netdev - Re: [PATCH v2 net-next] net: filter: export pkt_type

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Date:	Wed, 3 Sep 2014 19:43:10 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Denis Kirjanov <kda@...ux-powerpc.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Markos Chandras <markos.chandras@...tec.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH v2 net-next] net: filter: export pkt_type_offset() helper

On Wed, Sep 3, 2014 at 7:05 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2014-09-04 at 03:25 +0200, Hannes Frederic Sowa wrote:
>
>> Can't we add an address marker to struct sk_buff?
>>
>> Several possibilities are available:
>>
>> ptrdiff_t pkt_type_offset[0]  before the pkt_type flags field
>>
>> If one wants to make it more expressive:
>> typedef struct {} mark_struct_offset;
>> and add
>> mark_struct_offset pkt_type_offset;
>> at appropriate places
>>
>> Or maybe an anonymous union?
>>
>> pkt_type_offset would become a simple offsetof(struct sk_buff,
>> pkt_type_offset) then and there is no need for BUG_ON then.
>
>
> You can try, and make sure this works on all gcc compilers, even the one
> Andrew Morton uses.
>
> And of course, you need to not introduce holes doing so.

good points.
I think Hannes's idea is the best. It will help to get rid of helper altogether.
For my bpf syscall proposal I've used anonymous unions and tested
them with gcc 4.2 - 4.9 on x64/i386/arm32/sparc64 and clang.
All compilers were doing just fine. So it should work.
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