Date:	Thu, 13 Dec 2012 14:15:40 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] uprobes: Kill the pointless inode/uc checks in
	register/unregister

On 12/13, Srikar Dronamraju wrote:
>
> * Oleg Nesterov <oleg@...hat.com> [2012-12-10 20:12:32]:
>
> > On 12/10, Srikar Dronamraju wrote:
> > >
> > > * Oleg Nesterov <oleg@...hat.com> [2012-11-23 21:28:06]:
> > >
> > > > register/unregister verifies that inode/uc != NULL. For what?
> > > > This really looks like "hide the potential problem", the caller
> > > > should pass the valid data.
> > > >
> > >
> > > Agree that users should pass valid data.
> > > I do understand that we expect the users to be knowledge-able.
> > > Also users are routed thro in-kernel api that does this check.
> > >
> > > However from an api perspective, if a user passes invalid data, do we
> > > want the system to crash.
> > >
> > > Esp if kernel can identify that users has indeed passed wrong info. I do agree
> > > that users can still pass invalid data that kernel maynot be able to
> > > identify in most cases.
> >
> > inode != NULL can't verify that it actually points to the valid inode,
> > NULL is only one example of invalid data.
> >
> > I agree, sometimes it makes sense to protect against the stupid mistakes,
> > but if we want to check against NULL we should do
> >
> > 	if (WARN_ON(!inode))
> > 		return;
> >
>
> agree, that warn_on is better than a simple check

and this one

	if (WARN_ON(inode < PAGE_OFFSET))

is even better ;)

> > Especially in uprobe_unregister(). The current code is really "hide
> > the possible problem" and nothing more. It is better to crash imho
> > than silently return.
> >
> > > > register() also checks uc->next == NULL, probably to prevent the
> > > > double-register but the caller can do other stupid/wrong things.
> > >
> > > Users can surely do more stupid things. But this is again something that
> > > kernel can identify. By allowing a double-register of a consumer, thats
> > > already registered, we might end up allowing circular loop of consumers.
> >
> > I understand. But in this case we should document that uc->next must
> > be cleared before uprobe_register(). Or add init_consumer().
> >
> > And we should change uprobe_unregister() to clear uc->next as well.
> > I think that the code like this
> >
> > 	uprobe_register(uc);
> > 	uprobe_unregister(uc);
> >
> > 	uprobe_register(uc);
> >
> > should work. Currently it doesn't because of this check.
> >
>
> yes, these should work and makes a case to nullify ->next on unregister.
>
> However, what if someone tries
>
> 	uprobe_register(uc1);
> 	uprobe_register(uc2);
> 	uprobe_register(uc1);
>
> i.e somebody tries to re-register uc1, while its active and has a valid
> next. After the re-registration of uc1, the uprobe->consumers will no more reference uc2.

Yes. And even without uprobe_register(uc2) the result won't be good.
This is like list_add(node).

> Should we leave this case as a fool shooting himself?

IMHO yes, or we should create init_consumer() or at least document that
the private ->next member should be nullified.

But let me repeat,

> > So I still think these checks are pointless and (at least in unregister)
> > even harmful.

Yes, but I am not going to argue if you want to keep these checks.

Oleg.

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