Linux下进程的创建过程分析(_do_fork do_fork详解)--Linux进程的管理与调度（八）

/* http://lxr.free-electrons.com/source/include/linux/sched.h?v=4.5#L2646  */
extern long _do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *, unsigned long);
extern long do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *);


/* linux2.5.32以后, 添加了TLS(Thread Local Storage)机制, 
    在最新的linux-4.2中添加了对CLONE_SETTLS 的支持 
    底层的_do_fork实现了对其的支持, 
    dansh*/
#ifndef CONFIG_HAVE_COPY_THREAD_TLS
/* For compatibility with architectures that call do_fork directly rather than
 * using the syscall entry points below. */
long do_fork(unsigned long clone_flags,
              unsigned long stack_start,
              unsigned long stack_size,
              int __user *parent_tidptr,
              int __user *child_tidptr)
{
        return _do_fork(clone_flags, stack_start, stack_size,
                        parent_tidptr, child_tidptr, 0);
}
#endif

asmlinkage long sys_fork(struct pt_regs regs)
{
    return do_fork(SIGCHLD, regs.rsp, &regs, 0);
}

#ifdef __ARCH_WANT_SYS_FORK
SYSCALL_DEFINE0(fork)
{
#ifdef CONFIG_MMU
        return _do_fork(SIGCHLD, 0, 0, NULL, NULL, 0);
#else
        /* can not support in nommu mode */
        return -EINVAL;
#endif
}
#endif

asmlinkage long sys_vfork(struct pt_regs regs)
{
    return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs.rsp, &regs, 0);
}

#ifdef __ARCH_WANT_SYS_VFORK
SYSCALL_DEFINE0(vfork)
{
        return _do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, 0,
                        0, NULL, NULL, 0);
}
#endif

casmlinkage int sys_clone(struct pt_regs regs)
{
    /* 注释中是i385下增加的代码, 其他体系结构无此定义
    unsigned long clone_flags;
    unsigned long newsp;

    clone_flags = regs.ebx;
    newsp = regs.ecx;*/
    if (!newsp)
        newsp = regs.esp;
    return do_fork(clone_flags, newsp, &regs, 0);
}

#ifdef __ARCH_WANT_SYS_CLONE
#ifdef CONFIG_CLONE_BACKWARDS
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
                 int __user *, parent_tidptr,
                 unsigned long, tls,
                 int __user *, child_tidptr)
#elif defined(CONFIG_CLONE_BACKWARDS2)
SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
                 int __user *, parent_tidptr,
                 int __user *, child_tidptr,
                 unsigned long, tls)
#elif defined(CONFIG_CLONE_BACKWARDS3)
SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
                int, stack_size,
                int __user *, parent_tidptr,
                int __user *, child_tidptr,
                unsigned long, tls)
#else
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
                 int __user *, parent_tidptr,
                 int __user *, child_tidptr,
                 unsigned long, tls)
#endif
{
        return _do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr, tls);
}
#endif

long _do_fork(unsigned long clone_flags,
      unsigned long stack_start,
      unsigned long stack_size,
      int __user *parent_tidptr,
      int __user *child_tidptr,
      unsigned long tls)
{
    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;
    }
    /*  复制进程描述符，copy_process()的返回值是一个 task_struct 指针  */
    p = copy_process(clone_flags, stack_start, stack_size,
         child_tidptr, NULL, trace, tls);
    /*
     * 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);
    /*  得到新创建的进程的pid信息  */
    pid = get_task_pid(p, PIDTYPE_PID);
    nr = pid_vnr(pid);

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

    /*  如果调用的 vfork()方法，初始化 vfork 完成处理信息 */
    if (clone_flags & CLONE_VFORK) {
        p->vfork_done = &vfork;
        init_completion(&vfork);
        get_task_struct(p);
    }
    /*  将子进程加入到调度器中，为其分配 CPU，准备执行  */
    wake_up_new_task(p);

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

    /*  如果是 vfork，将父进程加入至等待队列，等待子进程完成  */
    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;
}

/*
 * This creates a new process as a copy of the old one,
 * but does not actually start it yet.
 *
 * It copies the registers, and all the appropriate
 * parts of the process environment (as per the clone
 * flags). The actual kick-off is left to the caller.
 */
static struct task_struct *copy_process(unsigned long clone_flags,
                    unsigned long stack_start,
                    unsigned long stack_size,
                    int __user *child_tidptr,
                    struct pid *pid,
                    int trace,
                    unsigned long tls)
{
    int retval;
    struct task_struct *p;

    retval = security_task_create(clone_flags);
    if (retval)
        goto fork_out;
    //  复制当前的 task_struct
    retval = -ENOMEM;
    p = dup_task_struct(current);
    if (!p)
        goto fork_out;

    ftrace_graph_init_task(p);

