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

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Message-ID: <CAD6jFUSCXV7QdNAgZZNVDJ9FTFAvV0Qx+_Zyte6agnfr9-DdVw@mail.gmail.com>
Date:	Sat, 11 Aug 2012 09:11:34 +0200
From:	Daniel Borkmann <danborkmann@...earbox.net>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] af_packet: relax BUG statement in tpacket_destruct_skb

On Sat, Aug 11, 2012 at 1:54 AM, David Miller <davem@...emloft.net> wrote:
> 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.

Thanks for the feedback, I will resend a proper patch!

Best,
Daniel
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