Q:

What is the correct way to define a Netfilter hook function? 

I'm coding a kernel module (more specifically, a Netfilter module) for Linux.  I'm trying to make it compatible with a wide range of kernels, but the entry function is giving me trouble. 
From LXR, I can see that the nf_hookfn typedef changed in kernel 3.13. 
Linux 3.12 and before:
typedef unsigned int nf_hookfn(unsigned int hooknum, (...));

3.13 onwards:
typedef unsigned int nf_hookfn(const struct nf_hook_ops *ops, (...));

However, we have a Red Hat machine claiming to be using kernel 3.10.0-123.4.4.el7.x86_64, yet its netlink.h reads
typedef unsigned int nf_hookfn(const struct nf_hook_ops *ops, (...));

as if it were 3.13+ code. 
It's causing warnings on my module because it completely trainwrecks my attempt to define the function differently based on kernel version:
#if LINUX_VERSION_CODE >= KERNEL_VERSION(3, 13, 0)
#define HOOK_ARG_TYPE const struct nf_hook_ops *
#else
#define HOOK_ARG_TYPE unsigned int
#endif

What piece of documentation did I miss?  Nothing ever suggested to me that kernel API depends on BOTH kernel version AND distro, and it makes no sense. 
And more to the point, how can I fix this?  nf_hookfn is a typedef, not a macro, so I can't just drop it on my function definition. 
One thing that might make things easier is that I never use that one argument. 
Surely I'm not the first person ever experiencing this?  I mean nf_hookfn is the entry point for any Netfilter module; you'd think they broke thousands of drivers by changing it. 

A:

In the end I just made an entire module dedicated to this:
/**
 * The kernel API is far from static.  In particular, the Netfilter packet entry
 * function keeps changing.  nf_hook.c, the file where we declare our packet
 * entry function, has been quite difficult to read for a while now.  It's pretty
 * amusing, because we don't even use any of the noisy arguments. 
 *
 * This file declares a usable function header that abstracts away all those
 * useless arguments. 
 */

#include <linux/version.h>

/* If this is a Red Hat-based kernel (Red Hat, CentOS, Fedora, etc)... */
#ifdef RHEL_RELEASE_CODE

#if RHEL_RELEASE_CODE >= RHEL_RELEASE_VERSION(7, 2)
#define NF_CALLBACK(name, skb) unsigned int name( \
        const struct nf_hook_ops *ops, \
        struct sk_buff *skb, \
        const struct net_device *in, \
        const struct net_device *out, \
        const struct nf_hook_state *state) \

#elif RHEL_RELEASE_CODE >= RHEL_RELEASE_VERSION(7, 0)
#define NF_CALLBACK(name, skb) unsigned int name( \
        const struct nf_hook_ops *ops, \
        struct sk_buff *skb, \
        const struct net_device *in, \
        const struct net_device *out, \
        int (*okfn)(struct sk_buff *))

#else

/*
 * Sorry, I don't have headers for RHEL 6 and below because I'm in a bit of a
 * deadline right now. 
 * If this is causing you trouble, find `nf_hookfn` in your kernel headers
 * (typically in include/linux/netfilter.h) and add your version of the
 * NF_CALLBACK macro here. 
 * Also, kernel headers per version can be found here: http://vault.centos.org/
 */
#error "Sorry; this version of RHEL is not supported because it's kind of old." 

#endif /* RHEL_RELEASE_CODE >= x */

/* If this NOT a RedHat-based kernel (Ubuntu, Debian, SuSE, etc)... */
#else

#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 4, 0)
#define NF_CALLBACK(name, skb) unsigned int name( \
        void *priv, \
        struct sk_buff *skb, \
        const struct nf_hook_state *state)

#elif LINUX_VERSION_CODE >= KERNEL_VERSION(4, 1, 0)
#define NF_CALLBACK(name, skb) unsigned int name( \
        const struct nf_hook_ops *ops, \
        struct sk_buff *skb, \
        const struct nf_hook_state *state)

#elif LINUX_VERSION_CODE >= KERNEL_VERSION(3, 13, 0)
#define NF_CALLBACK(name, skb) unsigned int name( \
        const struct nf_hook_ops *ops, \
        struct sk_buff *skb, \
        const struct net_device *in, \
        const struct net_device *out, \
        int (*okfn)(struct sk_buff *))

#elif LINUX_VERSION_CODE >= KERNEL_VERSION(3, 0, 0)
#define NF_CALLBACK(name, skb) unsigned int name( \
        unsigned int hooknum, \
        struct sk_buff *skb, \
        const struct net_device *in, \
        const struct net_device *out, \
        int (*okfn)(struct sk_buff *))

#else
#error "Linux < 3.0 isn't supported at all." 

#endif /* LINUX_VERSION_CODE > n */

#endif /* RHEL or not RHEL */

So instead of this:
static unsigned int function_name((...), struct sk_buff *skb, (...))
{
    return do_something_with_skb(skb);
}

Do this:
static NF_CALLBACK(function_name, skb)
{
    return do_something_with_skb(skb);
}

