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

[Daniel Borkmann <daniel@xxxxxxxxxxxxx>]

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:

When you submit feel free to add:

Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>