long do_fork(unsigned long clone_flags,
          unsigned long stack_start,
          unsigned long stack_size,
          int __user *parent_tidptr,
          int __user *child_tidptr)
{
    struct task_struct *p;
    int trace = 0;
    long nr;

    /*
     * Determine whether and which event to report to ptracer.  When
     * called from kernel_thread or CLONE_UNTRACED is explicitly
     * requested, no event is reported; otherwise, report if the event
     * for the type of forking is enabled.
     */
    if (!(clone_flags & CLONE_UNTRACED)) {
        if (clone_flags & CLONE_VFORK)
            trace = PTRACE_EVENT_VFORK;
        else if ((clone_flags & CSIGNAL) != SIGCHLD)
            trace = PTRACE_EVENT_CLONE;
        else
            trace = PTRACE_EVENT_FORK;

        if (likely(!ptrace_event_enabled(current, trace)))
            trace = 0;
    }

    p = copy_process(clone_flags, stack_start, stack_size,
             child_tidptr, NULL, trace);
    /*
     * Do this prior waking up the new thread - the thread pointer
     * might get invalid after that point, if the thread exits quickly.
     */
    if (!IS_ERR(p)) {
        struct completion vfork;
        struct pid *pid;

        trace_sched_process_fork(current, p);

        pid = get_task_pid(p, PIDTYPE_PID);
        nr = pid_vnr(pid);

        if (clone_flags & CLONE_PARENT_SETTID)
            put_user(nr, parent_tidptr);

        if (clone_flags & CLONE_VFORK) {
            p->vfork_done = &vfork;
            init_completion(&vfork);
            get_task_struct(p);
        }

        wake_up_new_task(p);

        /* forking complete and child started to run, tell ptracer */
        if (unlikely(trace))
            ptrace_event_pid(trace, pid);

        if (clone_flags & CLONE_VFORK) {
            if (!wait_for_vfork_done(p, &vfork))
                ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid);
        }

        put_pid(pid);
    } else {
        nr = PTR_ERR(p);
    }
    return nr;
}

int do_execve(char * filename, char __user *__user *argv,
        char __user *__user *envp,     struct pt_regs * regs)
{
    struct linux_binprm *bprm;        // 保存和要执行的文件相关的数据
    struct file *file;
    int retval;
    int i;
    retval = -ENOMEM;
    bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
    if (!bprm)
        goto out_ret;
    // 打开要执行的文件，并检查其有效性（这里的检查并不完备）
    file = open_exec(filename);
    retval = PTR_ERR(file);
    if (IS_ERR(file))
        goto out_kfree;
    // 在多处理器系统中才执行，用以分配负载最低的CPU来执行新程序
    // 该函数在include/linux/sched.h文件中被定义如下：
    // #ifdef CONFIG_SMP
    // extern void sched_exec(void);
    // #else
    // #define sched_exec() {}
    // #endif
    sched_exec();
    // 填充linux_binprm结构
    bprm->p = PAGE_SIZE*MAX_ARG_PAGES-sizeof(void *);
    bprm->file = file;
    bprm->filename = filename;
    bprm->interp = filename;
    bprm->mm = mm_alloc();
    retval = -ENOMEM;
    if (!bprm->mm)
        goto out_file;
    // 检查当前进程是否在使用LDT，如果是则给新进程分配一个LDT
    retval = init_new_context(current, bprm->mm);
    if (retval  0)
        goto out_mm;
    // 继续填充linux_binprm结构
    bprm->argc = count(argv, bprm->p / sizeof(void *));
    if ((retval = bprm->argc)  0)
        goto out_mm;
    bprm->envc = count(envp, bprm->p / sizeof(void *));
    if ((retval = bprm->envc)  0)
        goto out_mm;
    retval = security_bprm_alloc(bprm);
    if (retval)
        goto out;
    // 检查文件是否可以被执行，填充linux_binprm结构中的e_uid和e_gid项
    // 使用可执行文件的前128个字节来填充linux_binprm结构中的buf项
    retval = prepare_binprm(bprm);
    if (retval  0)
        goto out;
    // 将文件名、环境变量和命令行参数拷贝到新分配的页面中
    retval = copy_strings_kernel(1, &bprm->filename, bprm);
    if (retval  0)
        goto out;
    bprm->exec = bprm->p;
    retval = copy_strings(bprm->envc, envp, bprm);
    if (retval  0)
        goto out;
    retval = copy_strings(bprm->argc, argv, bprm);
    if (retval  0)
        goto out;
    // 查询能够处理该可执行文件格式的处理函数，并调用相应的load_library方法进行处理
    retval = search_binary_handler(bprm,regs);
    if (retval >= 0) {
        free_arg_pages(bprm);
        // 执行成功
        security_bprm_free(bprm);
        acct_update_integrals(current);
        kfree(bprm);
        return retval;
    }
out:
    // 发生错误，返回inode，并释放资源
    for (i = 0 ; i  MAX_ARG_PAGES ; i++) {
        struct page * page = bprm->page;
        if (page)
            __free_page(page);
    }
    if (bprm->security)
        security_bprm_free(bprm);
out_mm:
    if (bprm->mm)
        mmdrop(bprm->mm);
out_file:
    if (bprm->file) {
        allow_write_access(bprm->file);
        fput(bprm->file);
    }
out_kfree:
    kfree(bprm);
out_ret:
    return retval;
}