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Message-ID: <CAM0EoMnPVxYA=7jn6AU7D3cJJbY5eeMLOxCrj4UJcFr=pCZ+Aw@mail.gmail.com>
Date: Mon, 25 Sep 2023 19:01:24 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: Victor Nogueira <victor@...atatu.com>, xiyou.wangcong@...il.com, jiri@...nulli.us,
davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
paulb@...dia.com, netdev@...r.kernel.org, kernel@...atatu.com,
martin.lau@...ux.dev, bpf@...r.kernel.org
Subject: Re: [PATCH net-next 1/1] net/sched: Disambiguate verdict from return code
On Fri, Sep 22, 2023 at 4:12 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 9/20/23 1:20 AM, Jamal Hadi Salim wrote:
> > On Tue, Sep 19, 2023 at 6:15 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> >>
> >> [ +Martin, bpf ]
> >>
> >> On 9/19/23 4:59 PM, Victor Nogueira wrote:
> >>> Currently there is no way to distinguish between an error and a
> >>> classification verdict. This patch adds the verdict field as a part of
> >>> struct tcf_result. That way, tcf_classify can return a proper
> >>> error number when it fails, and we keep the classification result
> >>> information encapsulated in struct tcf_result.
> >>>
> >>> Also add values SKB_DROP_REASON_TC_EGRESS_ERROR and
> >>> SKB_DROP_REASON_TC_INGRESS_ERROR to enum skb_drop_reason.
> >>> With that we can distinguish between a drop from a processing error versus
> >>> a drop from classification.
> >>>
> >>> Signed-off-by: Victor Nogueira <victor@...atatu.com>
> >>> ---
> >>> include/net/dropreason-core.h | 6 +++++
> >>> include/net/sch_generic.h | 7 ++++++
> >>> net/core/dev.c | 42 ++++++++++++++++++++++++++---------
> >>> net/sched/cls_api.c | 38 ++++++++++++++++++++-----------
> >>> net/sched/sch_cake.c | 32 +++++++++++++-------------
> >>> net/sched/sch_drr.c | 33 +++++++++++++--------------
> >>> net/sched/sch_ets.c | 6 +++--
> >>> net/sched/sch_fq_codel.c | 29 ++++++++++++------------
> >>> net/sched/sch_fq_pie.c | 28 +++++++++++------------
> >>> net/sched/sch_hfsc.c | 6 +++--
> >>> net/sched/sch_htb.c | 6 +++--
> >>> net/sched/sch_multiq.c | 6 +++--
> >>> net/sched/sch_prio.c | 7 ++++--
> >>> net/sched/sch_qfq.c | 34 +++++++++++++---------------
> >>> net/sched/sch_sfb.c | 29 ++++++++++++------------
> >>> net/sched/sch_sfq.c | 28 +++++++++++------------
> >>> 16 files changed, 195 insertions(+), 142 deletions(-)
> >>>
> >>> diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
> >>> index a587e83fc169..b1c069c8e7f2 100644
> >>> --- a/include/net/dropreason-core.h
> >>> +++ b/include/net/dropreason-core.h
> >>> @@ -80,6 +80,8 @@
> >>> FN(IPV6_NDISC_BAD_OPTIONS) \
> >>> FN(IPV6_NDISC_NS_OTHERHOST) \
> >>> FN(QUEUE_PURGE) \
> >>> + FN(TC_EGRESS_ERROR) \
> >>> + FN(TC_INGRESS_ERROR) \
> >>> FNe(MAX)
> >>>
> >>> /**
> >>> @@ -345,6 +347,10 @@ enum skb_drop_reason {
> >>> SKB_DROP_REASON_IPV6_NDISC_NS_OTHERHOST,
> >>> /** @SKB_DROP_REASON_QUEUE_PURGE: bulk free. */
> >>> SKB_DROP_REASON_QUEUE_PURGE,
> >>> + /** @SKB_DROP_REASON_TC_EGRESS_ERROR: dropped in TC egress HOOK due to error */
> >>> + SKB_DROP_REASON_TC_EGRESS_ERROR,
> >>> + /** @SKB_DROP_REASON_TC_INGRESS_ERROR: dropped in TC ingress HOOK due to error */
> >>> + SKB_DROP_REASON_TC_INGRESS_ERROR,
> >>> /**
> >>> * @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which
> >>> * shouldn't be used as a real 'reason' - only for tracing code gen
> >>> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> >>> index f232512505f8..9a3f71d2545e 100644
> >>> --- a/include/net/sch_generic.h
> >>> +++ b/include/net/sch_generic.h
> >>> @@ -326,6 +326,7 @@ struct Qdisc_ops {
> >>>
> >>>
> >>> struct tcf_result {
> >>> + u32 verdict;
> >>> union {
> >>> struct {
> >>> unsigned long class;
> >>> @@ -336,6 +337,12 @@ struct tcf_result {
> >>> };
> >>> };
> >>>
> >>> +static inline void tcf_result_set_verdict(struct tcf_result *res,
> >>> + const u32 verdict)
> >>> +{
> >>> + res->verdict = verdict;
> >>> +}
> >>> +
> >>> struct tcf_chain;
> >>>
> >>> struct tcf_proto_ops {
> >>> diff --git a/net/core/dev.c b/net/core/dev.c
> >>> index ccff2b6ef958..1450f4741d9b 100644
> >>> --- a/net/core/dev.c
> >>> +++ b/net/core/dev.c
> >>> @@ -3910,31 +3910,39 @@ EXPORT_SYMBOL_GPL(netdev_xmit_skip_txqueue);
> >>> #endif /* CONFIG_NET_EGRESS */
> >>>
> >>> #ifdef CONFIG_NET_XGRESS
> >>> -static int tc_run(struct tcx_entry *entry, struct sk_buff *skb)
> >>> +static int tc_run(struct tcx_entry *entry, struct sk_buff *skb,
> >>> + struct tcf_result *res)
> >>> {
> >>> - int ret = TC_ACT_UNSPEC;
> >>> + int ret = 0;
