Subject: Re: [PATCH net-next 1/1] net/sched: Disambiguate verdict from return code

From: Jakub Kicinski <[email-masked]>
Date: Fri, 6 Oct 2023 06:32:33 -0700

On Fri, 6 Oct 2023 13:18:40 +0200 Daniel Borkmann wrote:
> That should be possible with some work this way, agree. I've been toying a bit
> more on this issue, and actually there is an even better way which would cleanly
> solve all use cases and we likely would utilize it for bpf as well in future.
> I wasn't aware of it before, but the drop reason actually has per-subsystem infra
> already which so far only mac80211 and ovs makes use of.

FWIW I'm not sure if leaning into the subsys specific error codes for
something as core as TC is a good direction. I'd think that what
matters to the user is was it an intentional policy drop or some form
of an error / exception. More detailed info can come from stats.

Maybe I'm overly conservative because I don't care about debugging
mac80211 or ovs but do very much care about TC :) And I think Alastair 
(bpftrace) is working on auto-prettifying enums when bpftrace outputs
maps. So we can do something like:

$ bpftrace -e 'tracepoint:skb:kfree_skb { @[args->reason] = count(); }'
Attaching 1 probe...
^C

@[SKB_DROP_REASON_TC_INGRESS]: 2
@[SKB_CONSUMED]: 34

  ^^^^^^^^^^^^ names!!

Auto-magically.

Which will no longer work with the "pack multiple values into 
the reason" scheme of subsys-specific values :(

What I'm saying is that there is a trade-off here between providing 
as much info as possible vs basic user getting intelligible data..