Re: ip_conntrack_get

[Jesse Peng <jesse@deansoft.com.tw>]

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Henrik Nordstrom wrote:

>>I also don't understand the way ctinfo is calculate. This stuff...
>>
>>403 <http://lxr.linux.no/source/net/ipv4/netfilter/ip_conntrack_core.c#L403>         *//* ctinfo is the index of the nfct inside the conntrack *//*
>>404 <http://lxr.linux.no/source/net/ipv4/netfilter/ip_conntrack_core.c#L404>         *ctinfo = nfct - ct->infos;
>>    
>>
>
>Neither do I.. but then I have never needed to care as ip_conntrack_get()  
>and the conntrack core does a good job of hiding the magics in that area.. 
>it is probably quite simple reasons behind this odd looking construct.
>
>Regards
>Henrik
>
>
>  
>
Execuse me for the late response,for I thought my colleague would taken 
the responsibility. As I think, just same as me, Pablo must be very 
interested in learning netfilter ;)!

while a new ip_conntrack cache initiates, every member 
infos[i](nf_ct_info structure) then be charged with the same address 
value pointing to the ip_conntrack cache's begining address(it's the 
address of its first member nf_conntrack structure naming ct_general also).

code reference:ip_conntrack_core.c ->init_conntrack ->
for (i=0; i < IP_CT_NUMBER; i++)
conntrack->infos[i].master = &conntrack->ct_general;

either the first packet which initiates the ip_conntrack initiation or 
any subsequent packets belonging to this connection, after 
resolve_normal_ct, will get its member pointer structure nf_ct_info 
naming nfct burned the address pointing to the concerning infos[i](the i 
is right the ctinfo we figure out during resolve_normal_ct, as you know, 
which indecates the relation this packet with its belonging connection).

code reference:ip_conntrack_core.c ->resolve_normal_ct ->
skb->nfct = &h->ctrack->infos[*ctinfo];

ok! now with the address of info[i] the pskb get, we can figure out two 
things.First, following the pointer to its address, with the value it 
keeps, we get the address of the ip_conntrack.Second,following the 
ip_conntrack we can get the address of infos[0],then with 
infos[i]-infos[0](nfct-ct->infos),we get the i, which is right the 
*ctinfo we wanna know!

code reference:ip_conntrack_core.c ->__ip_conntrack_get ->
struct ip_conntrack *ct
= (struct ip_conntrack *)nfct->master; 
-------------------------------------->first

/* ctinfo is the index of the nfct inside the conntrack */
*ctinfo = nfct - ct->infos; -------------------------------------->second

I don't know if this answer your question? if not, it's time to improve 
my English expression ;)!



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