin_addr.s_addr = htonl(INADDR_ANY);
    serv.sin_port = htons(4567);

    ld = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (ld < 0 )
        perror("socket");

    ret = bind(ld, (struct sockaddr *) &serv, sizeof(serv));
    if (ret < 0)
        perror("bind");

    ret = setsockopt(ld, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse));
    if (ret < 0)
        perror("setsockopt");  

    ret = listen(ld, 1);
    if (ret < 0)
        perror("listen");

    ad = accept4(ld, NULL, NULL,  SOCK_CLOEXEC);
    printf("Parent accepted fd %d\n", ad); fflush(stdout);

    pid = fork();

    if (pid < 0)
        perror("fork");

    if (pid == 0) {

        char adbuf[2] = {0};
        char * params [3];

        adbuf[0] = ad + '0'; /* kluge for singe digit numbers */
        params[0] = "./child";
        params[1] = adbuf;
        params[2] = NULL;

        execv("./child", params);
        /* execv() does not return on success */
        perror("execv");
    }

    /* from man waitpid */
    do {
        w = waitpid(pid, &status, WUNTRACED | WCONTINUED);
        if (w == -1) {
            perror("waitpid");
            exit(EXIT_FAILURE);
        }

        if (WIFEXITED(status)) {
            printf("exited, status=%d\n", WEXITSTATUS(status));
        } else if (WIFSIGNALED(status)) {
            printf("killed by signal %d\n", WTERMSIG(status));
        } else if (WIFSTOPPED(status)) {
            printf("stopped by signal %d\n", WSTOPSIG(status));
        } else if (WIFCONTINUED(status)) {
            printf("continued\n");
        }
    } while (!WIFEXITED(status) && !WIFSIGNALED(status));
    exit(EXIT_SUCCESS);
}

child.c:

/*
  gcc -g -Wall -Wextra -std=c99 child.c -o child
 */
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main (int argc, char *argv[])
{
    char msg[] = "Hello\n";
    ssize_t written = 0;
    int ad = -1;

    if (argc < 2) {
        printf("Too few arguments to child\n");
        exit(EXIT_FAILURE);
    }
    ad = atoi(argv[1]); /* atoi() does not detect errors properly */
    printf("Child attempting to write to fd %d\n", ad); fflush(stdout);
    written = write(ad, msg, sizeof(msg));
    if (written != sizeof(msg)) {
        perror("write");
        exit(EXIT_FAILURE);
    }
    close(ad);
    exit(EXIT_SUCCESS);
}

用法：

1. 编译成a.out和child
2. 执行./a.out
3. telnet 本地主机 4567
4. 观察
5. 额外功劳：将accept4()flags 更改为 0 并重做 1-4

ps。我希望我没有做你的功课/剥夺你的学习经验。