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

Jamal Hadi Salim
2023-10-25T11:05:46Z

On Wed, Oct 25, 2023 at 6:01 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> Hi Ido,
>
> 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?

cheers,
jamal

> When you submit feel free to add:
>
> Acked-by: Daniel Borkmann <daniel@iogearbox.net>