linux-kernel - Re: [PATCH 2/3] ptrace children revamp

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Date:	Mon, 31 Mar 2008 13:12:15 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Roland McGrath <roland@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] ptrace children revamp

On 03/30, Roland McGrath wrote:
>
> +static void ptrace_exit(struct task_struct *parent, struct list_head *dead)
>  {
> -	if (p->pdeath_signal)
> -		/* We already hold the tasklist_lock here.  */
> -		group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
> +	struct task_struct *p, *n;
>  
> -	/* Move the child from its dying parent to the new one.  */
> -	if (unlikely(traced)) {
> -		/* Preserve ptrace links if someone else is tracing this child.  */
> -		list_del_init(&p->ptrace_list);
> -		if (p->parent != p->real_parent)
> -			list_add(&p->ptrace_list, &p->real_parent->ptrace_children);
> -	} else {
> -		/* If this child is being traced, then we're the one tracing it
> -		 * anyway, so let go of it.
> -		 */
> -		p->ptrace = 0;
> -		remove_parent(p);
> -		p->parent = p->real_parent;
> -		add_parent(p);
> +	list_for_each_entry_safe(p, n, &parent->ptrace_attach, ptrace_list) {
> +		__ptrace_unlink(p);
>  
> -		if (task_is_traced(p)) {
> -			/*
> -			 * If it was at a trace stop, turn it into
> -			 * a normal stop since it's no longer being
> -			 * traced.
> -			 */
> -			ptrace_untrace(p);
> +		/*
> +		 * If it's a zombie, our attachedness prevented normal
> +		 * parent notification or self-reaping.  Do notification
> +		 * now if it would have happened earlier.  If it should
> +		 * reap itself, add it to the @dead list.  We can't call
> +		 * release_task() here because we already hold tasklist_lock.
> +		 */
> +		if (p->exit_state == EXIT_ZOMBIE) {
> +			if (p->exit_signal != -1 && !thread_group_empty(p))
                                                    ^^^^^^^^^^^^^^^^^^^^^^^
I still think this is wrong. Please also look at my previous reply,
http://marc.info/?l=linux-kernel&m=120680153931177

> @@ -1481,6 +1478,15 @@ static int wait_consider_task(struct task_struct *parent,
>  	 */
>  	*retval = 0;
>
> +	if (unlikely(p->parent != parent)) {
> +		/*
> +		 * This child is hidden by someone else who did PTRACE_ATTACH.
> +		 * We aren't allowed to see it now, but eventually we will.
> +		 */
> +		*retval = 0;
> +		return 0;
> +	}

Looks wrong... Above this "p->parent != parent" check we have

	if (p->exit_state == EXIT_ZOMBIE && !delay_group_leader(p))
		return wait_task_zombie(p, options, infop, stat_addr, ru);

Suppose that p is not a group leader, it is traced by some unrelated task,
and now it is EXIT_ZOMBIE. We shouldn't release it (and return the "bogus"
pid to user-space).

Oleg.

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