Re: [PATCH net-next v2 7/9] net: openvswitch: do not notify drops inside sample

[Ilya Maximets <i.maximets@ovn.org>]

On 6/3/24 20:56, Adrian Moreno wrote:
> The OVS_ACTION_ATTR_SAMPLE action is, in essence,
> observability-oriented.
> 
> Apart from some corner case in which it's used a replacement of clone()
> for old kernels, it's really only used for sFlow, IPFIX and now,
> local emit_sample.
> 
> With this in mind, it doesn't make much sense to report
> OVS_DROP_LAST_ACTION inside sample actions.
> 
> For instance, if the flow:
> 
>   actions:sample(..,emit_sample(..)),2
> 
> triggers a OVS_DROP_LAST_ACTION skb drop event, it would be extremely
> confusing for users since the packet did reach its destination.
> 
> This patch makes internal action execution silently consume the skb
> instead of notifying a drop for this case.
> 
> Unfortunately, this patch does not remove all potential sources of
> confusion since, if the sample action itself is the last action, e.g:
> 
>     actions:sample(..,emit_sample(..))
> 
> we actually _should_ generate a OVS_DROP_LAST_ACTION event, but we aren't.
> 
> Sadly, this case is difficult to solve without breaking the
> optimization by which the skb is not cloned on last sample actions.
> But, given explicit drop actions are now supported, OVS can just add one
> after the last sample() and rewrite the flow as:
> 
>     actions:sample(..,emit_sample(..)),drop
> 
> Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
> ---
>  net/openvswitch/actions.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
> index 33f6d93ba5e4..54fc1abcff95 100644
> --- a/net/openvswitch/actions.c
> +++ b/net/openvswitch/actions.c
> @@ -82,6 +82,15 @@ static struct action_fifo __percpu *action_fifos;
>  static struct action_flow_keys __percpu *flow_keys;
>  static DEFINE_PER_CPU(int, exec_actions_level);
>  
> +static inline void ovs_drop_skb_last_action(struct sk_buff *skb)
> +{
> +	/* Do not emit packet drops inside sample(). */
> +	if (OVS_CB(skb)->probability)
> +		consume_skb(skb);
> +	else
> +		ovs_kfree_skb_reason(skb, OVS_DROP_LAST_ACTION);
> +}
> +
>  /* Make a clone of the 'key', using the pre-allocated percpu 'flow_keys'
>   * space. Return NULL if out of key spaces.
>   */
> @@ -1061,7 +1070,7 @@ static int sample(struct datapath *dp, struct sk_buff *skb,
>  	if ((arg->probability != U32_MAX) &&
>  	    (!arg->probability || get_random_u32() > arg->probability)) {
>  		if (last)
> -			ovs_kfree_skb_reason(skb, OVS_DROP_LAST_ACTION);
> +			ovs_drop_skb_last_action(skb);
>  		return 0;
>  	}
>  
> @@ -1579,7 +1588,7 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
>  		}
>  	}
>  
> -	ovs_kfree_skb_reason(skb, OVS_DROP_LAST_ACTION);
> +	ovs_drop_skb_last_action(skb);

I don't think I agree with this one.  If we have a sample() action with
a lot of different actions inside and we reached the end while the last
action didn't consume the skb, then we should report that.  E.g.
"sample(emit_sample(),push_vlan(),set(eth())),2"  should report that the
cloned skb was dropped.  "sample(push_vlan(),emit_sample())" should not.

The only actions that are actually consuming the skb are "output",
"userspace", "recirc" and now "emit_sample".  "output" and "recirc" are
consuming the skb "naturally" by stealing it when it is the last action.
"userspace" has an explicit check to consume the skb if it is the last
action.  "emit_sample" should have the similar check.  It should likely
be added at the point of action introduction instead of having a separate
patch.

Best regards, Ilya Maximets.