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

[Simon Horman]

On Mon, Jun 03, 2024 at 08:56:41PM +0200, 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@xxxxxxxxxx>

Reviewed-by: Simon Horman <horms@xxxxxxxxxx>