netdev - Re: [PATCH net] vlan: pull on __vlan_insert_tag error path and fix csum correction

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Date:	Fri, 01 Apr 2016 15:00:00 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	daniel@...earbox.net
Cc:	jiri@...nulli.us, alexei.starovoitov@...il.com, jesse@...nel.org,
	tom@...bertland.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] vlan: pull on __vlan_insert_tag error path and fix
 csum correction

From: Daniel Borkmann <daniel@...earbox.net>
Date: Fri,  1 Apr 2016 11:41:03 +0200

> Moreover, I noticed that when in the non-error path the __skb_pull()
> is done and the original offset to mac header was non-zero, we fixup
> from a wrong skb->data offset in the checksum complete processing.
> 
> So the skb_postpush_rcsum() really needs to be done before __skb_pull()
> where skb->data still points to the mac header start.

Ugh, what a mess, are you sure any of this is right even after your
change?  What happens (outside of the csum part) is this:

	__skb_push(offset);
	__vlan_insert_tag(); {
		skb_push(VLAN_HLEN);
	...
		memmove(skb->data, skb->data + VLAN_HLEN, 2 * ETH_ALEN);
	}
	__skb_pull(offset);

If I understand this correctly, the last pull will therefore put
skb->data pointing at vlan_ethhdr->h_vlan_TCI of the new VLAN header
pushed by __vlan_insert_tag().

That is assuming skb->data began right after the original ethernet
header.

To me, that postpull csum currently is absolutely in the correct spot,
because it's acting upon the pull done by __vlan_insert_tag(), not the
one done here by skb_vlan_push().

Right?

Can you tell me how you tested this?  Just curious...