Subject: Re: [ovs-dev] [PATCH net-next v2 5/9] net: openvswitch: add emit_sample action

From: Aaron Conole <[email-masked]>
Date: Tue, 11 Jun 2024 09:54:49 -0400

Adrián Moreno <[email-masked]> writes:

> On Mon, Jun 10, 2024 at 11:46:14AM GMT, Aaron Conole wrote:
>> Adrian Moreno <[email-masked]> writes:
>>
>> > Add support for a new action: emit_sample.
>> >
>> > This action accepts a u32 group id and a variable-length cookie and uses
>> > the psample multicast group to make the packet available for
>> > observability.
>> >
>> > The maximum length of the user-defined cookie is set to 16, same as
>> > tc_cookie, to discourage using cookies that will not be offloadable.
>> >
>> > Signed-off-by: Adrian Moreno <[email-masked]>
>> > ---
>>
>> I saw some of the nits Simon raised - I'll add one more below.
>>
>> I haven't gone through the series thoroughly enough to make a detailed
>> review.
>>
>> >  Documentation/netlink/specs/ovs_flow.yaml | 17 ++++++++
>> >  include/uapi/linux/openvswitch.h          | 25 ++++++++++++
>> >  net/openvswitch/actions.c                 | 50 +++++++++++++++++++++++
>> >  net/openvswitch/flow_netlink.c            | 33 ++++++++++++++-
>> >  4 files changed, 124 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/Documentation/netlink/specs/ovs_flow.yaml b/Documentation/netlink/specs/ovs_flow.yaml
>> > index 4fdfc6b5cae9..a7ab5593a24f 100644
>> > --- a/Documentation/netlink/specs/ovs_flow.yaml
>> > +++ b/Documentation/netlink/specs/ovs_flow.yaml
>> > @@ -727,6 +727,12 @@ attribute-sets:
>> >          name: dec-ttl
>> >          type: nest
>> >          nested-attributes: dec-ttl-attrs
>> > +      -
>> > +        name: emit-sample
>> > +        type: nest
>> > +        nested-attributes: emit-sample-attrs
>> > +        doc: |
>> > +          Sends a packet sample to psample for external observation.
>> >    -
>> >      name: tunnel-key-attrs
>> >      enum-name: ovs-tunnel-key-attr
>> > @@ -938,6 +944,17 @@ attribute-sets:
>> >        -
>> >          name: gbp
>> >          type: u32
>> > +  -
>> > +    name: emit-sample-attrs
>> > +    enum-name: ovs-emit-sample-attr
>> > +    name-prefix: ovs-emit-sample-attr-
>> > +    attributes:
>> > +      -
>> > +        name: group
>> > +        type: u32
>> > +      -
>> > +        name: cookie
>> > +        type: binary
>> >
>> >  operations:
>> >    name-prefix: ovs-flow-cmd-
>> > diff --git a/include/uapi/linux/openvswitch.h b/include/uapi/linux/openvswitch.h
>> > index efc82c318fa2..a0e9dde0584a 100644
>> > --- a/include/uapi/linux/openvswitch.h
>> > +++ b/include/uapi/linux/openvswitch.h
>> > @@ -914,6 +914,30 @@ struct check_pkt_len_arg {
>> >  };
>> >  #endif
>> >
>> > +#define OVS_EMIT_SAMPLE_COOKIE_MAX_SIZE 16
>> > +/**
>> > + * enum ovs_emit_sample_attr - Attributes for %OVS_ACTION_ATTR_EMIT_SAMPLE
>> > + * action.
>> > + *
>> > + * @OVS_EMIT_SAMPLE_ATTR_GROUP: 32-bit number to identify the source of the
>> > + * sample.
>> > + * @OVS_EMIT_SAMPLE_ATTR_COOKIE: A variable-length binary cookie that contains
