VFS in a multitasking scenario

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struct task_t {
    ...
    struct files_struct files;
    struct dentry* pwd;
}

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struct files_struct {
    int count;
    int next_fd;
    struct file** fd;
    struct file* fd_array[NR_OPEN_DEFAULT]; // default array of file objects
};

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void sys_open(struct trapframe* trapframe) {
    const char* pathname = (char*) trapframe->x[0];
    int flags = trapframe->x[1];
    struct file* f = vfs_open(pathname, flags);
    // open failed
    if (f == NULL) {
        trapframe->x[0] = -1;
        return;
    }
    // store fd in task struct
    int fd_num = current_task->files.next_fd;
    // open files more than fd array
    if (fd_num >= current_task->files.count) {
        int new_fd_array_size = current_task->files.count + NR_OPEN_DEFAULT;
        struct file** new_fd_array = (struct file**)kmalloc(sizeof(struct file*) * new_fd_array_size);
        for (int i = 0; i < current_task->files.count; i++) {
            new_fd_array[i] = current_task->files.fd[i];
        }
        current_task->files.fd = new_fd_array;
        current_task->files.count = new_fd_array_size;
    }
    current_task->files.fd[fd_num] = f;
    current_task->files.next_fd++;
    trapframe->x[0] = fd_num;
}

void sys_close(struct trapframe* trapframe) {
    int fd_num = (int)trapframe->x[0];
    if (fd_num < 0) {
        trapframe->x[0] = -1;
        return;
    }
    struct file* f = current_task->files.fd[fd_num];
    trapframe->x[0] = vfs_close(f);
    current_task->files.fd[fd_num] = NULL;
}

void sys_write(struct trapframe* trapframe) {
    int fd_num = (int)trapframe->x[0];
    char* buf = (char*)trapframe->x[1];
    if (fd_num < 0) {
        trapframe->x[0] = -1;
        return;
    }
    uint64_t len = (uint64_t)trapframe->x[2];
    struct file* f = current_task->files.fd[fd_num];
    if (f == NULL) {
        trapframe->x[0] = 0;
        return;
    }
    trapframe->x[0] = vfs_write(f, buf, len);
}

void sys_read(struct trapframe* trapframe) {
    int fd_num = (int)trapframe->x[0];
    char* buf = (char*)trapframe->x[1];
    if (fd_num < 0) {
        trapframe->x[0] = -1;
        return;
    }
    uint64_t len = (uint64_t)trapframe->x[2];
    struct file* f = current_task->files.fd[fd_num];
    if (f == NULL) {
        trapframe->x[0] = 0;
        return;
    }
    trapframe->x[0] = vfs_read(f, buf, len);
}

void sys_readdir(struct trapframe* trapframe) {
    int fd_num = (int)trapframe->x[0];
    if (fd_num < 0) {
        trapframe->x[0] = -1;
        return;
    }
    struct file* f = current_task->files.fd[fd_num];
    trapframe->x[0] = vfs_readdir(f);
}

void sys_mkdir(struct trapframe* trapframe) {
    const char* pathname = (char*) trapframe->x[0];
    trapframe->x[0] = vfs_mkdir(pathname);
}

void sys_chdir(struct trapframe* trapframe) {
    const char* pathname = (char*) trapframe->x[0];
    trapframe->x[0] = vfs_chdir(pathname);
}

void sys_mount(struct trapframe* trapframe) {
    const char* device = (char*) trapframe->x[0];
    const char* mountpoint = (char*) trapframe->x[1];
    const char* filesystem = (char*) trapframe->x[2];
    trapframe->x[0] = vfs_mount(device, mountpoint, filesystem);
}

void sys_umount(struct trapframe* trapframe) {
    const char* mountpoint = (char*) trapframe->x[0];
    trapframe->x[0] = vfs_umount(mountpoint);
}