> >>> #ifdef CONFIG_NET_CLS_ACT
> >>> struct mini_Qdisc *miniq = rcu_dereference_bh(entry->miniq);
> >>> - struct tcf_result res;
> >>>
> >>> - if (!miniq)
> >>> + if (!miniq) {
> >>> + tcf_result_set_verdict(res, TC_ACT_UNSPEC);
> >>> return ret;
> >>> + }
> >>>
> >>> tc_skb_cb(skb)->mru = 0;
> >>> tc_skb_cb(skb)->post_ct = false;
> >>>
> >>> mini_qdisc_bstats_cpu_update(miniq, skb);
> >>> - ret = tcf_classify(skb, miniq->block, miniq->filter_list, &res, false);
> >>> + ret = tcf_classify(skb, miniq->block, miniq->filter_list, res, false);
> >>> + if (ret < 0) {
> >>> + mini_qdisc_qstats_cpu_drop(miniq);
> >>> + return ret;
> >>> + }
> >>> /* Only tcf related quirks below. */
> >>> - switch (ret) {
> >>> + switch (res->verdict) {
> >>> case TC_ACT_SHOT:
> >>> mini_qdisc_qstats_cpu_drop(miniq);
> >>> break;
> >>> case TC_ACT_OK:
> >>> case TC_ACT_RECLASSIFY:
> >>> - skb->tc_index = TC_H_MIN(res.classid);
> >>> + skb->tc_index = TC_H_MIN(res->classid);
> >>> break;
> >>> }
> >>> +#else
> >>> + tcf_result_set_verdict(res, TC_ACT_UNSPEC);
> >>> #endif /* CONFIG_NET_CLS_ACT */
> >>> return ret;
> >>> }
> >>> @@ -3977,6 +3985,7 @@ sch_handle_ingress(struct sk_buff *skb, struct packet_type **pt_prev, int *ret,
> >>> struct net_device *orig_dev, bool *another)
> >>> {
> >>> struct bpf_mprog_entry *entry = rcu_dereference_bh(skb->dev->tcx_ingress);
> >>> + struct tcf_result res = {0};
> >>> int sch_ret;
> >>>
> >>> if (!entry)
> >>> @@ -3994,9 +4003,14 @@ sch_handle_ingress(struct sk_buff *skb, struct packet_type **pt_prev, int *ret,
> >>> if (sch_ret != TC_ACT_UNSPEC)
> >>> goto ingress_verdict;
> >>> }
> >>> - sch_ret = tc_run(tcx_entry(entry), skb);
> >>> + sch_ret = tc_run(tcx_entry(entry), skb, &res);
> >>> + if (sch_ret < 0) {
> >>> + kfree_skb_reason(skb, SKB_DROP_REASON_TC_INGRESS_ERROR);
> >>> + *ret = NET_RX_DROP;
> >>> + return NULL;
> >>> + }
> >>> ingress_verdict:
> >>> - switch (sch_ret) {
> >>> + switch (res.verdict) {
> >>
> >> This breaks tcx, please move all this logic into tc_run(). No changes to sch_handle_ingress()
> >> or sch_handle_egress should be necessary, you can then just remap the return code to TC_ACT_SHOT
> >> in such case.
> >
> > I think it is valuable to have a good reason code like
> > SKB_DROP_REASON_TC_XXX_ERROR to disambiguate between errors vs
> > verdicts in the case of tc_run() variant.
> > For tcx_run(), does this look ok (for consistency)?:
> >
> > if (static_branch_unlikely(&tcx_needed_key)) {
> > sch_ret = tcx_run(entry, skb, true);
> > if (sch_ret != TC_ACT_UNSPEC) {
> > res.verdict = sch_ret;
> > goto ingress_verdict;
> > }
> > }
>
> In the above case we don't have 'internal' errors which you want to trace, so I would
> also love to avoid the cost of zeroing struct tcf_result res which should be 3x 8b for
> every packet.
We can move the zeroing inside tc_run() but we declare it in the same
spot as we do right now. You will still need to set res.verdict as
above.
Would that work for you?
> I was more thinking like something below could be a better choice. I presume your main
> goal is to trace where these errors originated in the first place, so it might even be
> useful to capture the actual return code as well.
The main motivation is a few syzkaller bugs which resulted in not
disambiguating between errors being returned and sometimes
TC_ACT_SHOT.
> Then you can use perf script, bpf and whatnot to gather further insights into what
> happened while being less invasive and avoiding the need to extend struct tcf_result.
>
We could use trace instead - the reason we have the skb reason is
being used in the other spots (does this trace require ebpf to be
usable?).
> This would be quite similar to trace_xdp_exception() as well, and I think you can guarantee
> that in fast path all errors are < TC_ACT_UNSPEC anyway.
>
I am not sure i followed. 0 means success, result codes are returned in res now.
cheers,
jamal
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 85df22f05c38..4089d195144d 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3925,6 +3925,10 @@ static int tc_run(struct tcx_entry *entry, struct sk_buff *skb)
>
> mini_qdisc_bstats_cpu_update(miniq, skb);
> ret = tcf_classify(skb, miniq->block, miniq->filter_list, &res, false);
> + if (unlikely(ret < TC_ACT_UNSPEC)) {
> + trace_tc_exception(skb->dev, skb->tc_at_ingress, ret);
> + ret = TC_ACT_SHOT;
> + }
> /* Only tcf related quirks below. */
> switch (ret) {
> case TC_ACT_SHOT:
>
> Best,
> Daniel
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