netdev - Re: [PATCH v10 net-next 1/3] filter: add Extended BPF interpreter and converter

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Date:	Fri, 14 Mar 2014 08:37:27 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Pablo Neira Ayuso <pablo@...filter.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, Will Drewry <wad@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	Jesse Gross <jesse@...ira.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <edumazet@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Pekka Enberg <penberg@....fi>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v10 net-next 1/3] filter: add Extended BPF interpreter and converter

On Fri, Mar 14, 2014 at 5:58 AM, Pablo Neira Ayuso <pablo@...filter.org> wrote:
> On Wed, Mar 12, 2014 at 02:43:32PM -0700, Alexei Starovoitov wrote:
>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>> index e568c8ef896b..6e6aab5e062b 100644
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -25,20 +25,45 @@ struct sock;
>>  struct sk_filter
>>  {
>>       atomic_t                refcnt;
>> -     unsigned int            len;    /* Number of filter blocks */
>> +     /* len - number of insns in sock_filter program
>> +      * len_ext - number of insns in socket_filter_ext program
>> +      * jited - true if either original or extended program was JITed
>> +      * orig_prog - original sock_filter program if not NULL
>> +      */
>> +     unsigned int            len;
>> +     unsigned int            len_ext;
>> +     unsigned int            jited:1;
>
> This is consuming 4 bytes just to store the jited bit. I think you can
> scratch that bit from len, given the maximum filter length for bpf. I
> think the the jited bit change that David suggested have to come in
> first place as a separated patch in the series.

It was reviewed so many times that I would prefer not to break it
apart just to split it for single 'jited' bitfield, though I agree with taking
one bit from len.
I actually proposed it in 'bool vs bitfield' thread few days ago.
I think it can be done as a separate commit after this one goes in.

>> +     struct sock_filter      *orig_prog;
>
> If your new extended filtering is not used, this consumes 8 extra
> bytes + len_ext (bytes) in x86_64. I think a more generic way to make
> this is that you can move the original bpf filter and its length at
> the bottom of this structure after insns to store something like:
>
> struct sk_bpf_compat {
>         struct sock_filter      *prog;
>         unsigned int            len;
> };
>
> This would be only allocated when you filtering approach is used. For
> that you'll need some enum in sk_filter to indicate the filtering
> approach, but we'll save 8 bytes per filter in the end with regards to
> this current patch.

this is also can be done as separate commit after this one.
Though I don't like the idea, because access to 'prog' and 'len'
becomes very complicated. In every place we need a helper
function to calculate an offset to this 'sk_bpf_compat',
then typecast that memory location, etc.
Imo single pointer is much cleaner.

Thanks
Alexei
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