linux-kernel - Re: [PATCH 3/7] uprobes: Kill the pointless inode/uc checks in register/unregister

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Message-ID: <20121213140807.GF29086@linux.vnet.ibm.com>
Date:	Thu, 13 Dec 2012 19:38:07 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.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

> > >
> > > 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 ;)

Okay.

> 
> > > 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.
> 

Okay, Since we agree that its a user mistake. So lets document this and
continue with what you propose.

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