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

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Message-ID: <20121213103548.GC29086@linux.vnet.ibm.com>
Date:	Thu, 13 Dec 2012 16:05:48 +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

* 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 

> 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.

Should we leave this case as a fool shooting himself?
 
> So I still think these checks are pointless and (at least in unregister)
> even harmful.
> 

-- 
thanks and regards
Srikar

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