linux-kernel - Re: [PATCH 10/11] ptrace: move JOBCTL_TRAPPING wait to wait(2) and ptrace_check

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Date:	Wed, 11 May 2011 19:00:58 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	jan.kratochvil@...hat.com, vda.linux@...glemail.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu
Subject: Re: [PATCH 10/11] ptrace: move JOBCTL_TRAPPING wait to wait(2) and
	ptrace_check_attach()

On 05/11, Oleg Nesterov wrote:
>
> You know, I'd wish I could find the serious bugs in this patch. The
> code becomes really hairy. -EAGAIN in do_wait() doesn't make it more
> simple ;)

Mwahaha! I seem to see the bug ;)

	bool ptrace_wait_trapping(struct task_struct *child)
		__releases(&child->sighand->siglock)
		__releases(&tasklist_lock)
	{
		if (likely(!(child->jobctl & JOBCTL_TRAPPING)))
			return false;

		spin_unlock_irq(&child->sighand->siglock);
		get_task_struct(child);
		read_unlock(&tasklist_lock);

--> WINDOW

		wait_event(current->signal->wait_chldexit,
			   !(child->jobctl & JOBCTL_TRAPPING));
		put_task_struct(child);
		return true;
	}

When the caller is do_wait(), we can't assume we are the tracer when
we drop tasklist. Original tracer can detach, then another unrelated
process can attach again and provoke JOBCTL_TRAPPING. wait_event()
can hang forever.

Unfortunately, this is easy to fix :/

Oleg.

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