netdev - Re: [PATCH net-next] net: filter: rename 'struct sk_filter' to 'struct bpf

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Date:	Fri, 25 Jul 2014 14:50:32 -0400
From:	Willem de Bruijn <willemb@...gle.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Pablo Neira Ayuso <pablo@...filter.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Network Development <netdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	netfilter-devel <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: filter: rename 'struct sk_filter' to
 'struct bpf_prog'

On Fri, Jul 25, 2014 at 2:43 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
> On Fri, Jul 25, 2014 at 11:32 AM, Willem de Bruijn <willemb@...gle.com> 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.
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