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Date:	Fri, 25 Jul 2014 12:02:11 -0700
From:	Alexei Starovoitov <>
To:	Willem de Bruijn <>
Cc:	Pablo Neira Ayuso <>,
	Daniel Borkmann <>,
	"David S. Miller" <>,
	Network Development <>,
	linux-kernel <>,
	netfilter-devel <>
Subject: Re: [PATCH net-next] net: filter: rename 'struct sk_filter' to
 'struct bpf_prog'

On Fri, Jul 25, 2014 at 11:58 AM, Alexei Starovoitov <> wrote:
> On Fri, Jul 25, 2014 at 11:50 AM, Willem de Bruijn <> wrote:
>> On Fri, Jul 25, 2014 at 2:43 PM, Alexei Starovoitov <> wrote:
>>> On Fri, Jul 25, 2014 at 11:32 AM, Willem de Bruijn <> wrote:
>>>>>> This follows a convention in include/uapi/linux/netfilter/*.h that
>>>>>> likely predates the introduction of uapi. A search for "Used
>>>>>> internally by the kernel" shows many more examples. I should not have
>>>>>> included filter.h, however. The common behavior when using pointers
>>>>>> to kernel-internal structures is to have a forward declaration. I suggest
>>>>>> making that change, instead of changing to void *. This avoids having
>>>>>> to add casts where xt_bpf_info is used in net/netfilter/xt_bpf.c:
>>>>> that will not avoid typecast.
>>>>> Either 'void *' approach or extra 'struct sk_filter;' approach, both need
>>>>> type casts to 'struct bpf_prog' in xt_bpf.c
>>>>> (because of SK_RUN_FILTER macro)
>>>>> Therefore I prefer extra 'struct sk_filter;' approach.
>>>> I hadn't noticed that your patch makes the same change that I
>>>> proposed. Nothing in userspace should touch that pointer, so it is
>>>> fine to change its type to struct bpf_prog* at the same time. No need
>>>> for typecasts.
>>> really? I don't think it's a good idea to expose kernel struct type
>>> to user space. How is it even going to compile?
>> a forward declaration.
>>> #include <linux/filter.h> brings different files in kernel and in user space.
>>> struct bpf_prog is undefined in user space and compiler will complain.
>>> Adding 'struct bpf_prog;' will be ugly.
>>> imo the lesser evil is adding 'struct sk_filter;' and doing type casts
>>> in kernel.
>> but the exact same argument applies to sk_filter. If that struct is
>> renamed everywhere else, then the result will only be more confusing.
>> A forward declaration is the standard workaround to all such cases in
>> include/uapi/linux/netfilter. See for instance xt_connlimit.h. This is
>> sufficient to allow userspace build to succeed, without exposing any
>> kernel structure detail. If you don't even want to leak the name, then
>> let's make it void *. Keeping a declaration for sk_filter, while
>> sk_filter is renamed everywhere else is the least good option, in my
>> opinion.
> well, since you the author of this bit and you're ok with 'void *', I'm ok
> with it too :) Just typecast in kernel is still needed because of
> SK_RUN_FILTER() macro...

just tried with 'void *', actually all three type casts are needed...
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