CODEWITHSUNDEEP
linuxlinux-kernelnetfilter
Yd Ahhrk
Yd Ahhrk

Reputation: 1138

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.

Upvotes: 3

Views: 3533

Answers (2)

Yd Ahhrk
Yd Ahhrk

Reputation: 1138

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);
}

Upvotes: 3

askb
askb

Reputation: 6784

Its possibly that you are using a ko, which is compiled for the newer version of the kernel. If the module is not being shipped as a RH, then you may need to work with vendor to get this resolved.

Upvotes: 1

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