feat(fdtable): add CLOEXEC flag

[Stone749990226]

Add support for the CLOEXEC flag in file descriptors.

[Stone749990226]

test script:

main.c:

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

int main()
{
    int fd;
    pid_t pid;

    // Create a temporary file
    fd = open("test_file.txt", O_CREAT | O_RDWR, 0644);
    if (fd == -1) {
        perror("open failed");
        exit(EXIT_FAILURE);
    }

    // Print the file descriptor
    printf("Parent process: Opened file descriptor %d\n", fd);

    // Set FD_CLOEXEC flag (Close on exec)
    if (fcntl(fd, F_SETFD, FD_CLOEXEC) == -1) {
        perror("fcntl F_SETFD failed");
        close(fd);
        exit(EXIT_FAILURE);
    }

    // Create a child process
    pid = fork();
    if (pid == -1) {
        perror("fork failed");
        close(fd);
        exit(EXIT_FAILURE);
    }

    if (pid == 0) {
        // Child process: Execute another program
        printf("Child process: Executing 'child_test'...\n");
        char fd_str[16];
        snprintf(fd_str, sizeof(fd_str), "%d", fd); // Convert file descriptor to string
        execl("./test_cloexec_child_aarch64", "./test_cloexec_child_aarch64", fd_str, NULL);

        // If execl returns, something went wrong
        perror("execl failed");
        close(fd);
        exit(EXIT_FAILURE);
    } else {
        // Parent process: Wait for the child process to complete
        printf("Parent process: Waiting for child to complete...\n");
        int status;
        waitpid(pid, &status, 0);

        if (WIFEXITED(status)) {
            printf("Parent process: Child exited with status %d\n", WEXITSTATUS(status));
        } else {
            printf("Parent process: Child did not exit normally\n");
        }

        // Cleanup
        close(fd);
        unlink("test_file.txt"); // Delete the temporary file
        printf("Parent process: Done.\n");
    }

    return 0;
}

test_cloexec_child.c: This program is compiled into ./test_cloexec_child_aarch64, which will be called in main.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
    // Check if the file descriptor is passed as an argument
    if (argc < 2)
    {
        fprintf(stderr, "Usage: %s <fd>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    // Convert the argument to an integer (file descriptor)
    int fd = atoi(argv[1]); // Get the file descriptor passed as argument
    printf("Child process: Got file descriptor %d\n", fd);

    // Try to read from the file descriptor
    char buffer[16];
    ssize_t n = read(fd, buffer, sizeof(buffer));

    if (n == -1)
    {
        // If reading fails, it means FD_CLOEXEC is working, and the file descriptor is closed on exec
        perror("Child process: read failed");
        printf("Child process: File descriptor %d is not valid (expected if FD_CLOEXEC works).\n", fd);
        exit(EXIT_SUCCESS);
    }
    else
    {
        // If reading succeeds, something is wrong because the file descriptor should be closed
        printf("Child process: Unexpectedly read %zd bytes from file descriptor %d\n", n, fd);
        exit(EXIT_FAILURE);
    }
}

if CLOEXEC flag works, the output will be:

Parent process: Opened file descriptor 3
Parent process: Waiting for child to complete...
Child process: Executing 'child_test'...
Child process: read failed: Bad file descriptor
child get fd: 3Child process: File descriptor 3 is not valid (expected if FD_CLOEXEC works).
Parent process: Child exited with status 0
Parent process: Done.

else, the output will print:

Parent process: Opened file descriptor 3
Parent process: Waiting for child to complete...
Child process: Executing 'child_test'...
child get fd: 3
Child process: Unexpectedly read 0 bytes from file descriptor 3
Parent process: Child exited with status 1
Parent process: Done.