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

On Thu, 29 Nov 2012 15:59:08 +0000
David Woodhouse [off-list ref] wrote:

On Thu, 2012-11-29 at 10:37 -0500, chas williams - CONTRACTOR wrote:

quoted

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.

quoted

 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.

quoted

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.

quoted

 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.