Date: Thu, 6 Jul 2023 09:57:34 -0400
From: Eric Garver <eric@...ver.life>
To: Aaron Conole <aconole@...hat.com>
Cc: netdev@...r.kernel.org, dev@...nvswitch.org,
	Ilya Maximets <i.maximets@....org>
Subject: Re: [ovs-dev] [PATCH net-next 2/2] net: openvswitch: add drop action

On Thu, Jul 06, 2023 at 08:54:16AM -0400, Aaron Conole wrote:
> Eric Garver <eric@...ver.life> writes:
> 
> > This adds an explicit drop action. This is used by OVS to drop packets
> > for which it cannot determine what to do. An explicit action in the
> > kernel allows passing the reason _why_ the packet is being dropped. We
> > can then use perf tracing to match on the drop reason.
> >
> > e.g. trace all OVS dropped skbs
> >
> >  # perf trace -e skb:kfree_skb --filter="reason >= 0x30000"
> >  [..]
> >  106.023 ping/2465 skb:kfree_skb(skbaddr: 0xffffa0e8765f2000, \
> >   location:0xffffffffc0d9b462, protocol: 2048, reason: 196610)
> >
> > reason: 196610 --> 0x30002 (OVS_XLATE_RECURSION_TOO_DEEP)
> >
> > Signed-off-by: Eric Garver <eric@...ver.life>
> > ---
> >  include/uapi/linux/openvswitch.h                    |  2 ++
> >  net/openvswitch/actions.c                           | 13 +++++++++++++
> >  net/openvswitch/flow_netlink.c                      | 12 +++++++++++-
> >  .../testing/selftests/net/openvswitch/ovs-dpctl.py  |  3 +++
> >  4 files changed, 29 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/uapi/linux/openvswitch.h b/include/uapi/linux/openvswitch.h
> > index e94870e77ee9..a967dbca3574 100644
> > --- a/include/uapi/linux/openvswitch.h
> > +++ b/include/uapi/linux/openvswitch.h
> > @@ -965,6 +965,7 @@ struct check_pkt_len_arg {
> >   * start of the packet or at the start of the l3 header depending on the value
> >   * of l3 tunnel flag in the tun_flags field of OVS_ACTION_ATTR_ADD_MPLS
> >   * argument.
> > + * @OVS_ACTION_ATTR_DROP: Explicit drop action.
> >   *
> >   * Only a single header can be set with a single %OVS_ACTION_ATTR_SET.  Not all
> >   * fields within a header are modifiable, e.g. the IPv4 protocol and fragment
> > @@ -1002,6 +1003,7 @@ enum ovs_action_attr {
> >  	OVS_ACTION_ATTR_CHECK_PKT_LEN, /* Nested OVS_CHECK_PKT_LEN_ATTR_*. */
> >  	OVS_ACTION_ATTR_ADD_MPLS,     /* struct ovs_action_add_mpls. */
> >  	OVS_ACTION_ATTR_DEC_TTL,      /* Nested OVS_DEC_TTL_ATTR_*. */
> > +	OVS_ACTION_ATTR_DROP,         /* u32 xlate_error. */
> >  
> >  	__OVS_ACTION_ATTR_MAX,	      /* Nothing past this will be accepted
> >  				       * from userspace. */
> > diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
> > index cab1e02b63e0..4ad9a45dc042 100644
> > --- a/net/openvswitch/actions.c
> > +++ b/net/openvswitch/actions.c
> > @@ -32,6 +32,7 @@
> >  #include "vport.h"
> >  #include "flow_netlink.h"
> >  #include "openvswitch_trace.h"
> > +#include "drop.h"
> >  
> >  struct deferred_action {
> >  	struct sk_buff *skb;
> > @@ -1477,6 +1478,18 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
> >  				return dec_ttl_exception_handler(dp, skb,
> >  								 key, a);
> >  			break;
> > +
> > +		case OVS_ACTION_ATTR_DROP:
> > +			u32 reason = nla_get_u32(a);
> > +
> > +			reason |= SKB_DROP_REASON_SUBSYS_OPENVSWITCH <<
> > +					SKB_DROP_REASON_SUBSYS_SHIFT;
> > +
> > +			if (reason == OVS_XLATE_OK)
> > +				break;
> > +
> > +			kfree_skb_reason(skb, reason);
> > +			return 0;
> >  		}
> >  
> >  		if (unlikely(err)) {
> > diff --git a/net/openvswitch/flow_netlink.c b/net/openvswitch/flow_netlink.c
> > index 41116361433d..23d39eae9a0d 100644
> > --- a/net/openvswitch/flow_netlink.c
> > +++ b/net/openvswitch/flow_netlink.c
> > @@ -39,6 +39,7 @@
> >  #include <net/erspan.h>
> >  
> >  #include "flow_netlink.h"
> > +#include "drop.h"
> >  
> >  struct ovs_len_tbl {
> >  	int len;
> > @@ -61,6 +62,7 @@ static bool actions_may_change_flow(const struct nlattr *actions)
> >  		case OVS_ACTION_ATTR_RECIRC:
> >  		case OVS_ACTION_ATTR_TRUNC:
> >  		case OVS_ACTION_ATTR_USERSPACE:
> > +		case OVS_ACTION_ATTR_DROP:
> >  			break;
> >  
> >  		case OVS_ACTION_ATTR_CT:
> > @@ -2394,7 +2396,7 @@ static void ovs_nla_free_nested_actions(const struct nlattr *actions, int len)
> >  	/* Whenever new actions are added, the need to update this
> >  	 * function should be considered.
> >  	 */
> > -	BUILD_BUG_ON(OVS_ACTION_ATTR_MAX != 23);
> > +	BUILD_BUG_ON(OVS_ACTION_ATTR_MAX != 24);
> >  
> >  	if (!actions)
> >  		return;
> > @@ -3182,6 +3184,7 @@ static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
> >  			[OVS_ACTION_ATTR_CHECK_PKT_LEN] = (u32)-1,
> >  			[OVS_ACTION_ATTR_ADD_MPLS] = sizeof(struct ovs_action_add_mpls),
> >  			[OVS_ACTION_ATTR_DEC_TTL] = (u32)-1,
> > +			[OVS_ACTION_ATTR_DROP] = sizeof(u32),
> >  		};
> >  		const struct ovs_action_push_vlan *vlan;
> >  		int type = nla_type(a);
> > @@ -3453,6 +3456,13 @@ static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
> >  			skip_copy = true;
> >  			break;
> >  
> > +		case OVS_ACTION_ATTR_DROP:
> > +			if (nla_get_u32(a) >=
> > +			    u32_get_bits(OVS_XLATE_MAX,
> > +					 ~SKB_DROP_REASON_SUBSYS_MASK))
> > +				return -EINVAL;
> > +			break;
> > +
> 
> If there's a case where the userspace sends a drop reason that isn't
> known to the kernel, we will reject the flow, and the only "close" drop
> will be OVS_XLATE_OK, which would be wrong.  Is there a reason to do
> this?  For example, userspace might get new support for some kind of
> flows and during that time might have a new xlate drop reason.  Maybe we
> can have a reason code that OVS knows will exist, so that if this fails,
> it can at least fall back to that?

