netdev - Re: [patch 11/11] netfilter warning fix

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Date:	Wed, 07 Feb 2007 07:43:09 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Ingo Molnar <mingo@...e.hu>
CC:	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	netdev@...r.kernel.org, dipankar@...ibm.com, paulmck@...ibm.com
Subject: Re: [patch 11/11] netfilter warning fix

Ingo Molnar wrote:
> * David Miller <davem@...emloft.net> wrote:
> 
>>net/netfilter/nf_conntrack_core.c, calls:
>>
>>	l4proto = __nf_ct_l4proto_find((u_int16_t)pf, protonum);
>>
>>whichs assumes that preemption is disabled.

Yes, that is certainly broken.

> you are right - i mistakenly read that mail only up to the point where 
> you point out the (slightly) buggy NF_CT_STATIC_INC use and missed your 
> final point about other coding having implicit preempt_disable() 
> assumptions.
> 
> I've looked at __nf_ct_l4proto_find() and it's not obvious to me what 
> the hidden preempt_disable() assumption is. Its main use seems to be of 
> nf_ct_protos[] array, which is protected by nf_conntrack_lock. I'm 
> wondering whether what you say suggests that it's safe to call 
> __nf_ct_l4proto_find() without the nf_conntrack_lock locked (as read or 
> as write), and if it's safe, how it protects against simultaneous 
> modifications to the nf_ct_protos[] array. 
> 
> Ahh ... unregister does a synchronize_net(), right? That means that 
> removal of the pointer only happens if all CPUs have gone through a 
> quiescent state.
> 
> this means that this particular use could be fixed by converting the 
> preempt_disable()/enable() pair in nf_ct_l4proto_find_get() to 
> rcu_read_lock()/unlock(), correct? 

That is another bug (all uses of preempt_disable in netfilter
actually), but calling __nf_ct_l[34]proto_find without
rcu_read_lock is broken as well.

> Furthermore, every user of 
> synchronize_net() [and synchronize_rcu() in general] needs to be 
> reviewed.

I'll take care of netfilter.
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