>> > + * user-defined metadata. The maximum length is 16 bytes.
>> > + *
>> > + * Sends the packet to the psample multicast group with the specified group and
>> > + * cookie. It is possible to combine this action with the
>> > + * %OVS_ACTION_ATTR_TRUNC action to limit the size of the packet being emitted.
>> > + */
>> > +enum ovs_emit_sample_attr {
>> > +	OVS_EMIT_SAMPLE_ATTR_UNPSEC,
>> > +	OVS_EMIT_SAMPLE_ATTR_GROUP,	/* u32 number. */
>> > +	OVS_EMIT_SAMPLE_ATTR_COOKIE,	/* Optional, user specified cookie. */
>> > +	__OVS_EMIT_SAMPLE_ATTR_MAX
>> > +};
>> > +
>> > +#define OVS_EMIT_SAMPLE_ATTR_MAX (__OVS_EMIT_SAMPLE_ATTR_MAX - 1)
>> > +
>> > +
>> >  /**
>> >   * enum ovs_action_attr - Action types.
>> >   *
>> > @@ -1004,6 +1028,7 @@ enum ovs_action_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 error code. */
>> > +	OVS_ACTION_ATTR_EMIT_SAMPLE,  /* Nested OVS_EMIT_SAMPLE_ATTR_*. */
>> >
>> >  	__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 964225580824..3b4dba0ded59 100644
>> > --- a/net/openvswitch/actions.c
>> > +++ b/net/openvswitch/actions.c
>> > @@ -24,6 +24,11 @@
>> >  #include <net/checksum.h>
>> >  #include <net/dsfield.h>
>> >  #include <net/mpls.h>
>> > +
>> > +#if IS_ENABLED(CONFIG_PSAMPLE)
>> > +#include <net/psample.h>
>> > +#endif
>> > +
>> >  #include <net/sctp/checksum.h>
>> >
>> >  #include "datapath.h"
>> > @@ -1299,6 +1304,46 @@ static int execute_dec_ttl(struct sk_buff *skb, struct sw_flow_key *key)
>> >  	return 0;
>> >  }
>> >
>> > +static int execute_emit_sample(struct datapath *dp, struct sk_buff *skb,
>> > +			       const struct sw_flow_key *key,
>> > +			       const struct nlattr *attr)
>> > +{
>> > +#if IS_ENABLED(CONFIG_PSAMPLE)
>> > +	struct psample_group psample_group = {};
>> > +	struct psample_metadata md = {};
>> > +	struct vport *input_vport;
>> > +	const struct nlattr *a;
>> > +	int rem;
>> > +
>> > +	for (a = nla_data(attr), rem = nla_len(attr); rem > 0;
>> > +	     a = nla_next(a, &rem)) {
>> > +		switch (nla_type(a)) {
>> > +		case OVS_EMIT_SAMPLE_ATTR_GROUP:
>> > +			psample_group.group_num = nla_get_u32(a);
>> > +			break;
>> > +
>> > +		case OVS_EMIT_SAMPLE_ATTR_COOKIE:
>> > +			md.user_cookie = nla_data(a);
>> > +			md.user_cookie_len = nla_len(a);
>> > +			break;
>> > +		}
>> > +	}
>> > +
>> > +	psample_group.net = ovs_dp_get_net(dp);
>> > +
>> > +	input_vport = ovs_vport_rcu(dp, key->phy.in_port);
>> > +	if (!input_vport)
>> > +		input_vport = ovs_vport_rcu(dp, OVSP_LOCAL);
>> > +
>> > +	md.in_ifindex = input_vport->dev->ifindex;
>> > +	md.trunc_size = skb->len - OVS_CB(skb)->cutlen;
>> > +
>> > +	psample_sample_packet(&psample_group, skb, 0, &md);
>> > +#endif
>> > +
>> > +	return 0;
>>
>> Why this return here?  Doesn't seem used anywhere else.
>>
>
> It is being used in "do_execute_actions", right?
> All non-skb-consuming actions set the value of "err" and break from the
> switch-case so that the the packet is dropped with OVS_DROP_ACTION_ERROR reason.
>
> Am i missing something?

