pty下的SSH仍然是来自终端的标准

Question

我目前正在学习C中的伪终端功能，我能够创建pty的主、从端，在叉之后，我已经将stdin、stdout、stderr设置为fd从，在exec仍然从终端提示输入密码而不是从stdin(fds)中读取密码之后。当使用ssh复制流程会导致此行为时，其他程序将按预期工作。

当在子进程中使用setsid(); ioctl(0, TIOCSCTTY, 1);时，这个问题可以修复--在execvp之前，我找不到原因，这是一种预期的行为吗？

#define _XOPEN_SOURCE 600
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#define __USE_BSD
#include <termios.h>
#include <sys/select.h>
#include <sys/ioctl.h>
#include <string.h>

int main(int ac, char *av[])
{
    int fdm, fds;
    int rc;
    char input[1024], output[150];

    // Check arguments
    if (ac <= 1)
    {
        fprintf(stderr, "Usage: %s program_name [parameters]\n", av[0]);
        exit(1);
    }

    fdm = posix_openpt(O_RDWR);
    if (fdm < 0)
    {
        fprintf(stderr, "Error %d on posix_openpt()\n", errno);
        return 1;
    }

    rc = grantpt(fdm);
    if (rc != 0)
    {
        fprintf(stderr, "Error %d on grantpt()\n", errno);
        return 1;
    }

    rc = unlockpt(fdm);
    if (rc != 0)
    {
        fprintf(stderr, "Error %d on unlockpt()\n", errno);
        return 1;
    }

    // Open the slave side ot the PTY
    fds = open(ptsname(fdm), O_RDWR);

    // Create the child process
    if (fork())
    {
        fd_set fd_in;

        // FATHER

        // Close the slave side of the PTY
        close(fds);
        int pass_entered = 0;
        while (1)
        {
            FD_ZERO(&fd_in);
            FD_SET(fdm, &fd_in);
            rc = select(fdm + 1, &fd_in, NULL, NULL, NULL);
            // If data on master side of PTY
            if (FD_ISSET(fdm, &fd_in))
            {
                rc = read(fdm, output, sizeof(input));
                if (rc > 0)
                {
                    // Send data on standard output
                    if (!pass_entered)
                    {
                        write(fdm, "password\n", 10);
                        pass_entered = 1;
                    }
                    write(2, "<<", 2);
                    write(2, output, rc);
                    write(2, ">>", 2);
                    int n = write(fdm, "id\n", 3);
                }
            }
        }
    }
    else
    {
        // CHILD

        // Close the master side of the PTY
        close(fdm);
        // The slave side of the PTY becomes the standard input and outputs of the child process
        close(0); // Close standard input (current terminal)
        close(1); // Close standard output (current terminal)
        close(2); // Close standard error (current terminal)

        dup2(fds, 0); // PTY becomes standard input (0)
        dup2(fds, 1); // PTY becomes standard output (1)
        dup2(fds, 2); // PTY becomes standard error (2)

        close(fds);
        // setsid();
        // ioctl(0, TIOCSCTTY, 1);

        {
            char **child_av;
            int i;

            // Build the command line
            child_av = (char **)malloc(ac * sizeof(char *));
            for (i = 1; i < ac; i++)
            {
                child_av[i - 1] = strdup(av[i]);
            }
            child_av[i - 1] = NULL;
            rc = execvp(child_av[0], child_av);
        }

        return 1;
    }
    return 0;
} // main

// gcc test_pty.c  && ./a.out python3 /tty_test.py //works
// gcc test_pty.c  && ./a.out su root              //works
// gcc test_pty.c  && ./a.out ssh user@localhost   //fails

Answer 1

"Unix环境下的高级编程“(APUE)显示了用于pty设置的不同代码。特别是，子节点总是执行一个setsid(2)来创建一个新会话。此外，还注意到FreeBSD (可能还有其他*BSD系统)需要TIOCSCTTY调用才能建立控制终端：

// from http://www.apuebook.com/src.3e.tar.gz -- lib/ptyfork.c
        if ((pid = fork()) < 0) {
                return(-1);
        } else if (pid == 0) {          /* child */
                if (setsid() < 0)
                        err_sys("setsid error");

                /*
                 * System V acquires controlling terminal on open().
                 */
                if ((fds = ptys_open(pts_name)) < 0)
                        err_sys("can't open slave pty");
                close(fdm);             /* all done with master in child */

#if     defined(BSD)
                /*
                 * TIOCSCTTY is the BSD way to acquire a controlling terminal.
                 */
                if (ioctl(fds, TIOCSCTTY, (char *)0) < 0)
                        err_sys("TIOCSCTTY error");
#endif

除了示例ptym_open代码之外，您还需要深入挖掘定位pty/main.c和ptys_open代码，以查看APUE代码在什么时候执行什么任务。

您的孩子执行代码似乎不必要地复杂；

    {
        char **child_av;
        int i;

        // Build the command line
        child_av = (char **)malloc(ac * sizeof(char *));
        for (i = 1; i < ac; i++)
        {
            child_av[i - 1] = strdup(av[i]);
        }
        child_av[i - 1] = NULL;
        rc = execvp(child_av[0], child_av);
    }

可能被替换为

av++;
execvp(av[0], av);

它将直接使用现有的参数，而不是重复它们。作为另一种选择，APUE显示

execvp(argv[optind], &argv[optind]);

因为他们已经用getopt处理了选项。