Re: [PATCH] sys_getppid oopses on debug kernel (v2)

[Kirill Korotaev <dev@sw.ru>]

> > Although I'm not sure it's needed for this problem. A getppid() which does
> >
> > asmlinkage long sys_getppid(void)
> > {
> > 	int pid;
> >
> > 	read_lock(&tasklist_lock);
> > 	pid = current->group_leader->real_parent->tgid;
> > 	read_unlock(&tasklist_lock);
> >
> > 	return pid;
> > }
> >
> > seems like a fine implementation to me ;)
>
>
> Why do we need to use ->group_leader? All threads should have the same
> ->real_parent.
I'm not sure this is true for old LinuxThreads...

> Why do we need tasklist_lock? I think rcu_read_lock() is enough.
>
> In other words, do you see any problems with this code
>
> 	smlinkage long sys_getppid(void)
> 	{
> 		int pid;
>
> 		rcu_read_lock();
> 		pid = rcu_dereference(current->real_parent)->tgid;
> 		rcu_read_unlock();
>
> 		return pid;
> 	}
>
> ? Yes, we may read a stale value for ->real_parent, but the memory
> can't be freed while we are under rcu_read_lock(). And in this case
> the returned value is ok because the task could be reparented just
> after return anyway.
Your patch doesn't cure the problem.
rcu_read_lock just disables preemtion and rcu_dereference
introduces memory barrier. _None_ of this _prevents_
another CPU from freeing old real_parent in parallel with your dereference.

You can minimize the probability very much by making local_irq_disable()/enable()
around the code in question, but still it won't be a real fix (at least due to NMIs).

Kirill

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