Re: [PATCH net-next V2 1/5] flow_dissector: For stripped vlan, get vlan info from skb->vlan_tci

Wed, Aug 17, 2016 at 01:05:33PM CEST, hadarh@dev.mellanox.co.il wrote:

On Wed, Aug 17, 2016 at 1:46 PM, Jiri Pirko [off-list ref] wrote:

quoted

Wed, Aug 17, 2016 at 12:36:10PM CEST, hadarh@mellanox.com wrote:

quoted

Early in the datapath skb_vlan_untag function is called, stripped
the vlan from the skb and set skb->vlan_tci and skb->vlan_proto fields.

The current dissection doesn't handle stripped vlan packets correctly.
In some flows, vlan doesn't exist in skb->data anymore when applying
flow dissection on the skb, fix that.

In case vlan info wasn't stripped before applying flow_dissector (RPS
flow for example), or in case of skb with multiple vlans (e.g. 802.1ad),
get the vlan info from skb->data. The flow_dissector correctly skips
any number of vlans and stores only the first level vlan.

Fixes: 0744dd00c1b1 ('net: introduce skb_flow_dissect()')
Signed-off-by: Hadar Hen Zion <redacted>
---
net/core/flow_dissector.c | 34 ++++++++++++++++++++++++++--------
1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 91028ae..362d693 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c

@@ -119,12 +119,14 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
      struct flow_dissector_key_ports *key_ports;
      struct flow_dissector_key_tags *key_tags;
      struct flow_dissector_key_keyid *key_keyid;
+      bool skip_vlan = false;
      u8 ip_proto = 0;
      bool ret = false;

      if (!data) {
              data = skb->data;
-              proto = skb->protocol;
+              proto = skb_vlan_tag_present(skb) ?
+                       skb->vlan_proto : skb->protocol;
              nhoff = skb_network_offset(skb);
              hlen = skb_headlen(skb);
      }

@@ -243,23 +245,39 @@ ipv6:
      case htons(ETH_P_8021AD):
      case htons(ETH_P_8021Q): {
              const struct vlan_hdr *vlan;
-              struct vlan_hdr _vlan;

-              vlan = __skb_header_pointer(skb, nhoff, sizeof(_vlan), data, hlen, &_vlan);
-              if (!vlan)
-                      goto out_bad;
+              if (skb_vlan_tag_present(skb))
+                      proto = skb->protocol;
+
+              if (!skb_vlan_tag_present(skb) ||
+                  proto == cpu_to_be16(ETH_P_8021Q) ||
+                  proto == cpu_to_be16(ETH_P_8021AD)) {

How this can happen? Could you give me an example?

This can happen in 2 cases:

1. vlan wasn't stripped yet from the skb.
In RPS flow for example, get_rps_cpu function is using flow-dissector
before vlan_untag is called by __netif_receive_skb_core.

right, sigh...

2. skb with multiple vlan tags.
Only the first vlan is stripped while the inner vlans are still in skb->data.
In this case skb->vlan_proto is 802.1AD and skb->protocol is 802.1Q
(for example) so I have to take the next header from skb->data.

Hmm I think that whoever removes the outermost vlan from skb->vlan_*
should strip the next header into skb->vlan_*