    //初始化互斥变量
    rt_mutex_init_task(p);

#ifdef CONFIG_PROVE_LOCKING
    DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled);
    DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
#endif

    //检查进程数是否超过限制，由操作系统定义
    retval = -EAGAIN;
    if (atomic_read(&p->real_cred->user->processes) >=
            task_rlimit(p, RLIMIT_NPROC)) {
        if (p->real_cred->user != INIT_USER &&
            !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
            goto bad_fork_free;
    }
    current->flags &= ~PF_NPROC_EXCEEDED;

    retval = copy_creds(p, clone_flags);
    if (retval < 0)
        goto bad_fork_free;

    /*
     * If multiple threads are within copy_process(), then this check
     * triggers too late. This doesn't hurt, the check is only there
     * to stop root fork bombs.
     */
    //检查进程数是否超过 max_threads 由内存大小决定
    retval = -EAGAIN;
    if (nr_threads >= max_threads)
        goto bad_fork_cleanup_count;

    delayacct_tsk_init(p);  /* Must remain after dup_task_struct() */
    p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER);
    p->flags |= PF_FORKNOEXEC;
    INIT_LIST_HEAD(&p->children);
    INIT_LIST_HEAD(&p->sibling);
    rcu_copy_process(p);
    p->vfork_done = NULL;

    //  初始化自旋锁
    spin_lock_init(&p->alloc_lock);
    //  初始化挂起信号
    init_sigpending(&p->pending);

    //  初始化 CPU 定时器
    posix_cpu_timers_init(p);
    //  ......

    /* Perform scheduler related setup. Assign this task to a CPU. 
        初始化进程数据结构，并把进程状态设置为 TASK_RUNNING
    */
    retval = sched_fork(clone_flags, p);
    if (retval)
        goto bad_fork_cleanup_policy;
    retval = perf_event_init_task(p);

    /*  复制所有进程信息，包括文件系统、信号处理函数、信号、内存管理等
       形式类似于copy_xxx的形式   */
    if (retval)
        goto bad_fork_cleanup_policy;
    retval = audit_alloc(p);
    if (retval)
        goto bad_fork_cleanup_perf;
    /* copy all the process information */
    shm_init_task(p);
    retval = copy_semundo(clone_flags, p);
    if (retval)
        goto bad_fork_cleanup_audit;
    retval = copy_files(clone_flags, p);
    if (retval)
        goto bad_fork_cleanup_semundo;
    retval = copy_fs(clone_flags, p);
    if (retval)
        goto bad_fork_cleanup_files;
    retval = copy_sighand(clone_flags, p);
    if (retval)
        goto bad_fork_cleanup_fs;
    retval = copy_signal(clone_flags, p);
    if (retval)
        goto bad_fork_cleanup_sighand;
    retval = copy_mm(clone_flags, p);
    if (retval)
        goto bad_fork_cleanup_signal;
    retval = copy_namespaces(clone_flags, p);
    if (retval)
        goto bad_fork_cleanup_mm;
    retval = copy_io(clone_flags, p);
    if (retval)
        goto bad_fork_cleanup_namespaces;
    /*    初始化子进程内核栈
        linux-4.2新增处理TLS
        之前版本是   retval = copy_thread(clone_flags, stack_start, stack_size, p);
        */
    retval = copy_thread_tls(clone_flags, stack_start, stack_size, p, tls);
    if (retval)
        goto bad_fork_cleanup_io;

    /*  为新进程分配新的pid  */
    if (pid != &init_struct_pid) {
        pid = alloc_pid(p->nsproxy->pid_ns_for_children);
        if (IS_ERR(pid)) {
            retval = PTR_ERR(pid);
            goto bad_fork_cleanup_io;
        }
    }

    /*  设置子进程的pid  */
    /* ok, now we should be set up.. */
    p->pid = pid_nr(pid);
    if (clone_flags & CLONE_THREAD) {
        p->exit_signal = -1;
        p->group_leader = current->group_leader;
        p->tgid = current->tgid;
    } else {
        if (clone_flags & CLONE_PARENT)
            p->exit_signal = current->group_leader->exit_signal;
        else
            p->exit_signal = (clone_flags & CSIGNAL);
        p->group_leader = p;
        p->tgid = p->pid;
    }

    p->nr_dirtied = 0;
    p->nr_dirtied_pause = 128 >> (PAGE_SHIFT - 10);
    p->dirty_paused_when = 0;

    p->pdeath_signal = 0;
    INIT_LIST_HEAD(&p->thread_group);
    p->task_works = NULL;

    /*
     * Make it visible to the rest of the system, but dont wake it up yet.
     * Need tasklist lock for parent etc handling!
     */
    write_lock_irq(&tasklist_lock);

