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Date: Sat, 4 Nov 2023 11:46:22 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Simon Horman <horms@...nel.org>
Cc: davem@...emloft.net, kuba@...nel.org, edumazet@...gle.com, 
	pabeni@...hat.com, jiri@...nulli.us, xiyou.wangcong@...il.com, 
	daniel@...earbox.net, idosch@...sch.org, netdev@...r.kernel.org
Subject: Re: [PATCH net 1/1] net, sched: Fix SKB_NOT_DROPPED_YET splat under
 debug config

On Sat, Nov 4, 2023 at 9:11 AM Simon Horman <horms@...nel.org> wrote:
>
> On Sat, Oct 28, 2023 at 01:16:10PM -0400, Jamal Hadi Salim wrote:
> > 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 disambiguating a legit error from a verdict with a bogus drop_reason
> >
> > [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]
> >
> > 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
> >
> > Ido reported:
> >
> >   [...] 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). [...]
> >
> >   [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
> >
> > What happens is that inside most classifiers the tcf_result is copied over
> > from a filter template e.g. *res = f->res which then implicitly overrides
> > the prior SKB_DROP_REASON_TC_{INGRESS,EGRESS} default drop code which was
> > set via sch_handle_{ingress,egress}() for kfree_skb_reason().
> >
> > Commit text above copied verbatim from Daniel. The general idea of the patch
> > is not very different from what Ido originally posted but instead done at the
> > cls_api codepath.
> >
> > Fixes: 54a59aed395c ("net, sched: Make tc-related drop reason more flexible")
> > Reported-by: Ido Schimmel <idosch@...sch.org>
> > Signed-off-by: Jamal Hadi Salim <jhs@...atatu.com>
> > Link: https://lore.kernel.org/netdev/ZTjY959R+AFXf3Xy@shredder
>
> Hi Jamal,
>
> FWIIW, I think it would be nicer to fix this the classifiers so they don't
> do this, which by my reading is what Daniel's patch did.
>

I dont believe it was nicer tbh.
In any case we are going to send cleanups to do this with skb cb.

> But, I don't feel strongly about this and I do tend to think the
> approach taken in this patch is a nice clean fix for net.
>
> Reviewed-by: Simon Horman <horms@...nel.org>
>
> BTW, this patch is marked as Not Applicable in patchwork.
> I am unsure why.

Weird.

cheers,
jamal

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