netdev - Re: [PATCH v2 3/3] pppoatm: protect against freeing of vcc

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Date:	Thu, 29 Nov 2012 11:11:22 -0500
From:	chas williams - CONTRACTOR <chas@....nrl.navy.mil>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	David Laight <David.Laight@...LAB.COM>,
	Krzysztof Mazur <krzysiek@...lesie.net>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	nathan@...verse.com.au
Subject: Re: [PATCH v2 3/3] pppoatm: protect against freeing of vcc

On Thu, 29 Nov 2012 15:59:08 +0000
David Woodhouse <dwmw2@...radead.org> wrote:

> On Thu, 2012-11-29 at 10:37 -0500, chas williams - CONTRACTOR wrote:
> > you shouldnt clear ATM_VF_ADDR until the vpi/vci is actually closed and
> > ready for reuse.  at this point, it isnt.
> 
> So I should always wait for the completion of my PKT_CLOSE and only
> clear ATM_VF_ADDR when it's actually done?
> 
> But can you define 'ready for reuse'? From the moment I clear
> ATM_VF_ADDR, another CPU may enter my popen() function to set up another
> VCC with the same parameters, and everything should work fine. The
> PKT_POPEN will end up on the queue *after* my PKT_PCLOSE for the old
> VCC. Any received packets will be dropped until the new VCC gets
> ATM_VF_READY set (by the popen function).
> 
> What's the actual failure mode, caused by me clearing ATM_VF_ADDR "too
> early"?

there may not be one (due to serialization from other parts of the
atm stack) but you "shouldn't" clear ATM_VF_ADDR until the vpi/vci pair
is ready for reuse.  by reuse, i mean that any previous rx/tx data in
the vpi/vci segmentation hardware has been removed/cleared.

> >  ATM_VF_READY should already be clear at this point but you should set
> > it before you queue your PKT_CLOSE. 
> 
> I should *set* it? Do you mean clear it? Yes, I see it's cleared by

sorry, i did mean clear it.

> vcc_destroy_socket()... but all the other ATM drivers also seem to clear
> it for themselves, and that would appear to be harmless.

yeah, like i said, it is spuriously cleared in the drivers and should
probably just be moved to under the control of the next layer up
completely.  drivers/atm should just handle the hardware side, not the
software side.

> > checking for ATM_VF_READY in find_vcc() is probably going to give you
> > grief as well since ATM_VF_READY isnt entirely under your control. 
> 
> That's fine. If *anyone* has cleared ATM_VF_READY, I stop sending
> packets up it. Or, more to the point, I stop using the damn thing at
> all. See commit 1f6ea6e511e5ec730d8e88651da1b7b6e8fd1333.
> 
> >  you need to be able to find the vcc until after pclose() is finished since
> > your tasklet might have a few packets it is still processing?
> 
> The whole point of that check is that the tasklet *won't* be able to
> find it any more, and it'll just discard incoming packets for the
> obsolescent VCC.

that's fine as long as you understand this.  in the case of the he, i
needed to be able to find the vcc until close() is finished so that i
can wakeup the sleeper in the close() routine that is waiting for the
reassembly queue to be cleared/reset.  also, i still needed to find the
vcc for the tx side during close() since i still might need to pop()
skb's that are being sent during the close() while i am still trying to
get the hardware to shutdown the transmit dma engine.

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