    /*  调用fork的进程为其父进程  */
    /* CLONE_PARENT re-uses the old parent */
    if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
        p->real_parent = current->real_parent;
        p->parent_exec_id = current->parent_exec_id;
    } else {
        p->real_parent = current;
        p->parent_exec_id = current->self_exec_id;
    }

    spin_lock(&current->sighand->siglock);

    // ......

    return p;
}

static struct task_struct *dup_task_struct(struct task_struct *orig)
{
    struct task_struct *tsk;
    struct thread_info *ti;
    int node = tsk_fork_get_node(orig);
    int err;

    //分配一个 task_struct 节点
    tsk = alloc_task_struct_node(node);
    if (!tsk)
        return NULL;

    //分配一个 thread_info 节点，包含进程的内核栈，ti 为栈底
    ti = alloc_thread_info_node(tsk, node);
    if (!ti)
        goto free_tsk;

    //将栈底的值赋给新节点的栈
    tsk->stack = ti;

    //……

    return tsk;

}

union thread_union {
   struct thread_info thread_info;
  unsigned long stack[THREAD_SIZE/sizeof(long)];
};

int sched_fork(unsigned long clone_flags, struct task_struct *p)
{
    unsigned long flags;
    int cpu = get_cpu();

    __sched_fork(clone_flags, p);

    //  将子进程状态设置为 TASK_RUNNING
    p->state = TASK_RUNNING;

    //  ……

    //  为子进程分配 CPU
    set_task_cpu(p, cpu);

    put_cpu();
    return 0;
}

#ifdef CONFIG_HAVE_COPY_THREAD_TLS
extern int copy_thread_tls(unsigned long, unsigned long, unsigned long,
            struct task_struct *, unsigned long);
#else
extern int copy_thread(unsigned long, unsigned long, unsigned long,
            struct task_struct *);

/* Architectures that haven't opted into copy_thread_tls get the tls argument
 * via pt_regs, so ignore the tls argument passed via C. */
static inline int copy_thread_tls(
        unsigned long clone_flags, unsigned long sp, unsigned long arg,
        struct task_struct *p, unsigned long tls)
{
    return copy_thread(clone_flags, sp, arg, p);
}
#endif

int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
    unsigned long arg, struct task_struct *p, unsigned long tls)
{
    struct pt_regs *childregs = task_pt_regs(p);
    struct task_struct *tsk;
    int err;
    /*  获取寄存器的信息  */
    p->thread.sp = (unsigned long) childregs;
    p->thread.sp0 = (unsigned long) (childregs+1);
    memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));

    if (unlikely(p->flags & PF_KTHREAD)) {
        /* kernel thread
            内核线程的设置  */
        memset(childregs, 0, sizeof(struct pt_regs));
        p->thread.ip = (unsigned long) ret_from_kernel_thread;
        task_user_gs(p) = __KERNEL_STACK_CANARY;
        childregs->ds = __USER_DS;
        childregs->es = __USER_DS;
        childregs->fs = __KERNEL_PERCPU;
        childregs->bx = sp;     /* function */
        childregs->bp = arg;
        childregs->orig_ax = -1;
        childregs->cs = __KERNEL_CS | get_kernel_rpl();
        childregs->flags = X86_EFLAGS_IF | X86_EFLAGS_FIXED;
        p->thread.io_bitmap_ptr = NULL;
        return 0;
    }
    /*  将当前寄存器信息复制给子进程  */
    *childregs = *current_pt_regs();
    /*  子进程 eax 置 0，因此fork 在子进程返回0  */
    childregs->ax = 0;
    if (sp)
        childregs->sp = sp;
    /*  子进程ip 设置为ret_from_fork，因此子进程从ret_from_fork开始执行  */
    p->thread.ip = (unsigned long) ret_from_fork;
    task_user_gs(p) = get_user_gs(current_pt_regs());

    p->thread.io_bitmap_ptr = NULL;
    tsk = current;
    err = -ENOMEM;

    if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
        p->thread.io_bitmap_ptr = kmemdup(tsk->thread.io_bitmap_ptr,
                        IO_BITMAP_BYTES, GFP_KERNEL);
        if (!p->thread.io_bitmap_ptr) {
            p->thread.io_bitmap_max = 0;
            return -ENOMEM;
        }
        set_tsk_thread_flag(p, TIF_IO_BITMAP);
    }

    err = 0;

    /*
     * Set a new TLS for the child thread?
     * 为进程设置一个新的TLS
     */
    if (clone_flags & CLONE_SETTLS)
        err = do_set_thread_area(p, -1,
            (struct user_desc __user *)tls, 0);

    if (err && p->thread.io_bitmap_ptr) {
        kfree(p->thread.io_bitmap_ptr);
        p->thread.io_bitmap_max = 0;
    }
    return err;
}