How can i use skb->cb in my custom kernel modules - linux

Question

I build a new kernel module(2.6.32 CentOS6.5) named "xt_hello.ko", and I want to send some custom data to nflog, so I have changed skb->cb in my module, and nflog can read my data correctly.
Question: I found cb field has been used in tcp netlink and etc, can my module make some bad influence for them?
Definition of sk_buff:
struct sk_buff {
/* These two members must be first. */
struct sk_buff *next;
struct sk_buff *prev;
struct sock *sk;
ktime_t tstamp;
struct net_device *dev;
unsigned long _skb_dst;
#ifdef CONFIG_XFRM
struct sec_path *sp;
#endif
/*
* This is the control buffer. It is free to use for every
* layer. Please put your private variables there. If you
* want to keep them across layers you have to do a skb_clone()
* first. This is owned by whoever has the skb queued ATM.
*/
char cb[48];
... skip ...
Definition of cb in netlink:
#define NETLINK_CB(skb) (*(struct netlink_skb_parms*)&((skb)->cb))
#define NETLINK_CREDS(skb) (&NETLINK_CB((skb)).creds)
Core code in my kernel module:
My definition:
struct xt_ndpi_cb {
u_int16_t protocol_detected ;
u_int16_t ndpi_proto;
}xt_ndpi_cb_t;
#define NDPI_CB(skb) (*(struct xt_ndpi_cb*)&((skb)->cb))
#define NDPI_CB_RECORD(skb,entry) NDPI_CB(skb).ndpi_proto = entry.VALUEA; NDPI_CB(skb).protocol_detected = entry.VALUEB;
/*core func*/
static bool ndpi_process_packet_tg(const struct sk_buff *_skb, const struct
xt_ndpi_tginfo *info, struct nf_conn *ct) {
...skip...
if (/*condition*/)
NDPI_CB_RECORD(_skb, entry)
}
Kernel module .c:
static bool ndpi_match(const struct sk_buff *skb, struct xt_action_param *par){
bool verdict;
struct nf_conn * ct;
enum ip_conntrack_info ctinfo;
const struct xt_ndpi_protocols *info = par->matchinfo;
ct = nf_ct_get(skb, &ctinfo);
if((ct == NULL) || (skb == NULL)) {
return(false);
#if LINUX_VERSION_CODE < KERNEL_VERSION(3,0,0)
} else if (nf_ct_is_untracked(skb)) {
#else
} else if(nf_ct_is_untracked(ct)) {
#endif
return false;
}
/*change cb in this func*/
verdict = ndpi_process_packet(skb, info, ct );
return(verdict);
}
static struct xt_match ndpi_regs[] __read_mostly = {
{
.name = "ndpi",
.revision = 0,
.family = NFPROTO_IPV4,
.match = ndpi_match,
.matchsize = sizeof(struct xt_ndpi_protocols),
.me = THIS_MODULE,
}
};
Use in xt_NFLOG.c:
static unsigned int
nflog_tg(struct sk_buff *skb, const struct xt_target_param *par)
{
char buf[64+16]; /**my buf/
const struct xt_nflog_info *info = par->targinfo;
...skip...
sprintf(buf,"%s MYID=%u",info->prefix,NDPI_CB(_skb).ndpi_proto); /*NFLOG can get right cb value*/
...skip...
}