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SubjectRe: [PATCH net-next v2] openvswitch: add TTL decrement action
On Tue, Dec 17, 2019 at 5:30 PM Nikolay Aleksandrov
<nikolay@cumulusnetworks.com> wrote:
>
> On 17/12/2019 17:51, Matteo Croce wrote:
> > New action to decrement TTL instead of setting it to a fixed value.
> > This action will decrement the TTL and, in case of expired TTL, drop it
> > or execute an action passed via a nested attribute.
> > The default TTL expired action is to drop the packet.
> >
> > Supports both IPv4 and IPv6 via the ttl and hop_limit fields, respectively.
> >
> > Tested with a corresponding change in the userspace:
> >
> > # ovs-dpctl dump-flows
> > in_port(2),eth(),eth_type(0x0800), packets:0, bytes:0, used:never, actions:dec_ttl{ttl<=1 action:(drop)},1,1
> > in_port(1),eth(),eth_type(0x0800), packets:0, bytes:0, used:never, actions:dec_ttl{ttl<=1 action:(drop)},1,2
> > in_port(1),eth(),eth_type(0x0806), packets:0, bytes:0, used:never, actions:2
> > in_port(2),eth(),eth_type(0x0806), packets:0, bytes:0, used:never, actions:1
> >
> > # ping -c1 192.168.0.2 -t 42
> > IP (tos 0x0, ttl 41, id 61647, offset 0, flags [DF], proto ICMP (1), length 84)
> > 192.168.0.1 > 192.168.0.2: ICMP echo request, id 386, seq 1, length 64
> > # ping -c1 192.168.0.2 -t 120
> > IP (tos 0x0, ttl 119, id 62070, offset 0, flags [DF], proto ICMP (1), length 84)
> > 192.168.0.1 > 192.168.0.2: ICMP echo request, id 388, seq 1, length 64
> > # ping -c1 192.168.0.2 -t 1
> > #
> >
> > Co-authored-by: Bindiya Kurle <bindiyakurle@gmail.com>
> > Signed-off-by: Bindiya Kurle <bindiyakurle@gmail.com>
> > Signed-off-by: Matteo Croce <mcroce@redhat.com>
> > ---
> > include/uapi/linux/openvswitch.h | 22 +++++++
> > net/openvswitch/actions.c | 71 +++++++++++++++++++++
> > net/openvswitch/flow_netlink.c | 105 +++++++++++++++++++++++++++++++
> > 3 files changed, 198 insertions(+)
> >
>
> Hi Matteo,
>
> [snip]
> > +}
> > +
> > /* When 'last' is true, sample() should always consume the 'skb'.
> > * Otherwise, sample() should keep 'skb' intact regardless what
> > * actions are executed within sample().
> > @@ -1176,6 +1201,44 @@ static int execute_check_pkt_len(struct datapath *dp, struct sk_buff *skb,
> > nla_len(actions), last, clone_flow_key);
> > }
> >
> > +static int execute_dec_ttl(struct sk_buff *skb, struct sw_flow_key *key)
> > +{
> > + int err;
> > +
> > + if (skb->protocol == htons(ETH_P_IPV6)) {
> > + struct ipv6hdr *nh = ipv6_hdr(skb);
> > +
> > + err = skb_ensure_writable(skb, skb_network_offset(skb) +
> > + sizeof(*nh));
>
> skb_ensure_writable() calls pskb_may_pull() which may reallocate so nh might become invalid.
> It seems the IPv4 version below is ok as the ptr is reloaded.
>

Right

> One q as I don't know ovs that much - can this action be called only with
> skb->protocol == ETH_P_IP/IPV6 ? I.e. Are we sure that if it's not v6, then it must be v4 ?
>

I'm adding a check in validate_and_copy_dec_ttl() so only ipv4/ipv6
packet will pass.

Thanks,

--
Matteo Croce
per aspera ad upstream

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