How to understand the function of pskb_may_pull()?

[lx]

hi Alberto:
        I'm confused with " paging skbs is a technique meant to ease the
work
of drivers.A driver might choose to page "too much" data.", please give
more explain.

Thank you.

2015-01-07 23:35 GMT+08:00 Alberto Leiva <ydahhrk at gmail.com>:

> I think in theory whatever random driver for whatever random hardware
> might create this situation. It's not illegal. Anyone can create a
> driver outside of the kernel tree, after all. In practice, of course,
> this is not the case, because anyone can tell it creates a significant
> amount of overhead when the inevitable pskb_may_pull() come into play,
> but you don't know what constrictive environments a driver out there
> might be operating on.
>
> As I understand it, paging skbs is a technique meant to ease the work
> of drivers.
> A driver might choose to page "too much" data.
> The kernel is prepared for this situation ('Be conservative in what
> you do, be liberal in what you accept from others' -
> http://en.wikipedia.org/wiki/Robustness_principle).
> That's all there is to it, I think.
>
> PS: From your sorta definition, you seem to be assuming
> pskb_may_pull() is only used to ensure the IP header is not paged. I
> understand this might not be the case, but just to make things clear:
> pskb_may_pull() can be used to copy *any* amount of bytes from the
> paged part to the head room of the packet (it's not tied to the IP
> header's length). For example, you could use it to also guarantee a
> TCP header by doing this:
> if (!pskb_may_pull(skb, sizeof(struct iphdr) + sizeof(struct tcphdr)))
>         goto inhdr_error;
>
>
> On Wed, Jan 7, 2015 at 2:28 AM, lx <lxlenovostar at gmail.com> wrote:
> > 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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> >
>
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