Re: [LARTC] Is negative offset possible in u32 ?

[Julian Anastasov <ja@ssi.bg>]

	Hello,

On 14 Jan 2003, Eric Leblond wrote:

> I look at the kernel code, I'm almost a real beginner here so I could
> wrote stupid things :
>
> I saw that in the file cls_u32.c we work with skb and use only
> skb->nh.raw. That's the network header, so we don't have any information
> about Ethernet header (it's in skb->mac.raw that we have the ethernet
> header and that the protocol is given).

	Everything starts from the net drivers (drivers/net/*.c):

- dev_alloc_skb() is used to reserve skb head space for the eth hdr
and data space for the packet

- NIC stores ethhdr and packet there

- eth_type_trans() is called to set skb->mac.raw and to skip
the eth header by incrementing skb->data with skb_pull()

- net/core/dev.c sets skb->nh.raw = skb->data and packet is
passed to upper layers where tc ingress works

> Furthermore (maybe i'm wrong cause of inverted stockage in memory) in
> the skbuf struct the ethernet header union follow the network header
> union so we should read something else.

tc sees:

skb:
+-------+	head
|	|		before ofs=-14: more reserved bytes
|	|		ofs=-14: dst mac
|	|		ofs=-8: src mac
|ethhdr	|		ofs=-2: eth proto
+-------+	data	ofs=+0, same as nh.raw
|packet	|
|	|
+-------+	tail
|	|
+-------+	end

As for u32_classify, nh.ptr is used as base offset, later the 'ptr'
is adjusted by signed int offsets, so it can touch lower addresses.

> Thus we can at least say that negative offset in u32 are really "tricky"
> and really non clean and as seems to show experiment that they don't
> work (?)

	This is a hack, of course (may be designed so). But
it works for me (tm)

Regards

--
Julian Anastasov <ja@ssi.bg>

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