netdev - Re: [PATCH net-next] af_packet: relax BUG statement in tpacket_destruct

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Message-Id: <20120810.165450.638010040921216276.davem@davemloft.net>
Date:	Fri, 10 Aug 2012 16:54:50 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	danborkmann@...earbox.net
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] af_packet: relax BUG statement in
 tpacket_destruct_skb

From: Daniel Borkmann <danborkmann@...earbox.net>
Date: Thu, 09 Aug 2012 18:39:05 +0200

> Here's a quote of the comment about the BUG macro from asm-generic/bug.h:
> 
>  Don't use BUG() or BUG_ON() unless there's really no way out; one
>  example might be detecting data structure corruption in the middle
>  of an operation that can't be backed out of.  If the (sub)system
>  can somehow continue operating, perhaps with reduced functionality,
>  it's probably not BUG-worthy.
> 
>  If you're tempted to BUG(), think again:  is completely giving up
>  really the *only* solution?  There are usually better options, where
>  users don't need to reboot ASAP and can mostly shut down cleanly.
> 
> In our case, the status flag of a ring buffer slot is managed from both sides,
> the kernel space and the user space. This means that even though the kernel
> side might work as expected, the user space screws up and changes this flag
> right between the send(2) is triggered when the flag is changed to
> TP_STATUS_SENDING and a given skb is destructed after some time. Then, this
> will hit the BUG macro. Instead, we relax this condition with a WARN_ON_ONCE
> macro, so that the user is aware of this situation. I've tested it and the
> system still behaves /stable/, so in accordance with the above comment, we
> should rather relax this behavior with a warning.
> 
> Signed-off-by: Daniel Borkmann <daniel.borkmann@....ee.ethz.ch>

I would like this check to simply be deleted completely.

As you said, it's a user changable value, therefore we cannot use it
for kernel side internal consistency checks at all.
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