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

From: Alexei Starovoitov
Date: Fri Jul 25 2014 - 13:27:22 EST

On Fri, Jul 25, 2014 at 6:53 AM, Willem de Bruijn <willemb@xxxxxxxxxx> wrote:
>>> >alternative fix for xt_bpf.h could be to replace:
>>> > /* only used in the kernel */
>>> > struct sk_filter *filter __attribute__((aligned(8)));
>>> >with
>>> > /* only used in the kernel */
>>> > void *filter __attribute__((aligned(8)));
>>> >
>>> >but this 'void *' approach may further break broken userspace,
>>> >whereas the fix implemented here is more seamless.
>>> Yep, that's not good, 'struct sk_filter' should never have been in a uapi
>>> file actually.
> 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.

> -#include <linux/filter.h>
> #include <linux/types.h>
> #define XT_BPF_MAX_NUM_INSTR 64
> +struct sk_filter;
> +
> struct xt_bpf_info {
> I can send this as a separate patch to net-next, if that helps.
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