Re: [PATCH rc1-mm 2/3] coredump: shutdown current process first

[Oleg Nesterov <oleg@tv-sign.ru>]

On 04/14, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@tv-sign.ru> writes:
> 
> > On 04/10, Roland McGrath wrote:
> >>
> >> I would be inclined to restructure the inner loop something like this:
> >> 
> >> 		p = g;
> >> 		while (unlikely(p->mm == NULL)) {
> >> 			p = next_thread(p);
> >> 			if (p == g)
> >> 				break;
> >> 		}
> >> 		if (p->mm == mm) {
> >> 			/*
> >> 			 * p->sighand can't disappear, but
> >> 			 * may be changed by de_thread()
> >> 			 */
> >> 			lock_task_sighand(p, &flags);
> >> 			zap_process(p);
> >> 			unlock_task_sighand(p, &flags);
> >> 		}
> >
> > Yes, I agree, this is much more understandable.
> 
> There is one piece of zap_threads that still makes me uncomfortable.
> 
> task_lock is used to protect p->mm.
> Therefore killing a process based upon p->mm == mm is racy
> with respect to sys_unshare I believe if we don't take
> task_lock.

Well, unshare(CLONE_VM) is not yet supported. Currently (as I see
it) mm->mmap_sem is enough to protect against changing ->mm. Yes,
exit_mm/exec_mmap take task_lock too, so it can be used as well.
Please correct my understanding.

I think it is better to take ->mmap_sem in sys_unshare, this path
is rare.

Oleg.