How to understand the function of pskb_may_pull()?

[lx]

hi all:
      The job of pskb_may_pull is to make sure that the area pointed to by
skb->data contains a block of
data at least as big as the IP header, since each IP packet (fragments
included) must include a complete IP
header.When we receive a packet , the kernel will call the pkb_may_pull(),
it looks like the following in line 395:

373 /*
374  *  Main IP Receive routine.
375  */
376 int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct
packet_type *pt, struct net_device *orig_dev)
377 {
378     const struct iphdr *iph;
379     u32 len;
380
381     /* When the interface is in promisc. mode, drop all the crap
382      * that it receives, do not try to analyse it.
383      */
384     if (skb->pkt_type == PACKET_OTHERHOST)
385         goto drop;
386
387
388     IP_UPD_PO_STATS_BH(dev_net(dev), IPSTATS_MIB_IN, skb->len);
389
390     if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL) {
391         IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INDISCARDS);
392         goto out;
393     }
394
395     if (!pskb_may_pull(skb, sizeof(struct iphdr)))
396         goto inhdr_error;
397
398     iph = ip_hdr(skb);
399

And the definition of pkb_may_pull() looks like the following:

1708 static inline int pskb_may_pull(struct sk_buff *skb, unsigned int len)
1709 {
1710     if (likely(len <= skb_headlen(skb)))
1711         return 1;
1712     if (unlikely(len > skb->len))
1713         return 0;
1714     return __pskb_pull_tail(skb, len - skb_headlen(skb)) != NULL;
1715 }

I just want to know which situation make the skb->data contains a block of
data less than the IP header?
I think this packet can't send by kernel at all.
Thank you.
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