Re: [PATCH -mm 1/2] ptrace_stop: fix the race with ptrace detach+attach

[Alexey Dobriyan <adobriyan@sw.ru>]

On Thu, Nov 22, 2007 at 07:14:59PM +0300, Oleg Nesterov wrote:
> If the tracer went away (may_ptrace_stop() failed), ptrace_stop() drops tasklist
> and then changes the ->state from TASK_TRACED to TASK_RUNNING.
> 
> This can fool another tracer which attaches to us in between. Change the ->state
> under tasklist_lock to ensure that ptrace_check_attach() can't wrongly succeed.

ptrace_check_attach? Both do read_lock -- can run in parallel, so how can it help?

> --- PT/kernel/signal.c~1_ptrace_stop	2007-11-21 21:41:02.000000000 +0300
> +++ PT/kernel/signal.c	2007-11-22 16:59:35.000000000 +0300
> @@ -1628,11 +1628,11 @@ static void ptrace_stop(int exit_code, i
>  	} else {
>  		/*
>  		 * By the time we got the lock, our tracer went away.
> -		 * Don't stop here.
> +		 * Don't drop the lock yet, another tracer may come.
>  		 */
> -		read_unlock(&tasklist_lock);
> -		set_current_state(TASK_RUNNING);
> +		__set_current_state(TASK_RUNNING);
>  		current->exit_code = nostop_code;
> +		read_unlock(&tasklist_lock);
>  	}
>  
>  	/*