Re: 4.20-rc6: WARNING: CPU: 30 PID: 197360 at net/core/flow_dissector.c:764 __skb_flow_dissect

[Willem de Bruijn]

On Thu, Dec 20, 2018 at 6:15 AM Ido Schimmel <idosch@xxxxxxxxxx> wrote:
>
> +Willem
>
> On Thu, Dec 20, 2018 at 08:45:40AM +0100, Christian Borntraeger wrote:
> > Folks,
> >
> > I got this warning today. I cant tell when and why this happened, so I do not know yet how to reproduce.
> > Maybe someone has a quick idea.
> >
> > [85109.572032] WARNING: CPU: 30 PID: 197360 at net/core/flow_dissector.c:764 __skb_flow_dissect+0x1f0/0x1318
>
> I managed to trigger this warning as well the other day, but from a
> different call path:
>
> [280155.348610]  fib_multipath_hash+0x28c/0x2d0
> [280155.348613]  ? fib_multipath_hash+0x28c/0x2d0
> [280155.348619]  fib_select_path+0x241/0x32f
> [280155.348622]  ? __fib_lookup+0x6a/0xb0
> [280155.348626]  ip_route_output_key_hash_rcu+0x650/0xa30
> [280155.348631]  ? __alloc_skb+0x9b/0x1d0
> [280155.348634]  inet_rtm_getroute+0x3f7/0xb80

inet_rtm_getroute builds a new packet with inet_rtm_getroute_build_skb
here without dev or sk.

> Problem is the synthesized skb for output route resolution does not have
> skb->dev or skb->sk set. When a multipath route is hit and
> net.ipv4.fib_multipath_hash_policy is set the flow dissector is called
> with this skb and the warning is triggered.
>
> I plan to fix it by setting skb->dev to net->loopback_dev.

The device can be chosen based on iif in inet_rtm_getroute? A first
thought, I don't know this code very well.

Let me know if you want me to take a stab at that patch. IPv6 probably
will need the same.

> I assume we
> want to keep this warning to prevent call paths which will otherwise
> silently fallback to standard flow dissector instead of the BPF one.

Indeed, the warning is there to sniff out paths that do not follow
what I thought was an invariant. If there are too many exceptions, I
may have to revisit that assumption. But for now, let's see if we can
address these edge cases.

> I'm not familiar with tap code, so someone else will need to patch this
> case, but it looks like:
>
> tap_sendmsg()
>     tap_get_user()
>         skb_probe_transport_header()
>             skb_flow_dissect_flow_keys_basic()
>                 __skb_flow_dissect()
>
> skb->dev is only set later in the code.

tap_get_user uses sock_alloc_send_pskb (through tap_alloc_skb) to
allocate the skb. So skb->sk should be set at the time of
skb_probe_transport_header. I'm not sure how this path triggers the
warning.