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Date: Thu, 02 Nov 2023 11:17:23 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Daniel Borkmann <daniel@...earbox.net>, Jamal Hadi Salim
 <jhs@...atatu.com>
Cc: kuba@...nel.org, idosch@...sch.org, netdev@...r.kernel.org, 
	bpf@...r.kernel.org
Subject: Re: [PATCH net-next] net, sched: Fix SKB_NOT_DROPPED_YET splat
 under debug config

On Fri, 2023-10-27 at 20:21 +0200, Daniel Borkmann wrote:
> On 10/27/23 7:24 PM, Jamal Hadi Salim wrote:
> > On Fri, Oct 27, 2023 at 9:51 AM Daniel Borkmann <daniel@...earbox.net> wrote:
> > > 
> > > 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().
> > > 
> > > Add a small helper tcf_set_result() and convert classifiers over to it.
> > > The latter leaves the drop code intact and classifiers, actions as well
> > > as the action engine in tcf_exts_exec() can then in future make use of
> > > tcf_set_drop_reason(), too.
> > > 
> > > Tested that the splat is fixed under CONFIG_DEBUG_NET=y with the repro.
> > > 
> > > Fixes: 54a59aed395c ("net, sched: Make tc-related drop reason more flexible")
> > > Reported-by: Ido Schimmel <idosch@...sch.org>
> > > Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> > > Cc: Jamal Hadi Salim <jhs@...atatu.com>
> > > Cc: Jakub Kicinski <kuba@...nel.org>
> > > Link: https://lore.kernel.org/netdev/ZTjY959R+AFXf3Xy@shredder
> > > ---
> > >   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));
> > > +}
> > 
> > I believe our bigger issue here is we are using this struct now for
> > both policy set by the control plane and for runtime decisions
> 
> Hm, but that was also either way in the original rfc.
> 
> > (drop_reason) - whereas the original assumption was this struct only
> > held set policy. In retrospect we should have put the verdict(which is
> > policy) here and return the error code (as was in the first patch). I
> > am also not sure humans would not make a mistake on "this field must
> > be at the end of the struct". Can we put some assert (or big comment
> > on the struct) to make sure someone does not overwrite this field?
> 
> Yeah that can be done.

FTR, I agree the comment or even better a build_bug_on() somewhere
should be better.

Thanks!

Paolo


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