You're correct. It will reject the flow.

Maybe we clamp the value to OVS_XLATE_MAX if it's unknown. That makes
the skb drop reason less helpful, but no less helpful than today ;). At
least we won't reject the flow.

We could alias OVS_XLATE_MAX to OVS_XLATE_UNKNOWN. I prefer an explicit
value for OVS_XLATE_UNKNOWN, e.g. (u16)-1.

> >  		default:
> >  			OVS_NLERR(log, "Unknown Action type %d", type);
> >  			return -EINVAL;
> > diff --git a/tools/testing/selftests/net/openvswitch/ovs-dpctl.py b/tools/testing/selftests/net/openvswitch/ovs-dpctl.py
> > index 1c8b36bc15d4..526ebad7d514 100644
> > --- a/tools/testing/selftests/net/openvswitch/ovs-dpctl.py
> > +++ b/tools/testing/selftests/net/openvswitch/ovs-dpctl.py
> > @@ -115,6 +115,7 @@ class ovsactions(nla):
> >          ("OVS_ACTION_ATTR_CHECK_PKT_LEN", "none"),
> >          ("OVS_ACTION_ATTR_ADD_MPLS", "none"),
> >          ("OVS_ACTION_ATTR_DEC_TTL", "none"),
> > +        ("OVS_ACTION_ATTR_DROP", "uint32"),
> >      )
> >  
> >      class ctact(nla):
> > @@ -261,6 +262,8 @@ class ovsactions(nla):
> >                      print_str += "recirc(0x%x)" % int(self.get_attr(field[0]))
> >                  elif field[0] == "OVS_ACTION_ATTR_TRUNC":
> >                      print_str += "trunc(%d)" % int(self.get_attr(field[0]))
> > +                elif field[0] == "OVS_ACTION_ATTR_DROP":
> > +                    print_str += "drop"
> 
> Can we also include the reason here?

It could. This mimics dpctl output, which does currently not include the
reason. So I went with parity.

I have proposed adding the reason to all dpctl "drop" output, but was
planning on that being a follow up to this work.

> >              elif field[1] == "flag":
> >                  if field[0] == "OVS_ACTION_ATTR_CT_CLEAR":
> >                      print_str += "ct_clear"
> 
> 

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