I think so.  For example, it isn't used when the function cannot
possibly error.

see the following cases:

OVS_ACTION_ATTR_HASH
OVS_ACTION_ATTR_TRUNC

As you note, these can consume SKB so also don't bother setting err,
because they will need to return anyway:

OVS_ACTION_ATTR_USERSPACE
OVS_ACTION_ATTR_OUTPUT
OVS_ACTION_ATTR_DROP

And even the following does a weird thing:

OVS_ACTION_ATTR_CT

because sometimes it will consume, and sometimes not.

I think if there isn't a possibility of error being generated (and I
guess from the code I see there isn't), then it shouldn't return a
useless code, since err will be 0 on each iteration of the loop.

>> > +}
>> > +
>> >  /* Execute a list of actions against 'skb'. */
>> >  static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
>> >  			      struct sw_flow_key *key,
>> > @@ -1502,6 +1547,11 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
>> >  			ovs_kfree_skb_reason(skb, reason);
>> >  			return 0;
>> >  		}
>> > +
>> > +		case OVS_ACTION_ATTR_EMIT_SAMPLE:
>> > +			err = execute_emit_sample(dp, skb, key, a);
>> > +			OVS_CB(skb)->cutlen = 0;
>> > +			break;
>> >  		}
>> >
>> >  		if (unlikely(err)) {
>> > diff --git a/net/openvswitch/flow_netlink.c b/net/openvswitch/flow_netlink.c
>> > index f224d9bcea5e..eb59ff9c8154 100644
>> > --- a/net/openvswitch/flow_netlink.c
>> > +++ b/net/openvswitch/flow_netlink.c
>> > @@ -64,6 +64,7 @@ static bool actions_may_change_flow(const struct nlattr *actions)
>> >  		case OVS_ACTION_ATTR_TRUNC:
>> >  		case OVS_ACTION_ATTR_USERSPACE:
>> >  		case OVS_ACTION_ATTR_DROP:
>> > +		case OVS_ACTION_ATTR_EMIT_SAMPLE:
>> >  			break;
>> >
>> >  		case OVS_ACTION_ATTR_CT:
>> > @@ -2409,7 +2410,7 @@ 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 != 24);
>> > +	BUILD_BUG_ON(OVS_ACTION_ATTR_MAX != 25);
>> >
>> >  	if (!actions)
>> >  		return;
>> > @@ -3157,6 +3158,29 @@ static int validate_and_copy_check_pkt_len(struct net *net,
>> >  	return 0;
>> >  }
>> >
>> > +static int validate_emit_sample(const struct nlattr *attr)
>> > +{
>> > +	static const struct nla_policy policy[OVS_EMIT_SAMPLE_ATTR_MAX + 1] = {
>> > +		[OVS_EMIT_SAMPLE_ATTR_GROUP] = { .type = NLA_U32 },
>> > +		[OVS_EMIT_SAMPLE_ATTR_COOKIE] = {
>> > +			.type = NLA_BINARY,
>> > +			.len = OVS_EMIT_SAMPLE_COOKIE_MAX_SIZE
>> > +		},
>> > +	};
>> > +	struct nlattr *a[OVS_EMIT_SAMPLE_ATTR_MAX  + 1];
>> > +	int err;
>> > +
>> > +	if (!IS_ENABLED(CONFIG_PSAMPLE))
>> > +		return -EOPNOTSUPP;
>> > +
>> > +	err = nla_parse_nested(a, OVS_EMIT_SAMPLE_ATTR_MAX, attr, policy,
>> > +			       NULL);
>> > +	if (err)
>> > +		return err;
>> > +
>> > +	return a[OVS_EMIT_SAMPLE_ATTR_GROUP] ? 0 : -EINVAL;
>> > +}
>> > +
>> >  static int copy_action(const struct nlattr *from,
>> >  		       struct sw_flow_actions **sfa, bool log)
>> >  {
>> > @@ -3212,6 +3236,7 @@ static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
>> >  			[OVS_ACTION_ATTR_ADD_MPLS] = sizeof(struct ovs_action_add_mpls),
>> >  			[OVS_ACTION_ATTR_DEC_TTL] = (u32)-1,
>> >  			[OVS_ACTION_ATTR_DROP] = sizeof(u32),
>> > +			[OVS_ACTION_ATTR_EMIT_SAMPLE] = (u32)-1,
>> >  		};
>> >  		const struct ovs_action_push_vlan *vlan;
>> >  		int type = nla_type(a);
>> > @@ -3490,6 +3515,12 @@ static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
>> >  				return -EINVAL;
>> >  			break;
>> >
>> > +		case OVS_ACTION_ATTR_EMIT_SAMPLE:
>> > +			err = validate_emit_sample(a);
>> > +			if (err)
>> > +				return err;
>> > +			break;
>> > +
>> >  		default:
>> >  			OVS_NLERR(log, "Unknown Action type %d", type);
>> >  			return -EINVAL;
>>
>
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