On 09/28/2016 01:56 PM, Shmulik Ladkani wrote:
On Wed, 28 Sep 2016 12:30:56 +0200, dan...@iogearbox.net wrote:
@@ -4608,6 +4608,8 @@ int skb_vlan_push(struct sk_buff *skb, __be16 vlan_proto,
u16 vlan_tci)
skb->protocol = skb->vlan_proto;
skb->mac_len += VLAN_HLEN;
+ if (offset)
+ offset += VLAN_HLEN;
skb_postpush_rcsum(skb, skb->data + (2 * ETH_ALEN), VLAN_HLEN);
__skb_pull(skb, offset);
This looks much better indeed than your v1 of this patch.
Yep, after some meditation and history digging I happened to notice I
was barking at the wrong tree.
So the issue might only be visible to act_vlan as the other remaining user of
skb_vlan_push().
Yes, this is correct. I'll amend the log message to express that.
The bug occurs for callers of skb_vlan_push() whose data is not
pointing at mac_header.
My only question would be:
what about __skb_vlan_pop(), wouldn't that then need the same adjustment
a la offset -= VLAN_HLEN?
Well, theoretically, yes; but caller may expect 2 different things:
(assuming tags are in-payload)
(1) suppose upon entry we have
DA,SA,0x8100,TCI,0x0800,
^ ^
mac_hdr data
initial offset is 18, and after current unwinding code we'll get
You mean data points after the 0x0800, right?
DA,SA,0x0800,4_bytes,
^ ^
mac_hdr data
which is probably incorrect, adjustment 'offset -= VLAN_HLEN' is needed.
(2) suppose upon entry we have
DA,SA,0x8100,TCI,0x0800
^ ^
mac_hdr data
initial offset is 14, and after current unwinding code we'll get
DA,SA,0x0800,
^ ^
mac_hdr data
which is probably what user has intended.
(had we adjusted offset to be 10, 'data' would point into SA)
From test I've made using act_vlan upon ingress on QinQ tags, existing call
provides data as in (2).
Thoughts?
Yeah, so we likely end up at 2) because of things like eth_type_trans()
that would only pull ETH_HLEN.
Couldn't we end up with 1) for the act_vlan case when we'd have the
offset-adjusted skb_vlan_push() fix from here, where we'd then redirect
to ingress where skb_vlan_pop() would be called? If I'm not missing
something, skb_vlan_push() would then point to the data location of 1)
and with your other proposed direct netif_receive_skb() patch, no
further skb->data adjustments would be done, right?
Another potential issue (but unrelated to this fix here) I just noticed
is, whether act_vlan might have the same problem as we fixed in 8065694e6519
("bpf: fix checksum for vlan push/pop helper"). So potentially, we could
end up fixing CHECKSUM_COMPLETE wrongly on ingress, since these 14 bytes
are already pulled out of the sum at that point.
Should we adjust "offset" back, only if resulting offset is >=14 ?
If also the checksum one might end up as an issue, maybe it's just best
to go through the pain and do the push/pull for data plus csum, so both
skb_vlan_*() functions see the frame starting from mac header temporarily?
Jiri, any thoughts?
