diskrm command causes segmentation fault and double free in fox32os 0.3.0

[Andpuv]

Hello,

First of all, congratulations on the project — it's really fascinating and well-designed!

While experimenting with the system, I encountered some emulator crashes when using the diskrm command in the shell provided with fox32os 0.3.0. Specifically:

• Attempting to remove a disk that was never mounted results in a segmentation fault.
• Attempting to remove an already removed disk causes a double free error, leading to an emulator abort.

I believe the issue originates from the following function:

void remove_disk(size_t id) {
    if (id > 3) { puts("attempting to access disk with ID > 3"); return; }
    printf("unmounting disk ID %d\n", (int) id);
    /* >>> Abort! <<< */
    fclose(disk_controller.disks[id].file);
    disk_controller.disks[id].size = 0;
}

The problem seems to stem from the lack of validation on disk_controller.disks[id].file. If the pointer is NULL, calling fclose results in undefined behavior. Additionally, once the file is closed, the pointer is not set to NULL, making a second call to remove_disk(id) cause a double free.

A potential fix could be:

void remove_disk(size_t id) {
    if (id > 3) { puts("attempting to access disk with ID > 3"); return; }
    printf("unmounting disk ID %d\n", (int) id);

    /* Prevent segmentation fault by ensuring the file pointer is valid */
    if (disk_controller.disks[id].file) {
        fclose(disk_controller.disks[id].file);
        disk_controller.disks[id].size = 0;

        /* Prevent double free by setting the pointer to NULL */
        disk_controller.disks[id].file = NULL;
    }
}

Additionally, if I understand correctly, disk 0 is the one from which the operating system boots. It might be a good idea to prevent its removal entirely, possibly by adding a check inside the OS shell command (diskrm.asm). A warning message could notify users that disk 0 cannot be removed.

Let me know what you think! Thanks for your hard work on this project.