Re: [PATCH net-next v2 1/2] net, sched: Make tc-related drop reason more flexible

Daniel Borkmann
2023-10-27T14:01:30Z

On 10/26/23 1:13 AM, Jamal Hadi Salim wrote:
> On Wed, Oct 25, 2023 at 9:46 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>> On 10/25/23 3:21 PM, Jamal Hadi Salim wrote:
>>> On Wed, Oct 25, 2023 at 7:52 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>>>> On 10/25/23 1:05 PM, Jamal Hadi Salim wrote:
>>>>> On Wed, Oct 25, 2023 at 6:01 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>>>>>> On 10/25/23 10:59 AM, Ido Schimmel wrote:
>>>>>>> On Mon, Oct 09, 2023 at 11:26:54AM +0200, Daniel Borkmann wrote:
>>>>>>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>>>>>>> index 606a366cc209..664426285fa3 100644
>>>>>>>> --- a/net/core/dev.c
>>>>>>>> +++ b/net/core/dev.c
>>>>>>>> @@ -3910,7 +3910,8 @@ 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,
>>>>>>>> + enum skb_drop_reason *drop_reason)
>>>>>>>> {
>>>>>>>> int ret = TC_ACT_UNSPEC;
>>>>>>>> #ifdef CONFIG_NET_CLS_ACT
>>>>>>>> @@ -3922,12 +3923,14 @@ static int tc_run(struct tcx_entry *entry, struct sk_buff *skb)
>>>>>>>>
>>>>>>>> tc_skb_cb(skb)->mru = 0;
>>>>>>>> tc_skb_cb(skb)->post_ct = false;
>>>>>>>> + res.drop_reason = *drop_reason;
>>>>>>>>
>>>>>>>> mini_qdisc_bstats_cpu_update(miniq, skb);
>>>>>>>> ret = tcf_classify(skb, miniq->block, miniq->filter_list, &res, false);
>>>>>>>> /* Only tcf related quirks below. */
>>>>>>>> switch (ret) {
>>>>>>>> case TC_ACT_SHOT:
>>>>>>>> + *drop_reason = res.drop_reason;
>>>>>>>
>>>>>>> Daniel,
>>>>>>>
>>>>>>> Getting the following splat [1] with CONFIG_DEBUG_NET=y and this
>>>>>>> reproducer [2]. Problem seems to be that classifiers clear 'struct
>>>>>>> tcf_result::drop_reason', thereby triggering the warning in
>>>>>>> __kfree_skb_reason() due to reason being 'SKB_NOT_DROPPED_YET' (0).
>>>>>>>
>>>>>>> Fixed by maintaining the original drop reason if the one returned from
>>>>>>> tcf_classify() is 'SKB_NOT_DROPPED_YET' [3]. I can submit this fix
>>>>>>> unless you have a better idea.
>>>>>>
>>>>>> Thanks for catching this, looks reasonable to me as a fix.
>>>>>>
>>>>>>> [1]
>>>>>>> WARNING: CPU: 0 PID: 181 at net/core/skbuff.c:1082 kfree_skb_reason+0x38/0x130
>>>>>>> Modules linked in:
>>>>>>> CPU: 0 PID: 181 Comm: mausezahn Not tainted 6.6.0-rc6-custom-ge43e6d9582e0 #682
>>>>>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-1.fc37 04/01/2014
>>>>>>> RIP: 0010:kfree_skb_reason+0x38/0x130
>>>>>>> [...]
>>>>>>> Call Trace:
>>>>>>> <IRQ>
>>>>>>> __netif_receive_skb_core.constprop.0+0x837/0xdb0
>>>>>>> __netif_receive_skb_one_core+0x3c/0x70
>>>>>>> process_backlog+0x95/0x130
>>>>>>> __napi_poll+0x25/0x1b0
>>>>>>> net_rx_action+0x29b/0x310
>>>>>>> __do_softirq+0xc0/0x29b
>>>>>>> do_softirq+0x43/0x60
>>>>>>> </IRQ>
>>>>>>>
>>>>>>> [2]
>>>>>>> #!/bin/bash
>>>>>>>
>>>>>>> ip link add name veth0 type veth peer name veth1
>>>>>>> ip link set dev veth0 up
>>>>>>> ip link set dev veth1 up
>>>>>>> tc qdisc add dev veth1 clsact
>>>>>>> tc filter add dev veth1 ingress pref 1 proto all flower dst_mac 00:11:22:33:44:55 action drop
>>>>>>> mausezahn veth0 -a own -b 00:11:22:33:44:55 -q -c 1
>>>>>>
>>>>>> I didn't know you're using mausezahn, nice :)
>>>>>>
>>>>>>> [3]
>>>>>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>>>>>> index a37a932a3e14..abd0b13f3f17 100644
>>>>>>> --- a/net/core/dev.c
>>>>>>> +++ b/net/core/dev.c
>>>>>>> @@ -3929,7 +3929,8 @@ static int tc_run(struct tcx_entry *entry, struct sk_buff *skb,
>>>>>>> /* Only tcf related quirks below. */
>>>>>>> switch (ret) {
>>>>>>> case TC_ACT_SHOT:
>>>>>>> - *drop_reason = res.drop_reason;
>>>>>>> + if (res.drop_reason != SKB_NOT_DROPPED_YET)
>>>>>>> + *drop_reason = res.drop_reason;
>>>>>>> mini_qdisc_qstats_cpu_drop(miniq);
>>>>>>> break;
>>>>>>> case TC_ACT_OK:
>>>>>>>
>>>>>
>>>>> Out of curiosity - how does the policy say "drop" but drop_reason does
