I'm currently trying to debug an ATM encapsulation layer that runs on-top of Ethernet. Basically the ATM cells are stored in order after the ethernet header. However I suspect the drivers naive approach to the sk_buffs is broken.

The driver blindly assumes that skb->data can be iterated through but looking at the kernel code for virtio_net.c:page_to_skb I see the following behaviour:

memcpy(hdr, p, hdr_len);
len -= hdr_len;
p += offset;
copy = len;

if (copy > skb_tailroom(skb))
        copy = skb_tailroom(skb);

memcpy(skb_put(skb, copy), p, copy);

Then further on:

while (len) {
        set_skb_frag(skb, page, offset, &len);
    page = (struct page *)page->private;
    offset = 0;
}

Which seems to show the buffer is fragmented with only the first portion being directly accessible from skb->data.

What should I be using to get at the underlying data. Ideally I want to peek a few bytes at arbitrary offsets into the ethernet packet before memcpy'ing chunks into a reassembly buffer I maintain. What should I be using to do this?

c network-programming linux-kernel skb
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