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Linux - Networking This forum is for any issue related to networks or networking.
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Old 10-20-2016, 09:56 PM   #1
jiufei
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Registered: Sep 2016
Posts: 4

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why ip_frag_reasm() should check the head's frag_list ?


for kernel V2.6.23

static struct sk_buff *ip_frag_reasm(struct ipq *qp, struct net_device *dev)
{
...
if (skb_shinfo(head)->frag_list) {
...
}
...
}

the above red font statement implies that there are maybe some fragments associated with the head skb in its frag_list, but I have no idea that why the head has something in its frag_list.

As we know, after ip_push_pending_frames, the first skb's frag_list is used to link all following fragments, but after ip_fragment() is called, all fragments are sent out one by one, so when the receiver recives a fragment coming from the network, how can we expect that fragment's frag_list still has fragments with it? in other words, under what condition in ip_frag_reasm(), the head fragment'frag_list does have following fragments?

for higher versions above 2.6.23, there are still equivalent codes to the above example.

can any one help me with this question?

Last edited by jiufei; 10-20-2016 at 10:01 PM.
 
Old 10-28-2016, 12:19 PM   #2
dijetlo
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Registered: Jan 2009
Location: RHELtopia....
Distribution: Solaris 11.2/Slackware/RHEL/
Posts: 1,491
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I think it might be a requirement of the use case, in the following method they are writing the skb to a paging file so they are attempting to resolve/exclude the possibility of additional bytes in the fragment list. Seems like it's roughly equivalent to a python 'assert' sanity check (i.e. assuming this thing isn't hosed and half of it is in the fragment list)
 
  


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