>>>>> not reflect it?
>>>>
>>>> Ido, Jamal, wdyt about this alternative approach - these were the locations I could
>>>> find from an initial glance (compile-tested) :
>>>>
>>>> From a3d46a55aac484372b60b783cb6a3c98a0fef75c Mon Sep 17 00:00:00 2001
>>>> From: Daniel Borkmann <daniel@iogearbox.net>
>>>> Date: Wed, 25 Oct 2023 11:43:44 +0000
>>>> Subject: [PATCH] net, sched: fix..
>>>>
>>>> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
>>>> ---
>>>> include/net/pkt_cls.h | 12 ++++++++++++
>>>> net/sched/cls_basic.c | 2 +-
>>>> net/sched/cls_bpf.c | 2 +-
>>>> net/sched/cls_flower.c | 2 +-
>>>> net/sched/cls_fw.c | 2 +-
>>>> net/sched/cls_matchall.c | 2 +-
>>>> net/sched/cls_route.c | 4 ++--
>>>> net/sched/cls_u32.c | 2 +-
>>>> 8 files changed, 20 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/include/net/pkt_cls.h b/include/net/pkt_cls.h
>>>> index a76c9171db0e..31d8e8587824 100644
>>>> --- a/include/net/pkt_cls.h
>>>> +++ b/include/net/pkt_cls.h
>>>> @@ -160,6 +160,18 @@ static inline void tcf_set_drop_reason(struct tcf_result *res,
>>>> res->drop_reason = reason;
>>>> }
>>>>
>>>> +static inline void tcf_set_result(struct tcf_result *to,
>>>> + const struct tcf_result *from)
>>>> +{
>>>> + /* tcf_result's drop_reason which is the last member must be
>>>> + * preserved and cannot be copied from the cls'es tcf_result
>>>> + * template given this is carried all the way and potentially
>>>> + * set to a concrete tc drop reason upon error or intentional
>>>> + * drop. See tcf_set_drop_reason() locations.
>>>> + */
>>>> + memcpy(to, from, offsetof(typeof(*to), drop_reason));
>>>> +}
>>>> +
>>>
>>> Daniel, IMO, doing this at cls_api is best instead (like what Victors
>>> or my original patch did). Iam ~30K feet right now with a lousy
>>> keyboard - you can either do it, or i or Victor can send the patch by
>>> end of day today. There are missing cases which were covered by Victor
>>> and possibly something else will pop up next.
>>
>> Sure, if you have sth clean and simple for today, go for it. Otherwise I
>> can cook a proper one out of this as a fix and ship it tomorrow AM, so we
>> have a fix for the splat in CONFIG_DEBUG_NET kernels and you can still
>> refactor later.
>
> I was thinking something along these lines:
>
> --- a/net/sched/cls_api.c
> +++ b/net/sched/cls_api.c
> @@ -1662,6 +1662,7 @@ static inline int __tcf_classify(struct sk_buff *skb,
> const int max_reclassify_loop = 16;
> const struct tcf_proto *first_tp;
> int limit = 0;
> + u32 reason_code = res->drop_reason;
>
> reclassify:
> #endif
> @@ -1712,8 +1713,11 @@ static inline int __tcf_classify(struct sk_buff *skb,
> goto reset;
> }
> #endif
> - if (err >= 0)
> + if (err >= 0) {
> + if (err == TC_ACT_SHOT) /* policy drop */
> + tcf_set_drop_reason(res, orig_reason);
> return err;
> + }
> }
>
> if (unlikely(n)) {
>
> But tbh, i am struggling with the whole approach you took in the
> earlier patch - i.e setting the drop reason from cls_act and then
> expecting it not to be changed on policy drops; while it works for
> clsact, it is not going to work for the rest of the qdiscs - unless we
> change all the qdisc enqueues to take an extra param for drop_reason.
> Thoughts?

The downside of the above would be that you then cannot make use of
tcf_set_drop_reason() further down the tc engine, for example, within
classifiers/actions, e.g. tcf_action_exec() where you also have drops
like:

[...]
} else if (TC_ACT_EXT_CMP(ret, TC_ACT_GOTO_CHAIN)) {
if (unlikely(!rcu_access_pointer(a->goto_chain))) {
net_warn_ratelimited("can't go to NULL chain!\n");
return TC_ACT_SHOT;
}
tcf_action_goto_chain_exec(a, res);
}
[...]

If I spot this correctly, qdiscs also use tcf_classify() as well, and
so far none of them have kfree_skb_reason() support, but if you plan to
add it you can just set a default res.drop_reason from there as well
since tcf_classify() already takes the res param.

Thanks,
Daniel