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

[ebiederm@xmission.com (Eric W. Biederman)]

Oleg Nesterov <oleg@tv-sign.ru> writes:

> 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.

So what has me unsettled is that task_lock is used to
protect p->mm.  The other place this could be a problem
is exit_mm.  But it does appear that deliberately takes the mm_sem
to prevent this problem.  So it looks like I was just missed
that trick.

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

Agreed.

Eric