Re: [bpf, xdp] headroom - was: Re: Question about to KMSAN: uninit-value in can_receive

[Oliver Hartkopp]

On 06.01.26 00:26, Jakub Kicinski wrote:
> On Mon, 5 Jan 2026 14:47:08 +0100 Oliver Hartkopp wrote:
> > For the ifindex I would propose to store it in struct skb_shared_info:
> >
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 86737076101d..f7233b8f461c 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -604,10 +604,15 @@ struct skb_shared_info {
> >                   struct xsk_tx_metadata_compl xsk_meta;
> >           };
> >           unsigned int    gso_type;
> >           u32             tskey;
> >
> > +#if IS_ENABLED(CONFIG_CAN)
> > +       /* initial CAN iif to avoid routing back to it (can-gw) */
> > +       int can_iif;
> > +#endif
> > +
> >           /*
> >            * Warning : all fields before dataref are cleared in __alloc_skb()
> >            */
> >           atomic_t        dataref;
> >
> > Would this be a suitable approach to get rid of struct can_skb_priv in
> > your opinion?
>
> Possibly a naive question but why is skb_iif not working here?

With the CAN gateway (net/can/gw.c) incoming CAN frames can be modified 
and forwarded to other CAN interfaces via dev_queue_xmit(skb).

When such skb is echo'ed back after successful transmission via 
netif_rx() this leads to skb->skb_iif = skb->dev->ifindex;

To prevent a loopback the CAN frame must not be sent back to the 
originating interface - even when it has been routed to different CAN 
interfaces in the meantime (which always overwrites skb_iif).

Therefore we need to maintain the "real original" incoming interface.

can_iif could also be named skb_initial_iif when someone else would need 
such an information too.

Best regards,
Oliver