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

Daniel Borkmann
2023-10-25T13:49:06Z

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.

Thanks,
Daniel