linux-kernel - Re: [PATCH net-next] fix unsafe set_memory

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Date:	Thu, 3 Oct 2013 04:57:07 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Xi Wang <xi.wang@...il.com>, x86@...nel.org,
	Eric Dumazet <edumazet@...gle.com>,
	linux-kernel@...r.kernel.org,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH net-next] fix unsafe set_memory_rw from softirq

On Wed, Oct 2, 2013 at 9:57 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2013-10-02 at 21:53 -0700, Eric Dumazet wrote:
>> On Wed, 2013-10-02 at 21:44 -0700, Alexei Starovoitov wrote:
>>
>> > I think ifdef config_x86 is a bit ugly inside struct sk_filter, but
>> > don't mind whichever way.
>>
>> Its not fair to make sk_filter bigger, because it means that simple (non
>> JIT) filter might need an extra cache line.
>>
>> You could presumably use the following layout instead :
>>
>> struct sk_filter
>> {
>>         atomic_t                refcnt;
>>         struct rcu_head         rcu;
>>       struct work_struct      work;
>>
>>         unsigned int            len ____cacheline_aligned;    /* Number of filter blocks */
>>         unsigned int            (*bpf_func)(const struct sk_buff *skb,
>>                                             const struct sock_filter *filter);
>>         struct sock_filter      insns[0];
>> };
>
> And since @len is not used by sk_run_filter() use :
>
> struct sk_filter {
>         atomic_t                refcnt;
>         int                     len; /* number of filter blocks */
>         struct rcu_head         rcu;
>         struct work_struct      work;
>
>         unsigned int            (*bpf_func)(const struct sk_buff *skb,
>                                             const struct sock_filter *filter) ____cacheline_aligned;
>         struct sock_filter      insns[0];
> };

yes. make sense to avoid first insn cache miss inside sk_run_filter()
at the expense
of 8-byte gap between work and bpf_func (on x86_64 w/o lockdep)

Probably even better to overlap work and insns fields.
Pro: sk_filter size the same, no impact on non-jit case
Con: would be harder to understand the code

another problem is that kfree(sk_filter) inside
sk_filter_release_rcu() needs to move inside bpf_jit_free().
so self nack. Let me fix these issues and respin

Thanks
Alexei
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