Currently, the kfree_skb_reason() in sch_handle_{ingress,egress}() can only
express a basic SKB_DROP_REASON_TC_INGRESS or SKB_DROP_REASON_TC_EGRESS reason.

Victor kicked-off an initial proposal to make this more flexible by disambiguating
verdict from return code by moving the verdict into struct tcf_result and
letting tcf_classify() return a negative error. If hit, then two new drop
reasons were added in the proposal, that is SKB_DROP_REASON_TC_INGRESS_ERROR
as well as SKB_DROP_REASON_TC_EGRESS_ERROR. Further analysis of the actual
error codes would have required to attach to tcf_classify via kprobe/kretprobe
to more deeply debug skb and the returned error.

In order to make the kfree_skb_reason() in sch_handle_{ingress,egress}() more
extensible, it can be addressed in a more straight forward way, that is: Instead
of placing the verdict into struct tcf_result, we can just put the drop reason
in there, which does not require changes throughout various classful schedulers
given the existing verdict logic can stay as is.

Then, SKB_DROP_REASON_TC_ERROR{,_*} can be added to the enum skb_drop_reason
to disambiguate between an error or an intentional drop. New drop reason error
codes can be added successively to the tc code base.

For internal error locations which have not yet been annotated with a
SKB_DROP_REASON_TC_ERROR{,_*}, the fallback is SKB_DROP_REASON_TC_INGRESS and
SKB_DROP_REASON_TC_EGRESS, respectively. Generic errors could be marked with a
SKB_DROP_REASON_TC_ERROR code until they are converted to more specific ones
if it is found that they would be useful for troubleshooting.

While drop reasons have infrastructure for subsystem specific error codes which
are currently used by mac80211 and ovs, Jakub mentioned that it is preferred
for tc to use the enum skb_drop_reason core codes given it is a better fit and
currently the tooling support is better, too.

With regards to the latter:

  [...] 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. [...]

Add a small helper tcf_set_drop_reason() which can be used to set the drop reason
into the tcf_result.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Victor Nogueira <victor@mojatatu.com>
Link: https://lore.kernel.org/netdev/20231006063233.74345d36@kernel.orgReviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20231009092655.22025-1-daniel@iogearbox.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>